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ATTINY25/45/85(V) Datasheet by Microchip Technology

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Features
High Performance, Low Power AVR® 8-Bit Microcontroller
Advanced RISC Architecture
120 Powerful Instructions – Most Single Clock Cycle Execution
32 x 8 General Purpose Working Registers
Fully Static Operation
Non-volatile Program and Data Memories
2/4/8K Bytes of In-System Programmable Program Memory Flash
Endurance: 10,000 Write/Erase Cycles
128/256/512 Bytes In-System Programmable EEPROM
Endurance: 100,000 Write/Erase Cycles
128/256/512 Bytes Internal SRAM
Programming Lock for Self-Programming Flash Program and EEPROM Data Security
Peripheral Features
8-bit Timer/Counter with Prescaler and Two PWM Channels
8-bit High Speed Timer/Counter with Separate Prescaler
2 High Frequency PWM Outputs with Separate Output Compare Registers
Programmable Dead Time Generator
USI – Universal Serial Interface with Start Condition Detector
10-bit ADC
4 Single Ended Channels
2 Differential ADC Channel Pairs with Programmable Gain (1x, 20x)
Temperature Measurement
Programmable Watchdog Timer with Separate On-chip Oscillator
On-chip Analog Comparator
Special Microcontroller Features
debugWIRE On-chip Debug System
In-System Programmable via SPI Port
External and Internal Interrupt Sources
Low Power Idle, ADC Noise Reduction, and Power-down Modes
Enhanced Power-on Reset Circuit
Programmable Brown-out Detection Circuit
Internal Calibrated Oscillator
I/O and Packages
Six Programmable I/O Lines
8-pin PDIP, 8-pin SOIC, 20-pad QFN/MLF, and 8-pin TSSOP (only ATtiny45/V)
Operating Voltage
1.8 - 5.5V for ATtiny25V/45V/85V
2.7 - 5.5V for ATtiny25/45/85
Speed Grade
ATtiny25V/45V/85V: 0 – 4 MHz @ 1.8 - 5.5V, 0 - 10 MHz @ 2.7 - 5.5V
ATtiny25/45/85: 0 – 10 MHz @ 2.7 - 5.5V, 0 - 20 MHz @ 4.5 - 5.5V
Industrial Temperature Range
Low Power Consumption
Active Mode:
1 MHz, 1.8V: 300 µA
Power-down Mode:
0.1 µA at 1.8V
Atmel 8-bit AVR Microcontroller with 2/4/8K
Bytes In-System Programmable Flash
ATtiny25/V / ATtiny45/V / ATtiny85/V
Rev. 2586Q–AVR–08/2013
jjjj E1 E1 E3 I: FH’H—H—H—V LHJLHJLI UZD
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1. Pin Configurations
Figure 1-1. Pinout ATtiny25/45/85
1.1 Pin Descriptions
1.1.1 VCC
Supply voltage.
1.1.2 GND
Ground.
1.1.3 Port B (PB5:PB0)
Port B is a 6-bit bi-directional I/O port with internal pull-up resistors (selected for each bit). The Port B output buffers
have symmetrical drive characteristics with both high sink and source capability. As inputs, Port B pins that are
externally pulled low will source current if the pull-up resistors are activated. The Port B pins are tri-stated when a
reset condition becomes active, even if the clock is not running.
1
2
3
4
8
7
6
5
(PCINT5/RESET/ADC0/dW) PB5
(PCINT3/XTAL1/CLKI/OC1B/ADC3) PB3
(PCINT4/XTAL2/CLKO/OC1B/ADC2) PB4
GND
VCC
PB2 (SCK/USCK/SCL/ADC1/T0/INT0/PCINT2)
PB1 (MISO/DO/AIN1/OC0B/OC1A/PCINT1)
PB0 (MOSI/DI/SDA/AIN0/OC0A/OC1A/AREF/PCINT0)
PDIP/SOIC/TSSOP
1
2
3
4
5
QFN/MLF
15
14
13
12
11
20
19
18
17
16
6
7
8
9
10
DNC
DNC
GND
DNC
DNC
DNC
DNC
DNC
DNC
DNC
NOTE: Bottom pad should be soldered to ground.
DNC: Do Not Connect
NOTE: TSSOP only for ATtiny45/V
(PCINT5/RESET/ADC0/dW) PB5
(PCINT3/XTAL1/CLKI/OC1B/ADC3) PB3
DNC
DNC
(PCINT4/XTAL2/CLKO/OC1B/ADC2) PB4
VCC
PB2 (SCK/USCK/SCL/ADC1/T0/INT0/PCINT2)
DNC
PB1 (MISO/DO/AIN1/OC0B/OC1A/PCINT1)
PB0 (MOSI/DI/SDA/AIN0/OC0A/OC1A/AREF/PCINT0)
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Port B also serves the functions of various special features of the ATtiny25/45/85 as listed in “Alternate Functions
of Port B” on page 60.
On ATtiny25, the programmable I/O ports PB3 and PB4 (pins 2 and 3) are exchanged in ATtiny15 Compatibility
Mode for supporting the backward compatibility with ATtiny15.
1.1.4 RESET
Reset input. A low level on this pin for longer than the minimum pulse length will generate a reset, even if the clock
is not running and provided the reset pin has not been disabled. The minimum pulse length is given in Table 21-4
on page 165. Shorter pulses are not guaranteed to generate a reset.
The reset pin can also be used as a (weak) I/O pin.
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2. Overview
The ATtiny25/45/85 is a low-power CMOS 8-bit microcontroller based on the AVR enhanced RISC architecture. By
executing powerful instructions in a single clock cycle, the ATtiny25/45/85 achieves throughputs approaching 1
MIPS per MHz allowing the system designer to optimize power consumption versus processing speed.
2.1 Block Diagram
Figure 2-1. Block Diagram
The AVR core combines a rich instruction set with 32 general purpose working registers. All 32 registers are
directly connected to the Arithmetic Logic Unit (ALU), allowing two independent registers to be accessed in one
single instruction executed in one clock cycle. The resulting architecture is more code efficient while achieving
throughputs up to ten times faster than conventional CISC microcontrollers.
PROGRAM
COUNTER
CALIBRATED
INTERNAL
OSCILLATOR
WATCHDOG
TIMER
STACK
POINTER
PROGRAM
FLASH SRAM
MCU CONTROL
REGISTER
GENERAL
PURPOSE
REGISTERS
INSTRUCTION
REGISTER
TIMER/
COUNTER0
SERIAL
UNIVERSAL
INTERFACE
TIMER/
COUNTER1
INSTRUCTION
DECODER
DATA DIR.
REG.PORT B
DATA REGISTER
PORT B
PROGRAMMING
LOGIC
TIMING AND
CONTROL
MCU STATUS
REGISTER
STATUS
REGISTER
ALU
PORT B DRIVERS
PB[0:5]
VCC
GND
CONTROL
LINES
8-BIT DATABUS
Z
ADC /
ANALOG COMPARATOR
INTERRUPT
UNIT
DATA
EEPROM OSCILLATORS
Y
X
RESET
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The ATtiny25/45/85 provides the following features: 2/4/8K bytes of In-System Programmable Flash, 128/256/512
bytes EEPROM, 128/256/256 bytes SRAM, 6 general purpose I/O lines, 32 general purpose working registers, one
8-bit Timer/Counter with compare modes, one 8-bit high speed Timer/Counter, Universal Serial Interface, Internal
and External Interrupts, a 4-channel, 10-bit ADC, a programmable Watchdog Timer with internal Oscillator, and
three software selectable power saving modes. Idle mode stops the CPU while allowing the SRAM, Timer/Counter,
ADC, Analog Comparator, and Interrupt system to continue functioning. Power-down mode saves the register con-
tents, disabling all chip functions until the next Interrupt or Hardware Reset. ADC Noise Reduction mode stops the
CPU and all I/O modules except ADC, to minimize switching noise during ADC conversions.
The device is manufactured using Atmel’s high density non-volatile memory technology. The On-chip ISP Flash
allows the Program memory to be re-programmed In-System through an SPI serial interface, by a conventional
non-volatile memory programmer or by an On-chip boot code running on the AVR core.
The ATtiny25/45/85 AVR is supported with a full suite of program and system development tools including: C Com-
pilers, Macro Assemblers, Program Debugger/Simulators and Evaluation kits.
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3. About
3.1 Resources
A comprehensive set of development tools, application notes and datasheets are available for download on
http://www.atmel.com/avr.
3.2 Code Examples
This documentation contains simple code examples that briefly show how to use various parts of the device. These
code examples assume that the part specific header file is included before compilation. Be aware that not all C
compiler vendors include bit definitions in the header files and interrupt handling in C is compiler dependent.
Please confirm with the C compiler documentation for more details.
For I/O Registers located in the extended I/O map, “IN”, “OUT”, “SBIS”, “SBIC”, “CBI”, and “SBI” instructions must
be replaced with instructions that allow access to extended I/O. Typically, this means “LDS” and “STS” combined
with “SBRS”, “SBRC”, “SBR”, and “CBR”. Note that not all AVR devices include an extended I/O map.
3.3 Capacitive Touch Sensing
Atmel QTouch Library provides a simple to use solution for touch sensitive interfaces on Atmel AVR microcon-
trollers. The QTouch Library includes support for QTouch® and QMatrix® acquisition methods.
Touch sensing is easily added to any application by linking the QTouch Library and using the Application Program-
ming Interface (API) of the library to define the touch channels and sensors. The application then calls the API to
retrieve channel information and determine the state of the touch sensor.
The QTouch Library is free and can be downloaded from the Atmel website. For more information and details of
implementation, refer to the QTouch Library User Guide – also available from the Atmel website.
3.4 Data Retention
Reliability Qualification results show that the projected data retention failure rate is much less than 1 PPM over 20
years at 85°C or 100 years at 25°C.
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4. AVR CPU Core
4.1 Introduction
This section discusses the AVR core architecture in general. The main function of the CPU core is to ensure cor-
rect program execution. The CPU must therefore be able to access memories, perform calculations, control
peripherals, and handle interrupts.
4.2 Architectural Overview
Figure 4-1. Block Diagram of the AVR Architecture
In order to maximize performance and parallelism, the AVR uses a Harvard architecture – with separate memories
and buses for program and data. Instructions in the Program memory are executed with a single level pipelining.
While one instruction is being executed, the next instruction is pre-fetched from the Program memory. This concept
enables instructions to be executed in every clock cycle. The Program memory is In-System Reprogrammable
Flash memory.
The fast-access Register File contains 32 x 8-bit general purpose working registers with a single clock cycle
access time. This allows single-cycle Arithmetic Logic Unit (ALU) operation. In a typical ALU operation, two oper-
Flash
Program
Memory
Instruction
Register
Instruction
Decoder
Program
Counter
Control Lines
32 x 8
General
Purpose
Registrers
ALU
Status
and Control
I/O Lines
EEPROM
Data Bus 8-bit
Data
SRAM
Direct Addressing
Indirect Addressing
Interrupt
Unit
Watchdog
Timer
Analog
Comparator
I/O Module 2
I/O Module1
I/O Module n
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ands are output from the Register File, the operation is executed, and the result is stored back in the Register File
– in one clock cycle.
Six of the 32 registers can be used as three 16-bit indirect address register pointers for Data Space addressing –
enabling efficient address calculations. One of the these address pointers can also be used as an address pointer
for look up tables in Flash Program memory. These added function registers are the 16-bit X-, Y-, and Z-register,
described later in this section.
The ALU supports arithmetic and logic operations between registers or between a constant and a register. Single
register operations can also be executed in the ALU. After an arithmetic operation, the Status Register is updated
to reflect information about the result of the operation.
Program flow is provided by conditional and unconditional jump and call instructions, able to directly address the
whole address space. Most AVR instructions have a single 16-bit word format, but there are also 32-bit
instructions.
During interrupts and subroutine calls, the return address Program Counter (PC) is stored on the Stack. The Stack
is effectively allocated in the general data SRAM, and consequently the Stack size is only limited by the total
SRAM size and the usage of the SRAM. All user programs must initialize the SP in the Reset routine (before sub-
routines or interrupts are executed). The Stack Pointer (SP) is read/write accessible in the I/O space. The data
SRAM can easily be accessed through the five different addressing modes supported in the AVR architecture.
The memory spaces in the AVR architecture are all linear and regular memory maps.
A flexible interrupt module has its control registers in the I/O space with an additional Global Interrupt Enable bit in
the Status Register. All interrupts have a separate Interrupt Vector in the Interrupt Vector table. The interrupts have
priority in accordance with their Interrupt Vector position. The lower the Interrupt Vector address, the higher the
priority.
The I/O memory space contains 64 addresses for CPU peripheral functions as Control Registers, SPI, and other
I/O functions. The I/O memory can be accessed directly, or as the Data Space locations following those of the Reg-
ister File, 0x20 - 0x5F.
4.3 ALU – Arithmetic Logic Unit
The high-performance AVR ALU operates in direct connection with all the 32 general purpose working registers.
Within a single clock cycle, arithmetic operations between general purpose registers or between a register and an
immediate are executed. The ALU operations are divided into three main categories – arithmetic, logical, and bit-
functions. Some implementations of the architecture also provide a powerful multiplier supporting both
signed/unsigned multiplication and fractional format. See the “Instruction Set” section for a detailed description.
4.4 Status Register
The Status Register contains information about the result of the most recently executed arithmetic instruction. This
information can be used for altering program flow in order to perform conditional operations. Note that the Status
Register is updated after all ALU operations, as specified in the Instruction Set Reference. This will in many cases
remove the need for using the dedicated compare instructions, resulting in faster and more compact code.
The Status Register is not automatically stored when entering an interrupt routine and restored when returning
from an interrupt. This must be handled by software.
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4.4.1 SREG – AVR Status Register
The AVR Status Register – SREG – is defined as:
Bit 7 – I: Global Interrupt Enable
The Global Interrupt Enable bit must be set for the interrupts to be enabled. The individual interrupt enable control
is then performed in separate control registers. If the Global Interrupt Enable Register is cleared, none of the inter-
rupts are enabled independent of the individual interrupt enable settings. The I-bit is cleared by hardware after an
interrupt has occurred, and is set by the RETI instruction to enable subsequent interrupts. The I-bit can also be set
and cleared by the application with the SEI and CLI instructions, as described in the instruction set reference.
Bit 6 – T: Bit Copy Storage
The Bit Copy instructions BLD (Bit LoaD) and BST (Bit STore) use the T-bit as source or destination for the oper-
ated bit. A bit from a register in the Register File can be copied into T by the BST instruction, and a bit in T can be
copied into a bit in a register in the Register File by the BLD instruction.
Bit 5 – H: Half Carry Flag
The Half Carry Flag H indicates a Half Carry in some arithmetic operations. Half Carry is useful in BCD arithmetic.
See the “Instruction Set Description” for detailed information.
Bit 4 – S: Sign Bit, S = N V
The S-bit is always an exclusive or between the Negative Flag N and the Two’s Complement Overflow Flag V. See
the “Instruction Set Description” for detailed information.
Bit 3 – V: Two’s Complement Overflow Flag
The Two’s Complement Overflow Flag V supports two’s complement arithmetics. See the “Instruction Set Descrip-
tion” for detailed information.
Bit 2 – N: Negative Flag
The Negative Flag N indicates a negative result in an arithmetic or logic operation. See the “Instruction Set
Description” for detailed information.
Bit 1 – Z: Zero Flag
The Zero Flag Z indicates a zero result in an arithmetic or logic operation. See the “Instruction Set Description” for
detailed information.
Bit 0 – C: Carry Flag
The Carry Flag C indicates a carry in an arithmetic or logic operation. See the “Instruction Set Description” for
detailed information.
Bit 76543210
0x3F ITHSVNZCSREG
Read/Write R/W R/W R/W R/W R/W R/W R/W R/W
Initial Value00000000
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4.5 General Purpose Register File
The Register File is optimized for the AVR Enhanced RISC instruction set. In order to achieve the required perfor-
mance and flexibility, the following input/output schemes are supported by the Register File:
One 8-bit output operand and one 8-bit result input
Two 8-bit output operands and one 8-bit result input
Two 8-bit output operands and one 16-bit result input
One 16-bit output operand and one 16-bit result input
Figure 4-2 shows the structure of the 32 general purpose working registers in the CPU.
Figure 4-2. AVR CPU General Purpose Working Registers
Most of the instructions operating on the Register File have direct access to all registers, and most of them are sin-
gle cycle instructions.
As shown in Figure 4-2, each register is also assigned a Data memory address, mapping them directly into the first
32 locations of the user Data Space. Although not being physically implemented as SRAM locations, this memory
organization provides great flexibility in access of the registers, as the X-, Y- and Z-pointer registers can be set to
index any register in the file.
4.5.1 The X-register, Y-register, and Z-register
The registers R26..R31 have some added functions to their general purpose usage. These registers are 16-bit
address pointers for indirect addressing of the data space. The three indirect address registers X, Y, and Z are
defined as described in Figure 4-3.
7 0 Addr.
R0 0x00
R1 0x01
R2 0x02
R13 0x0D
General R14 0x0E
Purpose R15 0x0F
Working R16 0x10
Registers R17 0x11
R26 0x1A X-register Low Byte
R27 0x1B X-register High Byte
R28 0x1C Y-register Low Byte
R29 0x1D Y-register High Byte
R30 0x1E Z-register Low Byte
R31 0x1F Z-register High Byte
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Figure 4-3. The X-, Y-, and Z-registers
In the different addressing modes these address registers have functions as fixed displacement, automatic incre-
ment, and automatic decrement (see the instruction set reference for details).
4.6 Stack Pointer
The Stack is mainly used for storing temporary data, for storing local variables and for storing return addresses
after interrupts and subroutine calls. The Stack Pointer Register always points to the top of the Stack. Note that the
Stack is implemented as growing from higher memory locations to lower memory locations. This implies that a
Stack PUSH command decreases the Stack Pointer.
The Stack Pointer points to the data SRAM Stack area where the Subroutine and Interrupt Stacks are located. This
Stack space in the data SRAM must be defined by the program before any subroutine calls are executed or inter-
rupts are enabled. The Stack Pointer must be set to point above 0x60. The Stack Pointer is decremented by one
when data is pushed onto the Stack with the PUSH instruction, and it is decremented by two when the return
address is pushed onto the Stack with subroutine call or interrupt. The Stack Pointer is incremented by one when
data is popped from the Stack with the POP instruction, and it is incremented by two when data is popped from the
Stack with return from subroutine RET or return from interrupt RETI.
The AVR Stack Pointer is implemented as two 8-bit registers in the I/O space. The number of bits actually used is
implementation dependent. Note that the data space in some implementations of the AVR architecture is so small
that only SPL is needed. In this case, the SPH Register will not be present.
4.6.1 SPH and SPL — Stack Pointer Register
4.7 Instruction Execution Timing
This section describes the general access timing concepts for instruction execution. The AVR CPU is driven by the
CPU clock clkCPU, directly generated from the selected clock source for the chip. No internal clock division is used.
Figure 4-4 shows the parallel instruction fetches and instruction executions enabled by the Harvard architecture
and the fast access Register File concept. This is the basic pipelining concept to obtain up to 1 MIPS per MHz with
the corresponding unique results for functions per cost, functions per clocks, and functions per power-unit.
15 XH XL 0
X-register 7 0 7 0
R27 (0x1B) R26 (0x1A)
15 YH YL 0
Y-register 7 0 7 0
R29 (0x1D) R28 (0x1C)
15 ZH ZL 0
Z-register 7 0 7 0
R31 (0x1F) R30 (0x1E)
Bit 151413121110 9 8
0x3E SP15 SP14 SP13 SP12 SP11 SP10 SP9 SP8 SPH
0x3D SP7 SP6 SP5 SP4 SP3 SP2 SP1 SP0 SPL
76543210
Read/Write R/W R/W R/W R/W R/W R/W R/W R/W
Read/Write R/W R/W R/W R/W R/W R/W R/W R/W
Initial Value RAMEND RAMEND RAMEND RAMEND RAMEND RAMEND RAMEND RAMEND
Initial Value RAMEND RAMEND RAMEND RAMEND RAMEND RAMEND RAMEND RAMEND
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Figure 4-4. The Parallel Instruction Fetches and Instruction Executions
Figure 4-5 shows the internal timing concept for the Register File. In a single clock cycle an ALU operation using
two register operands is executed, and the result is stored back to the destination register.
Figure 4-5. Single Cycle ALU Operation
4.8 Reset and Interrupt Handling
The AVR provides several different interrupt sources. These interrupts and the separate Reset Vector each have a
separate Program Vector in the Program memory space. All interrupts are assigned individual enable bits which
must be written logic one together with the Global Interrupt Enable bit in the Status Register in order to enable the
interrupt.
The lowest addresses in the Program memory space are by default defined as the Reset and Interrupt Vectors.
The complete list of vectors is shown in “Interrupts” on page 48. The list also determines the priority levels of the
different interrupts. The lower the address the higher is the priority level. RESET has the highest priority, and next
is INT0 – the External Interrupt Request 0.
When an interrupt occurs, the Global Interrupt Enable I-bit is cleared and all interrupts are disabled. The user soft-
ware can write logic one to the I-bit to enable nested interrupts. All enabled interrupts can then interrupt the current
interrupt routine. The I-bit is automatically set when a Return from Interrupt instruction – RETI – is executed.
There are basically two types of interrupts. The first type is triggered by an event that sets the Interrupt Flag. For
these interrupts, the Program Counter is vectored to the actual Interrupt Vector in order to execute the interrupt
handling routine, and hardware clears the corresponding Interrupt Flag. Interrupt Flags can also be cleared by writ-
ing a logic one to the flag bit position(s) to be cleared. If an interrupt condition occurs while the corresponding
interrupt enable bit is cleared, the Interrupt Flag will be set and remembered until the interrupt is enabled, or the
flag is cleared by software. Similarly, if one or more interrupt conditions occur while the Global Interrupt Enable bit
is cleared, the corresponding Interrupt Flag(s) will be set and remembered until the Global Interrupt Enable bit is
set, and will then be executed by order of priority.
clk
1st Instruction Fetch
1st Instruction Execute
2nd Instruction Fetch
2nd Instruction Execute
3rd Instruction Fetch
3rd Instruction Execute
4th Instruction Fetch
T1 T2 T3 T4
CPU
Total Execution Time
Register Operands Fetch
ALU Operation Execute
Result Write Back
T1 T2 T3 T4
clkCPU
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The second type of interrupts will trigger as long as the interrupt condition is present. These interrupts do not nec-
essarily have Interrupt Flags. If the interrupt condition disappears before the interrupt is enabled, the interrupt will
not be triggered.
When the AVR exits from an interrupt, it will always return to the main program and execute one more instruction
before any pending interrupt is served.
Note that the Status Register is not automatically stored when entering an interrupt routine, nor restored when
returning from an interrupt routine. This must be handled by software.
When using the CLI instruction to disable interrupts, the interrupts will be immediately disabled. No interrupt will be
executed after the CLI instruction, even if it occurs simultaneously with the CLI instruction. The following example
shows how this can be used to avoid interrupts during the timed EEPROM write sequence.
When using the SEI instruction to enable interrupts, the instruction following SEI will be executed before any pend-
ing interrupts, as shown in this example.
4.8.1 Interrupt Response Time
The interrupt execution response for all the enabled AVR interrupts is four clock cycles minimum. After four clock
cycles the Program Vector address for the actual interrupt handling routine is executed. During this four clock cycle
period, the Program Counter is pushed onto the Stack. The vector is normally a jump to the interrupt routine, and
this jump takes three clock cycles. If an interrupt occurs during execution of a multi-cycle instruction, this instruction
is completed before the interrupt is served. If an interrupt occurs when the MCU is in sleep mode, the interrupt exe-
cution response time is increased by four clock cycles. This increase comes in addition to the start-up time from the
selected sleep mode.
Assembly Code Example
in r16, SREG ; store SREG value
cli ; disable interrupts during timed sequence
sbi EECR, EEMPE ; start EEPROM write
sbi EECR, EEPE
out SREG, r16 ; restore SREG value (I-bit)
C Code Example
char cSREG;
cSREG = SREG; /* store SREG value */
/* disable interrupts during timed sequence */
_CLI();
EECR |= (1<<EEMPE); /* start EEPROM write */
EECR |= (1<<EEPE);
SREG = cSREG; /* restore SREG value (I-bit) */
Assembly Code Example
sei ; set Global Interrupt Enable
sleep; enter sleep, waiting for interrupt
; note: will enter sleep before any pending
; interrupt(s)
C Code Example
_SEI(); /* set Global Interrupt Enable */
_SLEEP(); /* enter sleep, waiting for interrupt */
/* note: will enter sleep before any pending interrupt(s) */
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A return from an interrupt handling routine takes four clock cycles. During these four clock cycles, the Program
Counter (two bytes) is popped back from the Stack, the Stack Pointer is incremented by two, and the I-bit in SREG
is set.
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5. AVR Memories
This section describes the different memories in the ATtiny25/45/85. The AVR architecture has two main memory
spaces, the Data memory and the Program memory space. In addition, the ATtiny25/45/85 features an EEPROM
Memory for data storage. All three memory spaces are linear and regular.
5.1 In-System Re-programmable Flash Program Memory
The ATtiny25/45/85 contains 2/4/8K bytes On-chip In-System Reprogrammable Flash memory for program stor-
age. Since all AVR instructions are 16 or 32 bits wide, the Flash is organized as 1024/2048/4096 x 16.
The Flash memory has an endurance of at least 10,000 write/erase cycles. The ATtiny25/45/85 Program Counter
(PC) is 10/11/12 bits wide, thus addressing the 1024/2048/4096 Program memory locations. “Memory Program-
ming” on page 147 contains a detailed description on Flash data serial downloading using the SPI pins.
Constant tables can be allocated within the entire Program memory address space (see the LPM – Load Program
memory instruction description).
Timing diagrams for instruction fetch and execution are presented in “Instruction Execution Timing” on page 11.
Figure 5-1. Program Memory Map
5.2 SRAM Data Memory
Figure 5-2 shows how the ATtiny25/45/85 SRAM Memory is organized.
The lower 224/352/607 Data memory locations address both the Register File, the I/O memory and the internal
data SRAM. The first 32 locations address the Register File, the next 64 locations the standard I/O memory, and
the last 128/256/512 locations address the internal data SRAM.
The five different addressing modes for the Data memory cover: Direct, Indirect with Displacement, Indirect, Indi-
rect with Pre-decrement, and Indirect with Post-increment. In the Register File, registers R26 to R31 feature the
indirect addressing pointer registers.
The direct addressing reaches the entire data space.
The Indirect with Displacement mode reaches 63 address locations from the base address given by the Y- or Z-
register.
When using register indirect addressing modes with automatic pre-decrement and post-increment, the address
registers X, Y, and Z are decremented or incremented.
The 32 general purpose working registers, 64 I/O Registers, and the 128/256/512 bytes of internal data SRAM in
the ATtiny25/45/85 are all accessible through all these addressing modes. The Register File is described in “Gen-
eral Purpose Register File” on page 10.
0x0000
0x03FF/0x07FF/0x0FFF
Program Memory
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Figure 5-2. Data Memory Map
5.2.1 Data Memory Access Times
This section describes the general access timing concepts for internal memory access. The internal data SRAM
access is performed in two clkCPU cycles as described in Figure 5-3.
Figure 5-3. On-chip Data SRAM Access Cycles
5.3 EEPROM Data Memory
The ATtiny25/45/85 contains 128/256/512 bytes of data EEPROM memory. It is organized as a separate data
space, in which single bytes can be read and written. The EEPROM has an endurance of at least 100,000
write/erase cycles. The access between the EEPROM and the CPU is described in the following, specifying the
EEPROM Address Registers, the EEPROM Data Register, and the EEPROM Control Register. For details see
“Serial Downloading” on page 151.
5.3.1 EEPROM Read/Write Access
The EEPROM Access Registers are accessible in the I/O space.
The write access times for the EEPROM are given in Table 5-1 on page 21. A self-timing function, however, lets
the user software detect when the next byte can be written. If the user code contains instructions that write the
EEPROM, some precautions must be taken. In heavily filtered power supplies, VCC is likely to rise or fall slowly on
Power-up/down. This causes the device for some period of time to run at a voltage lower than specified as mini-
mum for the clock frequency used. See “Preventing EEPROM Corruption” on page 19 for details on how to avoid
problems in these situations.
In order to prevent unintentional EEPROM writes, a specific write procedure must be followed. Refer to “Atomic
Byte Programming” on page 17 and “Split Byte Programming” on page 17 for details on this.
32 Registers
64 I/O Registers
Internal SRAM
(128/256/512 x 8)
0x0000 - 0x001F
0x0020 - 0x005F
0x0DF/0x015F/0x025F
0x0060
Data Memory
clk
WR
RD
Data
Data
Address
Address valid
T1 T2 T3
Compute Address
Read Write
CPU
Memory Access Instruction Next Instruction
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When the EEPROM is read, the CPU is halted for four clock cycles before the next instruction is executed. When
the EEPROM is written, the CPU is halted for two clock cycles before the next instruction is executed.
5.3.2 Atomic Byte Programming
Using Atomic Byte Programming is the simplest mode. When writing a byte to the EEPROM, the user must write
the address into the EEAR Register and data into EEDR Register. If the EEPMn bits are zero, writing EEPE (within
four cycles after EEMPE is written) will trigger the erase/write operation. Both the erase and write cycle are done in
one operation and the total programming time is given in Table 5-1 on page 21. The EEPE bit remains set until the
erase and write operations are completed. While the device is busy with programming, it is not possible to do any
other EEPROM operations.
5.3.3 Split Byte Programming
It is possible to split the erase and write cycle in two different operations. This may be useful if the system requires
short access time for some limited period of time (typically if the power supply voltage falls). In order to take advan-
tage of this method, it is required that the locations to be written have been erased before the write operation. But
since the erase and write operations are split, it is possible to do the erase operations when the system allows
doing time-critical operations (typically after Power-up).
5.3.4 Erase
To erase a byte, the address must be written to EEAR. If the EEPMn bits are 0b01, writing the EEPE (within four
cycles after EEMPE is written) will trigger the erase operation only (programming time is given in Table 5-1 on
page 21). The EEPE bit remains set until the erase operation completes. While the device is busy programming, it
is not possible to do any other EEPROM operations.
5.3.5 Write
To write a location, the user must write the address into EEAR and the data into EEDR. If the EEPMn bits are
0b10, writing the EEPE (within four cycles after EEMPE is written) will trigger the write operation only (program-
ming time is given in Table 5-1 on page 21). The EEPE bit remains set until the write operation completes. If the
location to be written has not been erased before write, the data that is stored must be considered as lost. While
the device is busy with programming, it is not possible to do any other EEPROM operations.
The calibrated Oscillator is used to time the EEPROM accesses. Make sure the Oscillator frequency is within the
requirements described in “OSCCAL – Oscillator Calibration Register” on page 31.
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The following code examples show one assembly and one C function for erase, write, or atomic write of the
EEPROM. The examples assume that interrupts are controlled (e.g., by disabling interrupts globally) so that no
interrupts will occur during execution of these functions.
Assembly Code Example
EEPROM_write:
; Wait for completion of previous write
sbic EECR,EEPE
rjmp EEPROM_write
; Set Programming mode
ldi r16, (0<<EEPM1)|(0<<EEPM0)
out EECR, r16
; Set up address (r18:r17) in address register
out EEARH, r18
out EEARL, r17
; Write data (r19) to data register
out EEDR, r19
; Write logical one to EEMPE
sbi EECR,EEMPE
; Start eeprom write by setting EEPE
sbi EECR,EEPE
ret
C Code Example
void EEPROM_write(unsigned char ucAddress, unsigned char ucData)
{
/* Wait for completion of previous write */
while(EECR & (1<<EEPE))
;
/* Set Programming mode */
EECR = (0<<EEPM1)|(0<<EEPM0);
/* Set up address and data registers */
EEAR = ucAddress;
EEDR = ucData;
/* Write logical one to EEMPE */
EECR |= (1<<EEMPE);
/* Start eeprom write by setting EEPE */
EECR |= (1<<EEPE);
}
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The next code examples show assembly and C functions for reading the EEPROM. The examples assume that
interrupts are controlled so that no interrupts will occur during execution of these functions.
5.3.6 Preventing EEPROM Corruption
During periods of low VCC, the EEPROM data can be corrupted because the supply voltage is too low for the CPU
and the EEPROM to operate properly. These issues are the same as for board level systems using EEPROM, and
the same design solutions should be applied.
An EEPROM data corruption can be caused by two situations when the voltage is too low. First, a regular write
sequence to the EEPROM requires a minimum voltage to operate correctly. Secondly, the CPU itself can execute
instructions incorrectly, if the supply voltage is too low.
EEPROM data corruption can easily be avoided by following this design recommendation:
Keep the AVR RESET active (low) during periods of insufficient power supply voltage. This can be done by
enabling the internal Brown-out Detector (BOD). If the detection level of the internal BOD does not match the
needed detection level, an external low VCC reset protection circuit can be used. If a reset occurs while a write
operation is in progress, the write operation will be completed provided that the power supply voltage is sufficient.
5.4 I/O Memory
The I/O space definition of the ATtiny25/45/85 is shown in “Register Summary” on page 200.
Assembly Code Example
EEPROM_read:
; Wait for completion of previous write
sbic EECR,EEPE
rjmp EEPROM_read
; Set up address (r18:r17) in address register
out EEARH, r18
out EEARL, r17
; Start eeprom read by writing EERE
sbi EECR,EERE
; Read data from data register
in r16,EEDR
ret
C Code Example
unsigned char EEPROM_read(unsigned char ucAddress)
{
/* Wait for completion of previous write */
while(EECR & (1<<EEPE))
;
/* Set up address register */
EEAR = ucAddress;
/* Start eeprom read by writing EERE */
EECR |= (1<<EERE);
/* Return data from data register */
return EEDR;
}
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All ATtiny25/45/85 I/Os and peripherals are placed in the I/O space. All I/O locations may be accessed by the
LD/LDS/LDD and ST/STS/STD instructions, transferring data between the 32 general purpose working registers
and the I/O space. I/O Registers within the address range 0x00 - 0x1F are directly bit-accessible using the SBI and
CBI instructions. In these registers, the value of single bits can be checked by using the SBIS and SBIC instruc-
tions. Refer to the instruction set section for more details. When using the I/O specific commands IN and OUT, the
I/O addresses 0x00 - 0x3F must be used. When addressing I/O Registers as data space using LD and ST instruc-
tions, 0x20 must be added to these addresses.
For compatibility with future devices, reserved bits should be written to zero if accessed. Reserved I/O memory
addresses should never be written.
Some of the Status Flags are cleared by writing a logical one to them. Note that the CBI and SBI instructions will
only operate on the specified bit, and can therefore be used on registers containing such Status Flags. The CBI
and SBI instructions work with registers 0x00 to 0x1F only.
The I/O and Peripherals Control Registers are explained in later sections.
5.5 Register Description
5.5.1 EEARH – EEPROM Address Register
Bits 7:1 – Res: Reserved Bits
These bits are reserved for future use and will always read as zero.
Bits 0 – EEAR8: EEPROM Address
This is the most significant EEPROM address bit of ATtiny85. In devices with less EEPROM, i.e.
ATtiny25/ATtiny45, this bit is reserved and will always read zero. The initial value of the EEPROM Address Regis-
ter (EEAR) is undefined and a proper value must therefore be written before the EEPROM is accessed.
5.5.2 EEARL – EEPROM Address Register
Bit 7 – EEAR7: EEPROM Address
This is the most significant EEPROM address bit of ATtiny45. In devices with less EEPROM, i.e. ATtiny25, this bit
is reserved and will always read zero. The initial value of the EEPROM Address Register (EEAR) is undefined and
a proper value must therefore be written before the EEPROM is accessed.
Bits 6:0 – EEAR[6:0]: EEPROM Address
These are the (low) bits of the EEPROM Address Register. The EEPROM data bytes are addressed linearly in the
range 0...(128/256/512-1). The initial value of EEAR is undefined and a proper value must be therefore be written
before the EEPROM may be accessed.
Bit 76543210
0x1F – – – – –EEAR8EEARH
Read/Write RRRRRRRR/W
Initial Value0000000X/0
Bit
0x1E EEAR7 EEAR6 EEAR5 EEAR4 EEAR3 EEAR2 EEAR1 EEAR0 EEARL
Rear/Write R/W R/W R/W R/W R/W R/W R/W R/W
Initial Value XXXXXXXX
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5.5.3 EEDR – EEPROM Data Register
Bits 7:0 – EEDR[7:0]: EEPROM Data
For the EEPROM write operation the EEDR Register contains the data to be written to the EEPROM in the address
given by the EEAR Register. For the EEPROM read operation, the EEDR contains the data read out from the
EEPROM at the address given by EEAR.
5.5.4 EECR – EEPROM Control Register
Bit 7 – Res: Reserved Bit
This bit is reserved for future use and will always read as 0 in ATtiny25/45/85. For compatibility with future AVR
devices, always write this bit to zero. After reading, mask out this bit.
Bit 6 – Res: Reserved Bit
This bit is reserved in the ATtiny25/45/85 and will always read as zero.
Bits 5:4 – EEPM[1:0]: EEPROM Programming Mode Bits
The EEPROM Programming mode bits setting defines which programming action that will be triggered when writ-
ing EEPE. It is possible to program data in one atomic operation (erase the old value and program the new value)
or to split the Erase and Write operations in two different operations. The Programming times for the different
modes are shown in Table 5-1. While EEPE is set, any write to EEPMn will be ignored. During reset, the EEPMn
bits will be reset to 0b00 unless the EEPROM is busy programming.
Bit 3 – EERIE: EEPROM Ready Interrupt Enable
Writing EERIE to one enables the EEPROM Ready Interrupt if the I-bit in SREG is set. Writing EERIE to zero dis-
ables the interrupt. The EEPROM Ready Interrupt generates a constant interrupt when Non-volatile memory is
ready for programming.
Bit 2 – EEMPE: EEPROM Master Program Enable
The EEMPE bit determines whether writing EEPE to one will have effect or not.
When EEMPE is set, setting EEPE within four clock cycles will program the EEPROM at the selected address. If
EEMPE is zero, setting EEPE will have no effect. When EEMPE has been written to one by software, hardware
clears the bit to zero after four clock cycles.
Bit 76543210
0x1D EEDR7 EEDR6 EEDR5 EEDR4 EEDR3 EEDR2 EEDR1 EEDR0 EEDR
Read/Write R/W R/W R/W R/W R/W R/W R/W R/W
Initial Value00000000
Bit 76543210
0x1C EEPM1 EEPM0 EERIE EEMPE EEPE EERE EECR
Read/Write R R R/W R/W R/W R/W R/W R/W
Initial Value 0 0 X X 0 0 X 0
Table 5-1. EEPROM Mode Bits
EEPM1 EEPM0 Programming
Time Operation
0 0 3.4 ms Erase and Write in one operation (Atomic Operation)
0 1 1.8 ms Erase Only
1 0 1.8 ms Write Only
1 1 Reserved for future use
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Bit 1 – EEPE: EEPROM Program Enable
The EEPROM Program Enable Signal EEPE is the programming enable signal to the EEPROM. When EEPE is
written, the EEPROM will be programmed according to the EEPMn bits setting. The EEMPE bit must be written to
one before a logical one is written to EEPE, otherwise no EEPROM write takes place. When the write access time
has elapsed, the EEPE bit is cleared by hardware. When EEPE has been set, the CPU is halted for two cycles
before the next instruction is executed.
Bit 0 – EERE: EEPROM Read Enable
The EEPROM Read Enable Signal – EERE – is the read strobe to the EEPROM. When the correct address is set
up in the EEAR Register, the EERE bit must be written to one to trigger the EEPROM read. The EEPROM read
access takes one instruction, and the requested data is available immediately. When the EEPROM is read, the
CPU is halted for four cycles before the next instruction is executed. The user should poll the EEPE bit before start-
ing the read operation. If a write operation is in progress, it is neither possible to read the EEPROM, nor to change
the EEAR Register.
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6. System Clock and Clock Options
6.1 Clock Systems and their Distribution
Figure 6-1 presents the principal clock systems in the AVR and their distribution. All of the clocks need not be
active at a given time. In order to reduce power consumption, the clocks to modules not being used can be halted
by using different sleep modes, as described in “Power Management and Sleep Modes” on page 34. The clock
systems are detailed below.
Figure 6-1. Clock Distribution
6.1.1 CPU Clock – clkCPU
The CPU clock is routed to parts of the system concerned with operation of the AVR core. Examples of such mod-
ules are the General Purpose Register File, the Status Register and the Data memory holding the Stack Pointer.
Halting the CPU clock inhibits the core from performing general operations and calculations.
6.1.2 I/O Clock – clkI/O
The I/O clock is used by the majority of the I/O modules, like Timer/Counter. The I/O clock is also used by the
External Interrupt module, but note that some external interrupts are detected by asynchronous logic, allowing
such interrupts to be detected even if the I/O clock is halted.
6.1.3 Flash Clock – clkFLASH
The Flash clock controls operation of the Flash interface. The Flash clock is usually active simultaneously with the
CPU clock.
General I/O
Modules CPU Core RAM
clk
I/O
AVR Clock
Control Unit
clk
CPU
Flash and
EEPROM
clk
FLASH
Source clock
Watchdog Timer
Watchdog
Oscillator
Reset Logic
Clock
Multiplexer
Watchdog clock
Calibrated RC
Oscillator
Calibrated RC
Oscillator
External Clock
ADC
clk
ADC
Crystal
Oscillator
Low-Frequency
Crystal Oscillator
System Clock
Prescaler
PLL
Oscillator
clk
PCK
clk
PCK
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6.1.4 ADC Clock – clkADC
The ADC is provided with a dedicated clock domain. This allows halting the CPU and I/O clocks in order to reduce
noise generated by digital circuitry. This gives more accurate ADC conversion results.
6.1.5 Internal PLL for Fast Peripheral Clock Generation - clkPCK
The internal PLL in ATtiny25/45/85 generates a clock frequency that is 8x multiplied from a source input. By
default, the PLL uses the output of the internal, 8.0 MHz RC oscillator as source. Alternatively, if bit LSM of
PLLCSR is set the PLL will use the output of the RC oscillator divided by two. Thus the output of the PLL, the fast
peripheral clock is 64 MHz. The fast peripheral clock, or a clock prescaled from that, can be selected as the clock
source for Timer/Counter1 or as a system clock. See Figure 6-2. The frequency of the fast peripheral clock is
divided by two when LSM of PLLCSR is set, resulting in a clock frequency of 32 MHz. Note, that LSM can not be
set if PLLCLK is used as system clock.
Figure 6-2. PCK Clocking System.
The PLL is locked on the RC oscillator and adjusting the RC oscillator via OSCCAL register will adjust the fast
peripheral clock at the same time. However, even if the RC oscillator is taken to a higher frequency than 8 MHz,
the fast peripheral clock frequency saturates at 85 MHz (worst case) and remains oscillating at the maximum fre-
quency. It should be noted that the PLL in this case is not locked any longer with the RC oscillator clock. Therefore,
it is recommended not to take the OSCCAL adjustments to a higher frequency than 8 MHz in order to keep the PLL
in the correct operating range.
The internal PLL is enabled when:
The PLLE bit in the register PLLCSR is set.
The CKSEL fuse is programmed to ‘0001’.
The CKSEL fuse is programmed to ‘0011’.
The PLLCSR bit PLOCK is set when PLL is locked.
Both internal RC oscillator and PLL are switched off in power down and stand-by sleep modes.
6.1.6 Internal PLL in ATtiny15 Compatibility Mode
Since ATtiny25/45/85 is a migration device for ATtiny15 users there is an ATtiny15 compatibility mode for back-
ward compatibility. The ATtiny15 compatibility mode is selected by programming the CKSEL fuses to ‘0011’.
In the ATtiny15 compatibility mode the frequency of the internal RC oscillator is calibrated down to 6.4 MHz and the
multiplication factor of the PLL is set to 4x. See Figure 6-3. With these adjustments the clocking system is
ATtiny15-compatible and the resulting fast peripheral clock has a frequency of 25.6 MHz (same as in ATtiny15).
1/2
8 MHz
LSM
8.0 MHz
OSCILLATOR PLL
8x
CKSEL[3:0]PLLEOSCCAL
4 MHz
1/4
LOCK
DETECTOR
PRESCALER
CLKPS[3:0]
SYSTEM
CLOCK
PLOCK
PCK
OSCILLATORS
XTAL1
XTAL2
64 / 32 MHz
8 MHz
16 MHz
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Figure 6-3. PCK Clocking System in ATtiny15 Compatibility Mode.
Note that low speed mode is not implemented in ATtiny15 compatibility mode.
6.2 Clock Sources
The device has the following clock source options, selectable by Flash Fuse bits as shown below. The clock from
the selected source is input to the AVR clock generator, and routed to the appropriate modules.
Note: 1. For all fuses “1” means unprogrammed while “0” means programmed.
2. The device is shipped with this option selected.
3. This will select ATtiny15 Compatibility Mode, where system clock is divided by four, resulting in a 1.6 MHz clock fre-
quency. For more inormation, see “Calibrated Internal Oscillator” on page 27.
The various choices for each clocking option is given in the following sections. When the CPU wakes up from
Power-down, the selected clock source is used to time the start-up, ensuring stable Oscillator operation before
instruction execution starts. When the CPU starts from reset, there is an additional delay allowing the power to
reach a stable level before commencing normal operation. The Watchdog Oscillator is used for timing this real-time
part of the start-up time. The number of WDT Oscillator cycles used for each time-out is shown in Table 6-2.
Table 6-1. Device Clocking Options Select
Device Clocking Option CKSEL[3:0](1)
External Clock (see page 26) 0000
High Frequency PLL Clock (see page 26) 0001
Calibrated Internal Oscillator (see page 27) 0010(2)
Calibrated Internal Oscillator (see page 27) 0011(3)
Internal 128 kHz Oscillator (see page 28) 0100
Low-Frequency Crystal Oscillator (see page 29)0110
Crystal Oscillator / Ceramic Resonator (see page 29) 1000 1111
Reserved 0101, 0111
Table 6-2. Number of Watchdog Oscillator Cycles
Typ Time-out Number of Cycles
4 ms 512
64 ms 8K (8,192)
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6.2.1 External Clock
To drive the device from an external clock source, CLKI should be driven as shown in Figure 6-4. To run the device
on an external clock, the CKSEL Fuses must be programmed to “00”.
Figure 6-4. External Clock Drive Configuration
When this clock source is selected, start-up times are determined by the SUT Fuses as shown in Table 6-3.
When applying an external clock, it is required to avoid sudden changes in the applied clock frequency to ensure
stable operation of the MCU. A variation in frequency of more than 2% from one clock cycle to the next can lead to
unpredictable behavior. It is required to ensure that the MCU is kept in Reset during such changes in the clock
frequency.
Note that the System Clock Prescaler can be used to implement run-time changes of the internal clock frequency
while still ensuring stable operation. Refer to “System Clock Prescaler” on page 31 for details.
6.2.2 High Frequency PLL Clock
There is an internal PLL that provides nominally 64 MHz clock rate locked to the RC Oscillator for the use of the
Peripheral Timer/Counter1 and for the system clock source. When selected as a system clock source, by program-
ming the CKSEL fuses to ‘0001’, it is divided by four like shown in Table 6-4.
When this clock source is selected, start-up times are determined by the SUT fuses as shown in Table 6-5.
Table 6-3. Start-up Times for the External Clock Selection
SUT[1:0]
Start-up Time from
Power-down Additional Delay from
Reset Recommended Usage
00 6 CK 14CK BOD enabled
01 6 CK 14CK + 4 ms Fast rising power
10 6 CK 14CK + 64 ms Slowly rising power
11 Reserved
EXTERNAL
CLOCK
SIGNAL
CLKI
GND
Table 6-4. High Frequency PLL Clock Operating Modes
CKSEL[3:0] Nominal Frequency
0001 16 MHz
Table 6-5. Start-up Times for the High Frequency PLL Clock
SUT[1:0]
Start-up Time from
Power Down
Additional Delay from
Power-On Reset (VCC = 5.0V) Recommended
usage
00 14CK + 1K (1024) CK + 4 ms 4 ms BOD enabled
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6.2.3 Calibrated Internal Oscillator
By default, the Internal RC Oscillator provides an approximate 8.0 MHz clock. Though voltage and temperature
dependent, this clock can be very accurately calibrated by the user. See “Calibrated Internal RC Oscillator Accu-
racy” on page 164 and “Internal Oscillator Speed” on page 192 for more details. The device is shipped with the
CKDIV8 Fuse programmed. See “System Clock Prescaler” on page 31 for more details.
This clock may be selected as the system clock by programming the CKSEL Fuses as shown in Table 6-6 on page
27. If selected, it will operate with no external components. During reset, hardware loads the pre-programmed cali-
bration value into the OSCCAL Register and thereby automatically calibrates the RC Oscillator. The accuracy of
this calibration is shown as Factory calibration in Table 21-2 on page 164.
By changing the OSCCAL register from SW, see “OSCCAL – Oscillator Calibration Register” on page 31, it is pos-
sible to get a higher calibration accuracy than by using the factory calibration. The accuracy of this calibration is
shown as User calibration in Table 21-2 on page 164.
When this Oscillator is used as the chip clock, the Watchdog Oscillator will still be used for the Watchdog Timer
and for the Reset Time-out. For more information on the pre-programmed calibration value, see the section “Cali-
bration Bytes” on page 150.
The internal oscillator can also be set to provide a 6.4 MHz clock by writing CKSEL fuses to “0011”, as shown in
Table 6-6 below. This setting is reffered to as ATtiny15 Compatibility Mode and is intended to provide a calibrated
clock source at 6.4 MHz, as in ATtiny15. In ATtiny15 Compatibility Mode the PLL uses the internal oscillator run-
ning at 6.4 MHz to generate a 25.6 MHz peripheral clock signal for Timer/Counter1 (see “8-bit Timer/Counter1 in
ATtiny15 Mode” on page 95). Note that in this mode of operation the 6.4 MHz clock signal is always divided by
four, providing a 1.6 MHz system clock.
Note: 1. The device is shipped with this option selected.
2. This setting will select ATtiny15 Compatibility Mode, where system clock is divided by four, resulting in a 1.6 MHz
clock frequency.
01 14CK + 16K (16384) CK + 4 ms 4 ms Fast rising power
10 14CK + 1K (1024) CK + 64 ms 4 ms Slowly rising power
11 14CK + 16K (16384) CK + 64 ms 4 ms Slowly rising power
Table 6-5. Start-up Times for the High Frequency PLL Clock
SUT[1:0]
Start-up Time from
Power Down
Additional Delay from
Power-On Reset (VCC = 5.0V) Recommended
usage
Table 6-6. Internal Calibrated RC Oscillator Operating Modes
CKSEL[3:0] Nominal Frequency
0010(1) 8.0 MHz
0011(2) 6.4 MHz
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When the calibrated 8 MHz internal oscillator is selected as clock source the start-up times are determined by the
SUT Fuses as shown in Table 6-7 below.
Note: 1. If the RSTDISBL fuse is programmed, this start-up time will be increased to 14CK + 4 ms to ensure programming
mode can be entered.
2. The device is shipped with this option selected.
In ATtiny15 Compatibility Mode start-up times are determined by SUT fuses as shown in Table 6-8 below.
Note: 1. If the RSTDISBL fuse is programmed, this start-up time will be increased to 14CK + 4 ms to ensure programming
mode can be entered.
In summary, more information on ATtiny15 Compatibility Mode can be found in sections “Port B (PB5:PB0)” on
page 2, “Internal PLL in ATtiny15 Compatibility Mode” on page 24, “8-bit Timer/Counter1 in ATtiny15 Mode” on
page 95, “Limitations of debugWIRE” on page 140, “Calibration Bytes” on page 150 and in table “Clock Prescaler
Select” on page 33.
6.2.4 Internal 128 kHz Oscillator
The 128 kHz internal Oscillator is a low power Oscillator providing a clock of 128 kHz. The frequency is nominal at
3V and 25C. This clock may be select as the system clock by programming the CKSEL Fuses to “0100”.
When this clock source is selected, start-up times are determined by the SUT Fuses as shown in Table 6-9.
Table 6-7. Start-up Times for Internal Calibrated RC Oscillator Clock
SUT[1:0] Start-up Time
from Power-down Additional Delay from
Reset (VCC = 5.0V) Recommended Usage
00 6 CK 14CK(1) BOD enabled
01 6 CK 14CK + 4 ms Fast rising power
10(2) 6 CK 14CK + 64 ms Slowly rising power
11 Reserved
Table 6-8. Start-up Times for Internal Calibrated RC Oscillator Clock (in ATtiny15 Mode)
SUT[1:0] Start-up Time
from Power-down Additional Delay from
Reset (VCC = 5.0V) Recommended Usage
00 6 CK 14CK + 64 ms
01 6 CK 14CK + 64 ms
10 6 CK 14CK + 4 ms
11 1 CK 14CK(1)
Table 6-9. Start-up Times for the 128 kHz Internal Oscillator
SUT[1:0]
Start-up Time from
Power-down Additional Delay from
Reset Recommended Usage
00 6 CK 14CK(1) BOD enabled
01 6 CK 14CK + 4 ms Fast rising power
10 6 CK 14CK + 64 ms Slowly rising power
11 Reserved
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Note: 1. If the RSTDISBL fuse is programmed, this start-up time will be increased to 14CK + 4 ms to ensure programming
mode can be entered.
6.2.5 Low-Frequency Crystal Oscillator
To use a 32.768 kHz watch crystal as the clock source for the device, the Low-frequency Crystal Oscillator must be
selected by setting CKSEL fuses to ‘0110’. The crystal should be connected as shown in Figure 6-5. To find suit-
able load capacitance for a 32.768 kHz crysal, please consult the manufacturer’s datasheet.
When this oscillator is selected, start-up times are determined by the SUT fuses as shown in Table 6-10.
Note: 1. These options should be used only if frequency stability at start-up is not important.
The Low-frequency Crystal Oscillator provides an internal load capacitance, see Table 6-11 at each TOSC pin.
6.2.6 Crystal Oscillator / Ceramic Resonator
XTAL1 and XTAL2 are input and output, respectively, of an inverting amplifier which can be configured for use as
an On-chip Oscillator, as shown in Figure 6-5. Either a quartz crystal or a ceramic resonator may be used.
Figure 6-5. Crystal Oscillator Connections
C1 and C2 should always be equal for both crystals and resonators. The optimal value of the capacitors depends
on the crystal or resonator in use, the amount of stray capacitance, and the electromagnetic noise of the environ-
Table 6-10. Start-up Times for the Low Frequency Crystal Oscillator Clock Selection
SUT[1:0] Start-up Time from
Power Down Additional Delay from
Reset (VCC = 5.0V) Recommended usage
00 1K (1024) CK(1) 4 ms Fast rising power or BOD enabled
01 1K (1024) CK(1) 64 ms Slowly rising power
10 32K (32768) CK 64 ms Stable frequency at start-up
11 Reserved
Table 6-11. Capacitance of Low-Frequency Crystal Oscillator
Device 32 kHz Osc. Type Cap (Xtal1/Tosc1) Cap (Xtal2/Tosc2)
ATtiny25/45/85 System Osc. 16 pF 6 pF
XTAL2
XTAL1
GND
C2
C1
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ment. Some initial guidelines for choosing capacitors for use with crystals are given in Table 6-12 below. For
ceramic resonators, the capacitor values given by the manufacturer should be used.
Notes: 1. This option should not be used with crystals, only with ceramic resonators.
The Oscillator can operate in three different modes, each optimized for a specific frequency range. The operating
mode is selected by the fuses CKSEL[3:1] as shown in Table 6-12.
The CKSEL0 Fuse together with the SUT[1:0] Fuses select the start-up times as shown in Table 6-13.
Notes: 1. These options should only be used when not operating close to the maximum frequency of the device, and only if
frequency stability at start-up is not important for the application. These options are not suitable for crystals.
2. These options are intended for use with ceramic resonators and will ensure frequency stability at start-up. They can
also be used with crystals when not operating close to the maximum frequency of the device, and if frequency sta-
bility at start-up is not important for the application.
6.2.7 Default Clock Source
The device is shipped with CKSEL = “0010”, SUT = “10”, and CKDIV8 programmed. The default clock source set-
ting is therefore the Internal RC Oscillator running at 8 MHz with longest start-up time and an initial system clock
prescaling of 8, resulting in 1.0 MHz system clock. This default setting ensures that all users can make their
desired clock source setting using an In-System or High-voltage Programmer.
Table 6-12. Crystal Oscillator Operating Modes
CKSEL[3:1] Frequency Range (MHz) Recommended Range for Capacitors C1 and
C2 for Use with Crystals (pF)
100(1) 0.4 - 0.9
101 0.9 - 3.0 12 - 22
110 3.0 - 8.0 12 - 22
111 8.0 - 12 - 22
Table 6-13. Start-up Times for the Crystal Oscillator Clock Selection
CKSEL0 SUT[1:0] Start-up Time from
Power-down Additional Delay
from Reset Recommended Usage
0 00 258 CK(1) 14CK + 4 ms Ceramic resonator,
fast rising power
0 01 258 CK(1) 14CK + 64 ms Ceramic resonator,
slowly rising power
0 10 1K (1024) CK(2) 14CK Ceramic resonator,
BOD enabled
0 11 1K (1024)CK(2) 14CK + 4 ms Ceramic resonator,
fast rising power
1 00 1K (1024)CK(2) 14CK + 64 ms Ceramic resonator,
slowly rising power
1 01 16K (16384) CK 14CK Crystal Oscillator,
BOD enabled
1 10 16K (16384) CK 14CK + 4 ms Crystal Oscillator,
fast rising power
1 11 16K (16384) CK 14CK + 64 ms Crystal Oscillator,
slowly rising power
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6.3 System Clock Prescaler
The ATtiny25/45/85 system clock can be divided by setting the “CLKPR – Clock Prescale Register” on page 32.
This feature can be used to decrease power consumption when the requirement for processing power is low. This
can be used with all clock source options, and it will affect the clock frequency of the CPU and all synchronous
peripherals. clkI/O, clkADC, clkCPU, and clkFLASH are divided by a factor as shown in Table 6-15 on page 33.
6.3.1 Switching Time
When switching between prescaler settings, the System Clock Prescaler ensures that no glitches occur in the clock
system and that no intermediate frequency is higher than neither the clock frequency corresponding to the previous
setting, nor the clock frequency corresponding to the new setting.
The ripple counter that implements the prescaler runs at the frequency of the undivided clock, which may be faster
than the CPU’s clock frequency. Hence, it is not possible to determine the state of the prescaler – even if it were
readable, and the exact time it takes to switch from one clock division to another cannot be exactly predicted.
From the time the CLKPS values are written, it takes between T1 + T2 and T1 + 2*T2 before the new clock fre-
quency is active. In this interval, 2 active clock edges are produced. Here, T1 is the previous clock period, and T2
is the period corresponding to the new prescaler setting.
6.4 Clock Output Buffer
The device can output the system clock on the CLKO pin (when not used as XTAL2 pin). To enable the output, the
CKOUT Fuse has to be programmed. This mode is suitable when the chip clock is used to drive other circuits on
the system. Note that the clock will not be output during reset and that the normal operation of the I/O pin will be
overridden when the fuse is programmed. Internal RC Oscillator, WDT Oscillator, PLL, and external clock (CLKI)
can be selected when the clock is output on CLKO. Crystal oscillators (XTAL1, XTAL2) can not be used for clock
output on CLKO. If the System Clock Prescaler is used, it is the divided system clock that is output.
6.5 Register Description
6.5.1 OSCCAL – Oscillator Calibration Register
Bits 7:0 – CAL[7:0]: Oscillator Calibration Value
The Oscillator Calibration Register is used to trim the Calibrated Internal RC Oscillator to remove process varia-
tions from the oscillator frequency. A pre-programmed calibration value is automatically written to this register
during chip reset, giving the Factory calibrated frequency as specified in Table 21-2 on page 164. The application
software can write this register to change the oscillator frequency. The oscillator can be calibrated to frequencies
as specified in Table 21-2 on page 164. Calibration outside that range is not guaranteed.
Note that this oscillator is used to time EEPROM and Flash write accesses, and these write times will be affected
accordingly. If the EEPROM or Flash are written, do not calibrate to more than 8.8 MHz. Otherwise, the EEPROM
or Flash write may fail.
The CAL7 bit determines the range of operation for the oscillator. Setting this bit to 0 gives the lowest frequency
range, setting this bit to 1 gives the highest frequency range. The two frequency ranges are overlapping, in other
words a setting of OSCCAL = 0x7F gives a higher frequency than OSCCAL = 0x80.
The CAL[6:0] bits are used to tune the frequency within the selected range. A setting of 0x00 gives the lowest fre-
quency in that range, and a setting of 0x7F gives the highest frequency in the range.
Bit 76543210
0x31 CAL7 CAL6 CAL5 CAL4 CAL3 CAL2 CAL1 CAL0 OSCCAL
Read/Write R/W R/W R/W R/W R/W R/W R/W R/W
Initial Value Device Specific Calibration Value
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To ensure stable operation of the MCU the calibration value should be changed in small. A variation in frequency of
more than 2% from one cycle to the next can lead to unpredicatble behavior. Changes in OSCCAL should not
exceed 0x20 for each calibration. It is required to ensure that the MCU is kept in Reset during such changes in the
clock frequency
6.5.2 CLKPR – Clock Prescale Register
Bit 7 – CLKPCE: Clock Prescaler Change Enable
The CLKPCE bit must be written to logic one to enable change of the CLKPS bits. The CLKPCE bit is only updated
when the other bits in CLKPR are simultaniosly written to zero. CLKPCE is cleared by hardware four cycles after it
is written or when the CLKPS bits are written. Rewriting the CLKPCE bit within this time-out period does neither
extend the time-out period, nor clear the CLKPCE bit.
Bits 6:4 – Res: Reserved Bits
These bits are reserved bits in the ATtiny25/45/85 and will always read as zero.
Bits 3:0 – CLKPS[3:0]: Clock Prescaler Select Bits 3 - 0
These bits define the division factor between the selected clock source and the internal system clock. These bits
can be written run-time to vary the clock frequency to suit the application requirements. As the divider divides the
master clock input to the MCU, the speed of all synchronous peripherals is reduced when a division factor is used.
The division factors are given in Table 6-15.
To avoid unintentional changes of clock frequency, a special write procedure must be followed to change the
CLKPS bits:
1. Write the Clock Prescaler Change Enable (CLKPCE) bit to one and all other bits in CLKPR to zero.
2. Within four cycles, write the desired value to CLKPS while writing a zero to CLKPCE.
Interrupts must be disabled when changing prescaler setting to make sure the write procedure is not interrupted.
The CKDIV8 Fuse determines the initial value of the CLKPS bits. If CKDIV8 is unprogrammed, the CLKPS bits will
be reset to “0000”. If CKDIV8 is programmed, CLKPS bits are reset to “0011”, giving a division factor of eight at
start up. This feature should be used if the selected clock source has a higher frequency than the maximum fre-
quency of the device at the present operating conditions. Note that any value can be written to the CLKPS bits
regardless of the CKDIV8 Fuse setting. The Application software must ensure that a sufficient division factor is
Table 6-14. Internal RC Oscillator Frequency Range
OSCCAL Value Typical Lowest Frequency
with Respect to Nominal Frequency Typical Highest Frequency
with Respect to Nominal Frequency
0x00 50% 100%
0x3F 75% 150%
0x7F 100% 200%
Bit 76543210
0x26 CLKPCE CLKPS3 CLKPS2 CLKPS1 CLKPS0 CLKPR
Read/Write R/W R R R R/W R/W R/W R/W
Initial Value 0 0 0 0 See Bit Description
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chosen if the selcted clock source has a higher frequency than the maximum frequency of the device at the present
operating conditions. The device is shipped with the CKDIV8 Fuse programmed.
Note: The prescaler is disabled in ATtiny15 compatibility mode and neither writing to CLKPR, nor programming the CKDIV8
fuse has any effect on the system clock (which will always be 1.6 MHz).
Table 6-15. Clock Prescaler Select
CLKPS3 CLKPS2 CLKPS1 CLKPS0 Clock Division Factor
0000 1
0001 2
0010 4
0011 8
0100 16
0101 32
0110 64
0111 128
1000 256
1001 Reserved
1010 Reserved
1011 Reserved
1100 Reserved
1101 Reserved
1110 Reserved
1111 Reserved
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7. Power Management and Sleep Modes
The high performance and industry leading code efficiency makes the AVR microcontrollers an ideal choise for low
power applications. In addition, sleep modes enable the application to shut down unused modules in the MCU,
thereby saving power. The AVR provides various sleep modes allowing the user to tailor the power consumption to
the application’s requirements.
7.1 Sleep Modes
Figure 6-1 on page 23 presents the different clock systems and their distribution in ATtiny25/45/85. The figure is
helpful in selecting an appropriate sleep mode. Table 7-1 shows the different sleep modes and their wake up
sources.
Note: 1. For INT0, only level interrupt.
To enter any of the three sleep modes, the SE bit in MCUCR must be written to logic one and a SLEEP instruction
must be executed. The SM[1:0] bits in the MCUCR Register select which sleep mode (Idle, ADC Noise Reduction
or Power-down) will be activated by the SLEEP instruction. See Table 7-2 for a summary.
If an enabled interrupt occurs while the MCU is in a sleep mode, the MCU wakes up. The MCU is then halted for
four cycles in addition to the start-up time, executes the interrupt routine, and resumes execution from the instruc-
tion following SLEEP. The contents of the Register File and SRAM are unaltered when the device wakes up from
sleep. If a reset occurs during sleep mode, the MCU wakes up and executes from the Reset Vector.
Note that if a level triggered interrupt is used for wake-up the changed level must be held for some time to wake up
the MCU (and for the MCU to enter the interrupt service routine). See “External Interrupts” on page 49 for details.
7.1.1 Idle Mode
When the SM[1:0] bits are written to 00, the SLEEP instruction makes the MCU enter Idle mode, stopping the CPU
but allowing Analog Comparator, ADC, USI, Timer/Counter, Watchdog, and the interrupt system to continue oper-
ating. This sleep mode basically halts clkCPU and clkFLASH, while allowing the other clocks to run.
Idle mode enables the MCU to wake up from external triggered interrupts as well as internal ones like the Timer
Overflow. If wake-up from the Analog Comparator interrupt is not required, the Analog Comparator can be powered
down by setting the ACD bit in “ACSR – Analog Comparator Control and Status Register” on page 120. This will
reduce power consumption in Idle mode. If the ADC is enabled, a conversion starts automatically when this mode
is entered.
Table 7-1. Active Clock Domains and Wake-up Sources in the Different Sleep Modes
Active Clock Domains Oscillators Wake-up Sources
Sleep Mode
clkCPU
clkFLASH
clkIO
clkADC
clkPCK
Main Clock
Source Enabled
INT0 and
Pin Change
SPM/EEPROM
Ready
USI Start Condition
ADC
Other I/O
Watchdog
Interrupt
Idle XXX X XXXXXX
ADC Noise
Reduction XXX
(1) XXX X
Power-down X(1) XX
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7.1.2 ADC Noise Reduction Mode
When the SM[1:0] bits are written to 01, the SLEEP instruction makes the MCU enter ADC Noise Reduction mode,
stopping the CPU but allowing the ADC, the external interrupts, and the Watchdog to continue operating (if
enabled). This sleep mode halts clkI/O, clkCPU, and clkFLASH, while allowing the other clocks to run.
This improves the noise environment for the ADC, enabling higher resolution measurements. If the ADC is
enabled, a conversion starts automatically when this mode is entered. Apart form the ADC Conversion Complete
interrupt, only an External Reset, a Watchdog Reset, a Brown-out Reset, an SPM/EEPROM ready interrupt, an
external level interrupt on INT0 or a pin change interrupt can wake up the MCU from ADC Noise Reduction mode.
7.1.3 Power-down Mode
When the SM[1:0] bits are written to 10, the SLEEP instruction makes the MCU enter Power-down mode. In this
mode, the Oscillator is stopped, while the external interrupts, the USI start condition detection and the Watchdog
continue operating (if enabled). Only an External Reset, a Watchdog Reset, a Brown-out Reset, USI start condition
interupt, an external level interrupt on INT0 or a pin change interrupt can wake up the MCU. This sleep mode halts
all generated clocks, allowing operation of asynchronous modules only.
7.2 Software BOD Disable
When the Brown-out Detector (BOD) is enabled by BODLEVEL fuses (see Table 20-4 on page 148), the BOD is
actively monitoring the supply voltage during a sleep period. In some devices it is possible to save power by dis-
abling the BOD by software in Power-Down sleep mode. The sleep mode power consumption will then be at the
same level as when BOD is globally disabled by fuses.
If BOD is disabled by software, the BOD function is turned off immediately after entering the sleep mode. Upon
wake-up from sleep, BOD is automatically enabled again. This ensures safe operation in case the VCC level has
dropped during the sleep period.
When the BOD has been disabled, the wake-up time from sleep mode will be the same as that for wakeing up from
RESET. The user must manually configure the wake up times such that the bandgap reference has time to start
and the BOD is working correctly before the MCU continues executing code. See SUT[1:0] and CKSEL[3:0] fuse
bits in table “Fuse Low Byte” on page 149
BOD disable is controlled by the BODS (BOD Sleep) bit of MCU Control Register, see “MCUCR – MCU Control
Register” on page 37. Writing this bit to one turns off BOD in Power-Down, while writing a zero keeps the BOD
active. The default setting is zero, i.e. BOD active.
Writing to the BODS bit is controlled by a timed sequence and an enable bit, see “MCUCR – MCU Control Regis-
ter” on page 37.
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7.2.1 Limitations
BOD disable functionality has been implemented in the following devices, only:
ATtiny25, revision E, and newer
ATtiny45, revision D, and newer
ATtiny85, revision C, and newer
Revisions are marked on the device package and can be located as follows:
Bottom side of packages 8P3 and 8S2
Top side of package 20M1
7.3 Power Reduction Register
The Power Reduction Register (PRR), see “PRR – Power Reduction Register” on page 38, provides a method to
reduce power consumption by stopping the clock to individual peripherals. The current state of the peripheral is fro-
zen and the I/O registers can not be read or written. Resources used by the peripheral when stopping the clock will
remain occupied, hence the peripheral should in most cases be disabled before stopping the clock. Waking up a
module, which is done by clearing the bit in PRR, puts the module in the same state as before shutdown.
Module shutdown can be used in Idle mode and Active mode to significantly reduce the overall power consump-
tion. In all other sleep modes, the clock is already stopped. See “Supply Current of I/O modules” on page 177 for
examples.
7.4 Minimizing Power Consumption
There are several issues to consider when trying to minimize the power consumption in an AVR controlled system.
In general, sleep modes should be used as much as possible, and the sleep mode should be selected so that as
few as possible of the device’s functions are operating. All functions not needed should be disabled. In particular,
the following modules may need special consideration when trying to achieve the lowest possible power
consumption.
7.4.1 Analog to Digital Converter
If enabled, the ADC will be enabled in all sleep modes. To save power, the ADC should be disabled before entering
any sleep mode. When the ADC is turned off and on again, the next conversion will be an extended conversion.
Refer to “Analog to Digital Converter” on page 122 for details on ADC operation.
7.4.2 Analog Comparator
When entering Idle mode, the Analog Comparator should be disabled if not used. When entering ADC Noise
Reduction mode, the Analog Comparator should be disabled. In the other sleep modes, the Analog Comparator is
automatically disabled. However, if the Analog Comparator is set up to use the Internal Voltage Reference as
input, the Analog Comparator should be disabled in all sleep modes. Otherwise, the Internal Voltage Reference will
be enabled, independent of sleep mode. Refer to “Analog Comparator” on page 119 for details on how to configure
the Analog Comparator.
7.4.3 Brown-out Detector
If the Brown-out Detector is not needed in the application, this module should be turned off. If the Brown-out Detec-
tor is enabled by the BODLEVEL Fuses, it will be enabled in all sleep modes, and hence, always consume power.
In the deeper sleep modes, this will contribute significantly to the total current consumption. See “Brown-out Detec-
tion” on page 41 and “Software BOD Disable” on page 35 for details on how to configure the Brown-out Detector.
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7.4.4 Internal Voltage Reference
The Internal Voltage Reference will be enabled when needed by the Brown-out Detection, the Analog Comparator
or the ADC. If these modules are disabled as described in the sections above, the internal voltage reference will be
disabled and it will not be consuming power. When turned on again, the user must allow the reference to start up
before the output is used. If the reference is kept on in sleep mode, the output can be used immediately. Refer to
“Internal Voltage Reference” on page 42 for details on the start-up time.
7.4.5 Watchdog Timer
If the Watchdog Timer is not needed in the application, this module should be turned off. If the Watchdog Timer is
enabled, it will be enabled in all sleep modes, and hence, always consume power. In the deeper sleep modes, this
will contribute significantly to the total current consumption. Refer to “Watchdog Timer” on page 42 for details on
how to configure the Watchdog Timer.
7.4.6 Port Pins
When entering a sleep mode, all port pins should be configured to use minimum power. The most important thing
is then to ensure that no pins drive resistive loads. In sleep modes where both the I/O clock (clkI/O) and the ADC
clock (clkADC) are stopped, the input buffers of the device will be disabled. This ensures that no power is consumed
by the input logic when not needed. In some cases, the input logic is needed for detecting wake-up conditions, and
it will then be enabled. Refer to the section “Digital Input Enable and Sleep Modes” on page 57 for details on which
pins are enabled. If the input buffer is enabled and the input signal is left floating or has an analog signal level close
to VCC/2, the input buffer will use excessive power.
For analog input pins, the digital input buffer should be disabled at all times. An analog signal level close to VCC/2
on an input pin can cause significant current even in active mode. Digital input buffers can be disabled by writing to
the Digital Input Disable Register (DIDR0). Refer to “DIDR0 – Digital Input Disable Register 0” on page 121 for
details.
7.5 Register Description
7.5.1 MCUCR – MCU Control Register
The MCU Control Register contains control bits for power management.
Bit 7 – BODS: BOD Sleep
BOD disable functionality is available in some devices, only. See “Limitations” on page 36.
In order to disable BOD during sleep (see Table 7-1 on page 34) the BODS bit must be written to logic one. This is
controlled by a timed sequence and the enable bit, BODSE in MCUCR. First, both BODS and BODSE must be set
to one. Second, within four clock cycles, BODS must be set to one and BODSE must be set to zero. The BODS bit
is active three clock cycles after it is set. A sleep instruction must be executed while BODS is active in order to turn
off the BOD for the actual sleep mode. The BODS bit is automatically cleared after three clock cycles.
In devices where Sleeping BOD has not been implemented this bit is unused and will always read zero.
Bit 5 – SE: Sleep Enable
The SE bit must be written to logic one to make the MCU enter the sleep mode when the SLEEP instruction is exe-
cuted. To avoid the MCU entering the sleep mode unless it is the programmer’s purpose, it is recommended to
Bit 76543210
0x35 BODS PUD SE SM1 SM0 BODSE ISC01 ISC00 MCUCR
Read/Write R R/W R/W R/W R/W R R/W R/W
Initial Value00000000
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write the Sleep Enable (SE) bit to one just before the execution of the SLEEP instruction and to clear it immediately
after waking up.
Bits 4:3 – SM[1:0]: Sleep Mode Select Bits 1 and 0
These bits select between the three available sleep modes as shown in Table 7-2.
Bit 2 – BODSE: BOD Sleep Enable
BOD disable functionality is available in some devices, only. See “Limitations” on page 36.
The BODSE bit enables setting of BODS control bit, as explained on BODS bit description. BOD disable is con-
trolled by a timed sequence.
This bit is unused in devices where software BOD disable has not been implemented and will read as zero in those
devices.
7.5.2 PRR – Power Reduction Register
The Power Reduction Register provides a method to reduce power consumption by allowing peripheral clock sig-
nals to be disabled.
Bits 7:4 – Res: Reserved Bits
These bits are reserved bits in the ATtiny25/45/85 and will always read as zero.
Bit 3 – PRTIM1: Power Reduction Timer/Counter1
Writing a logic one to this bit shuts down the Timer/Counter1 module. When the Timer/Counter1 is enabled, opera-
tion will continue like before the shutdown.
Bit 2 – PRTIM0: Power Reduction Timer/Counter0
Writing a logic one to this bit shuts down the Timer/Counter0 module. When the Timer/Counter0 is enabled, opera-
tion will continue like before the shutdown.
Bit 1 – PRUSI: Power Reduction USI
Writing a logic one to this bit shuts down the USI by stopping the clock to the module. When waking up the USI
again, the USI should be re initialized to ensure proper operation.
Bit 0 – PRADC: Power Reduction ADC
Writing a logic one to this bit shuts down the ADC. The ADC must be disabled before shut down. Note that the ADC
clock is also used by some parts of the analog comparator, which means that the analogue comparator can not be
used when this bit is high.
Table 7-2. Sleep Mode Select
SM1 SM0 Sleep Mode
00Idle
0 1 ADC Noise Reduction
1 0 Power-down
11Reserved
Bit 76543 210
0x20 – – – – PRTIM1 PRTIM0 PRUSI PRADC PRR
Read/Write R R R R R/W R/W R/W R/W
Initial Value 0 0 0 0 0 0 0 0
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8. System Control and Reset
8.1 Resetting the AVR
During reset, all I/O Registers are set to their initial values, and the program starts execution from the Reset Vec-
tor. The instruction placed at the Reset Vector must be a RJMP – Relative Jump – instruction to the reset handling
routine. If the program never enables an interrupt source, the Interrupt Vectors are not used, and regular program
code can be placed at these locations. The circuit diagram in Figure 8-1 shows the reset logic. Electrical parame-
ters of the reset circuitry are given in “System and Reset Characteristics” on page 165.
Figure 8-1. Reset Logic
The I/O ports of the AVR are immediately reset to their initial state when a reset source goes active. This does not
require any clock source to be running.
After all reset sources have gone inactive, a delay counter is invoked, stretching the internal reset. This allows the
power to reach a stable level before normal operation starts. The time-out period of the delay counter is defined by
the user through the SUT and CKSEL Fuses. The different selections for the delay period are presented in “Clock
Sources” on page 25.
8.2 Reset Sources
The ATtiny25/45/85 has four sources of reset:
Power-on Reset. The MCU is reset when the supply voltage is below the Power-on Reset threshold (VPOT).
External Reset. The MCU is reset when a low level is present on the RESET pin for longer than the minimum
pulse length.
Watchdog Reset. The MCU is reset when the Watchdog Timer period expires and the Watchdog is enabled.
Brown-out Reset. The MCU is reset when the supply voltage VCC is below the Brown-out Reset threshold (VBOT)
and the Brown-out Detector is enabled.
MCU Status
Register (MCUSR)
Brown-out
Reset Circuit
BODLEVEL[2:0]
Delay Counters
Reset Circuit
RESET
VCC
Watchdog
Timer
INTERNAL RESET
COUNTER RESET
CKSEL[3:0]
CK
TIMEOUT
WDRF
BORF
EXTRF
PORF
DATA BUS
R
SQ
Clock
Generator
SPIKE
FILTER
Pull-up Resistor
Watchdog
Oscillator
SUT[1:0]
Power-on Reset
Circuit
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8.2.1 Power-on Reset
A Power-on Reset (POR) pulse is generated by an On-chip detection circuit. The detection level is defined in “Sys-
tem and Reset Characteristics” on page 165. The POR is activated whenever VCC is below the detection level. The
POR circuit can be used to trigger the Start-up Reset, as well as to detect a failure in supply voltage.
A Power-on Reset (POR) circuit ensures that the device is reset from Power-on. Reaching the Power-on Reset
threshold voltage invokes the delay counter, which determines how long the device is kept in RESET after VCC rise.
The RESET signal is activated again, without any delay, when VCC decreases below the detection level.
Figure 8-2. MCU Start-up, RESET Tied to VCC
Figure 8-3. MCU Start-up, RESET Extended Externally
8.2.2 External Reset
An External Reset is generated by a low level on the RESET pin if enabled. Reset pulses longer than the minimum
pulse width (see “System and Reset Characteristics” on page 165) will generate a reset, even if the clock is not
running. Shorter pulses are not guaranteed to generate a reset. When the applied signal reaches the Reset
Threshold Voltage – VRST – on its positive edge, the delay counter starts the MCU after the Time-out period – tTOUT
has expired.
V
RESET
TIME-OUT
INTERNAL
RESET
t
TOUT
V
POT
V
RST
CC
RESET
TIME-OUT
INTERNAL
RESET
t
TOUT
V
POT
V
RST
VCC
RESET T‘MEVOUT \NTERNAL RESET AtmeL Mr
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Figure 8-4. External Reset During Operation
8.2.3 Brown-out Detection
ATtiny25/45/85 has an On-chip Brown-out Detection (BOD) circuit for monitoring the VCC level during operation by
comparing it to a fixed trigger level. The trigger level for the BOD can be selected by the BODLEVEL Fuses. The
trigger level has a hysteresis to ensure spike free Brown-out Detection. The hysteresis on the detection level
should be interpreted as VBOT+ = VBOT + VHYST/2 and VBOT- = VBOT - VHYST/2.
When the BOD is enabled, and VCC decreases to a value below the trigger level (VBOT- in Figure 8-5), the Brown-
out Reset is immediately activated. When VCC increases above the trigger level (VBOT+ in Figure 8-5), the delay
counter starts the MCU after the Time-out period tTOUT has expired.
The BOD circuit will only detect a drop in VCC if the voltage stays below the trigger level for longer than tBOD given
in “System and Reset Characteristics” on page 165.
Figure 8-5. Brown-out Reset During Operation
8.2.4 Watchdog Reset
When the Watchdog times out, it will generate a short reset pulse of one CK cycle duration. On the falling edge of
this pulse, the delay timer starts counting the Time-out period tTOUT. Refer to “Watchdog Timer” on page 42 for
details on operation of the Watchdog Timer.
CC
V
CC
RESET
TIME-OUT
INTERNAL
RESET
V
BOT-
V
BOT+
tTOUT
AtmeL RESET VVDT ‘nME-OUT RESET T‘MEVOUT \NTERNAL RESET _,; 3— 1 Cyc‘e
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Figure 8-6. Watchdog Reset During Operation
8.3 Internal Voltage Reference
ATtiny25/45/85 features an internal bandgap reference. This reference is used for Brown-out Detection, and it can
be used as an input to the Analog Comparator or the ADC.
8.3.1 Voltage Reference Enable Signals and Start-up Time
The voltage reference has a start-up time that may influence the way it should be used. The start-up time is given
in “System and Reset Characteristics” on page 165. To save power, the reference is not always turned on. The ref-
erence is on during the following situations:
1. When the BOD is enabled (by programming the BODLEVEL[2:0] Fuse Bits).
2. When the bandgap reference is connected to the Analog Comparator (by setting the ACBG bit in ACSR).
3. When the ADC is enabled.
Thus, when the BOD is not enabled, after setting the ACBG bit or enabling the ADC, the user must always allow
the reference to start up before the output from the Analog Comparator or ADC is used. To reduce power con-
sumption in Power-down mode, the user can avoid the three conditions above to ensure that the reference is
turned off before entering Power-down mode.
8.4 Watchdog Timer
The Watchdog Timer is clocked from an On-chip Oscillator which runs at 128 kHz. By controlling the Watchdog
Timer prescaler, the Watchdog Reset interval can be adjusted as shown in Table 8-3 on page 46. The WDR –
Watchdog Reset – instruction resets the Watchdog Timer. The Watchdog Timer is also reset when it is disabled
and when a Chip Reset occurs. Ten different clock cycle periods can be selected to determine the reset period. If
the reset period expires without another Watchdog Reset, the ATtiny25/45/85 resets and executes from the Reset
Vector. For timing details on the Watchdog Reset, refer to Table 8-3 on page 46.
The Wathdog Timer can also be configured to generate an interrupt instead of a reset. This can be very helpful
when using the Watchdog to wake-up from Power-down.
CK
CC
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To prevent unintentional disabling of the Watchdog or unintentional change of time-out period, two different safety
levels are selected by the fuse WDTON as shown in Table 8-1 Refer to “Timed Sequences for Changing the Con-
figuration of the Watchdog Timer” on page 43 for details.
Figure 8-7. Watchdog Timer
8.4.1 Timed Sequences for Changing the Configuration of the Watchdog Timer
The sequence for changing configuration differs slightly between the two safety levels. Separate procedures are
described for each level.
8.4.1.1 Safety Level 1
In this mode, the Watchdog Timer is initially disabled, but can be enabled by writing the WDE bit to one without any
restriction. A timed sequence is needed when disabling an enabled Watchdog Timer. To disable an enabled
Watchdog Timer, the following procedure must be followed:
1. In the same operation, write a logic one to WDCE and WDE. A logic one must be written to WDE regard-
less of the previous value of the WDE bit.
2. Within the next four clock cycles, in the same operation, write the WDE and WDP bits as desired, but with
the WDCE bit cleared.
8.4.1.2 Safety Level 2
In this mode, the Watchdog Timer is always enabled, and the WDE bit will always read as one. A timed sequence
is needed when changing the Watchdog Time-out period. To change the Watchdog Time-out, the following proce-
dure must be followed:
1. In the same operation, write a logical one to WDCE and WDE. Even though the WDE always is set, the
WDE must be written to one to start the timed sequence.
2. Within the next four clock cycles, in the same operation, write the WDP bits as desired, but with the
WDCE bit cleared. The value written to the WDE bit is irrelevant.
Table 8-1. WDT Configuration as a Function of the Fuse Settings of WDTON
WDTON Safety
Level WDT Initial
State How to Disable the
WDT How to Change Time-
out
Unprogrammed 1 Disabled Timed sequence No limitations
Programmed 2 Enabled Always enabled Timed sequence
OSC/2K
OSC/4K
OSC/8K
OSC/16K
OSC/32K
OSC/64K
OSC/128K
OSC/256K
OSC/512K
OSC/1024K
MCU RESET
WATCHDOG
PRESCALER
128 kHz
OSCILLATOR
WATCHDOG
RESET
WDP0
WDP1
WDP2
WDP3
WDE
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8.4.2 Code Example
The following code example shows one assembly and one C function for turning off the WDT. The example
assumes that interrupts are controlled (e.g., by disabling interrupts globally) so that no interrupts will occur during
execution of these functions.
Note: 1. See “Code Examples” on page 6.
8.5 Register Description
8.5.1 MCUSR – MCU Status Register
The MCU Status Register provides information on which reset source caused an MCU Reset.
Bits 7:4 – Res: Reserved Bits
These bits are reserved bits in the ATtiny25/45/85 and will always read as zero.
Bit 3 – WDRF: Watchdog Reset Flag
Assembly Code Example(1)
WDT_off:
wdr
; Clear WDRF in MCUSR
ldi r16, (0<<WDRF)
out MCUSR, r16
; Write logical one to WDCE and WDE
; Keep old prescaler setting to prevent unintentional Watchdog Reset
in r16, WDTCR
ori r16, (1<<WDCE)|(1<<WDE)
out WDTCR, r16
; Turn off WDT
ldi r16, (0<<WDE)
out WDTCR, r16
ret
C Code Example(1)
void WDT_off(void)
{
_WDR();
/* Clear WDRF in MCUSR */
MCUSR = 0x00
/* Write logical one to WDCE and WDE */
WDTCR |= (1<<WDCE) | (1<<WDE);
/* Turn off WDT */
WDTCR = 0x00;
}
Bit 76543210
0x34 – – – – WDRF BORF EXTRF PORF MCUSR
Read/Write R R R R R/W R/W R/W R/W
Initial Value 0 0 0 0 See Bit Description
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This bit is set if a Watchdog Reset occurs. The bit is reset by a Power-on Reset, or by writing a logic zero to the
flag.
Bit 2 – BORF: Brown-out Reset Flag
This bit is set if a Brown-out Reset occurs. The bit is reset by a Power-on Reset, or by writing a logic zero to the
flag.
Bit 1 – EXTRF: External Reset Flag
This bit is set if an External Reset occurs. The bit is reset by a Power-on Reset, or by writing a logic zero to the flag.
Bit 0 – PORF: Power-on Reset Flag
This bit is set if a Power-on Reset occurs. The bit is reset only by writing a logic zero to the flag.
To make use of the Reset Flags to identify a reset condition, the user should read and then reset the MCUSR as
early as possible in the program. If the register is cleared before another reset occurs, the source of the reset can
be found by examining the Reset Flags.
8.5.2 WDTCR – Watchdog Timer Control Register
Bit 7 – WDIF: Watchdog Timeout Interrupt Flag
This bit is set when a time-out occurs in the Watchdog Timer and the Watchdog Timer is configured for interrupt.
WDIF is cleared by hardware when executing the corresponding interrupt handling vector. Alternatively, WDIF is
cleared by writing a logic one to the flag. When the I-bit in SREG and WDIE are set, the Watchdog Time-out Inter-
rupt is executed.
Bit 6 – WDIE: Watchdog Timeout Interrupt Enable
When this bit is written to one, WDE is cleared, and the I-bit in the Status Register is set, the Watchdog Time-out
Interrupt is enabled. In this mode the corresponding interrupt is executed instead of a reset if a timeout in the
Watchdog Timer occurs.
If WDE is set, WDIE is automatically cleared by hardware when a time-out occurs. This is useful for keeping the
Watchdog Reset security while using the interrupt. After the WDIE bit is cleared, the next time-out will generate a
reset. To avoid the Watchdog Reset, WDIE must be set after each interrupt.
Bit 76543210
0x21 WDIF WDIE WDP3 WDCE WDE WDP2 WDP1 WDP0 WDTCR
Read/Write R/W R/W R/W R/W R/W R/W R/W R/W
Initial Value0000X000
Table 8-2. Watchdog Timer Configuration
WDE WDIE Watchdog Timer State Action on Time-out
0 0 Stopped None
0 1 Running Interrupt
1 0 Running Reset
1 1 Running Interrupt
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Bit 4 – WDCE: Watchdog Change Enable
This bit must be set when the WDE bit is written to logic zero. Otherwise, the Watchdog will not be disabled. Once
written to one, hardware will clear this bit after four clock cycles. Refer to the description of the WDE bit for a
Watchdog disable procedure. This bit must also be set when changing the prescaler bits. See “Timed Sequences
for Changing the Configuration of the Watchdog Timer” on page 43.
Bit 3 – WDE: Watchdog Enable
When the WDE is written to logic one, the Watchdog Timer is enabled, and if the WDE is written to logic zero, the
Watchdog Timer function is disabled. WDE can only be cleared if the WDCE bit has logic level one. To disable an
enabled Watchdog Timer, the following procedure must be followed:
1. In the same operation, write a logic one to WDCE and WDE. A logic one must be written to WDE even
though it is set to one before the disable operation starts.
2. Within the next four clock cycles, write a logic 0 to WDE. This disables the Watchdog.
In safety level 2, it is not possible to disable the Watchdog Timer, even with the algorithm described above. See
“Timed Sequences for Changing the Configuration of the Watchdog Timer” on page 43.
In safety level 1, WDE is overridden by WDRF in MCUSR. See “MCUSR – MCU Status Register” on page 44 for
description of WDRF. This means that WDE is always set when WDRF is set. To clear WDE, WDRF must be
cleared before disabling the Watchdog with the procedure described above. This feature ensures multiple resets
during conditions causing failure, and a safe start-up after the failure.
Note: If the watchdog timer is not going to be used in the application, it is important to go through a watchdog disable proce-
dure in the initialization of the device. If the Watchdog is accidentally enabled, for example by a runaway pointer or
brown-out condition, the device will be reset, which in turn will lead to a new watchdog reset. To avoid this situation, the
application software should always clear the WDRF flag and the WDE control bit in the initialization routine.
Bits 5, 2:0 – WDP[3:0]: Watchdog Timer Prescaler 3, 2, 1, and 0
The WDP[3:0] bits determine the Watchdog Timer prescaling when the Watchdog Timer is enabled. The different
prescaling values and their corresponding Timeout Periods are shown in Table 8-3.
Table 8-3. Watchdog Timer Prescale Select
WDP3 WDP2 WDP1 WDP0 Number of WDT Oscillator
Cycles Typical Time-out at
VCC = 5.0V
0 0 0 0 2K (2048) cycles 16 ms
0 0 0 1 4K (4096) cycles 32 ms
0 0 1 0 8K (8192) cycles 64 ms
0 0 1 1 16K (16384) cycles 0.125 s
0 1 0 0 32K (32764) cycles 0.25 s
0 1 0 1 64K (65536) cycles 0.5 s
0 1 1 0 128K (131072) cycles 1.0 s
0 1 1 1 256K (262144) cycles 2.0 s
1 0 0 0 512K (524288) cycles 4.0 s
1 0 0 1 1024K (1048576) cycles 8.0 s
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Note: 1. If selected, one of the valid settings below 0b1010 will be used.
1010
Reserved(1)
1011
1100
1101
1110
1111
Table 8-3. Watchdog Timer Prescale Select (Continued)
WDP3 WDP2 WDP1 WDP0 Number of WDT Oscillator
Cycles Typical Time-out at
VCC = 5.0V
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9. Interrupts
This section describes the specifics of the interrupt handling as performed in ATtiny25/45/85. For a general expla-
nation of the AVR interrupt handling, refer to “Reset and Interrupt Handling” on page 12.
9.1 Interrupt Vectors in ATtiny25/45/85
The interrupt vectors of ATtiny25/45/85 are described in Table 9-1below.
If the program never enables an interrupt source, the Interrupt Vectors are not used, and regular program code can
be placed at these locations.
Table 9-1. Reset and Interrupt Vectors
Vector No. Program Address Source Interrupt Definition
1 0x0000 RESET External Pin, Power-on Reset,
Brown-out Reset, Watchdog Reset
2 0x0001 INT0 External Interrupt Request 0
3 0x0002 PCINT0 Pin Change Interrupt Request 0
4 0x0003 TIMER1_COMPA Timer/Counter1 Compare Match A
5 0x0004 TIMER1_OVF Timer/Counter1 Overflow
6 0x0005 TIMER0_OVF Timer/Counter0 Overflow
7 0x0006 EE_RDY EEPROM Ready
8 0x0007 ANA_COMP Analog Comparator
9 0x0008 ADC ADC Conversion Complete
10 0x0009 TIMER1_COMPB Timer/Counter1 Compare Match B
11 0x000A TIMER0_COMPA Timer/Counter0 Compare Match A
12 0x000B TIMER0_COMPB Timer/Counter0 Compare Match B
13 0x000C WDT Watchdog Time-out
14 0x000D USI_START USI START
15 0x000E USI_OVF USI Overflow
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A typical and general setup for interrupt vector addresses in ATtiny25/45/85 is shown in the program example
below.
Note: See “Code Examples” on page 6.
9.2 External Interrupts
The External Interrupts are triggered by the INT0 pin or any of the PCINT[5:0] pins. Observe that, if enabled, the
interrupts will trigger even if the INT0 or PCINT[5:0] pins are configured as outputs. This feature provides a way of
generating a software interrupt. Pin change interrupts PCI will trigger if any enabled PCINT[5:0] pin toggles. The
PCMSK Register control which pins contribute to the pin change interrupts. Pin change interrupts on PCINT[5:0]
are detected asynchronously. This implies that these interrupts can be used for waking the part also from sleep
modes other than Idle mode.
The INT0 interrupts can be triggered by a falling or rising edge or a low level. This is set up as indicated in the
specification for the MCU Control Register – MCUCR. When the INT0 interrupt is enabled and is configured as
level triggered, the interrupt will trigger as long as the pin is held low. Note that recognition of falling or rising edge
interrupts on INT0 requires the presence of an I/O clock, described in “Clock Systems and their Distribution” on
page 23.
9.2.1 Low Level Interrupt
A low level interrupt on INT0 is detected asynchronously. This implies that this interrupt can be used for waking the
part also from sleep modes other than Idle mode. The I/O clock is halted in all sleep modes except Idle mode.
Note that if a level triggered interrupt is used for wake-up from Power-down, the required level must be held long
enough for the MCU to complete the wake-up to trigger the level interrupt. If the level disappears before the end of
Assembly Code Example
.org 0x0000 ;Set address of next statement
rjmp RESET ; Address 0x0000
rjmp INT0_ISR ; Address 0x0001
rjmp PCINT0_ISR ; Address 0x0002
rjmp TIM1_COMPA_ISR ; Address 0x0003
rjmp TIM1_OVF_ISR ; Address 0x0004
rjmp TIM0_OVF_ISR ; Address 0x0005
rjmp EE_RDY_ISR ; Address 0x0006
rjmp ANA_COMP_ISR ; Address 0x0007
rjmp ADC_ISR ; Address 0x0008
rjmp TIM1_COMPB_ISR ; Address 0x0009
rjmp TIM0_COMPA_ISR ; Address 0x000A
rjmp TIM0_COMPB_ISR ; Address 0x000B
rjmp WDT_ISR ; Address 0x000C
rjmp USI_START_ISR ; Address 0x000D
rjmp USI_OVF_ISR ; Address 0x000E
RESET: ; Main program start
<instr> ; Address 0x000F
...
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the Start-up Time, the MCU will still wake up, but no interrupt will be generated. The start-up time is defined by the
SUT and CKSEL Fuses as described in “System Clock and Clock Options” on page 23.
If the low level on the interrupt pin is removed before the device has woken up then program execution will not be
diverted to the interrupt service routine but continue from the instruction following the SLEEP command.
9.2.2 Pin Change Interrupt Timing
An example of timing of a pin change interrupt is shown in Figure 9-1.
Figure 9-1. Timing of pin change interrupts
clk
PCINT(0)
pin_lat
pin_sync
pcint_in_(0)
pcint_syn
p
cint_setflag
PCIF
PCINT(0)
pin_sync pcint_syn
pin_lat
D Q
LE
pcint_setflag PC
IF
clk
clk
PCINT(0) in PCMSK(x)
pcint_in_(0) 0
x
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9.3 Register Description
9.3.1 MCUCR – MCU Control Register
The External Interrupt Control Register A contains control bits for interrupt sense control.
Bits 1:0 – ISC0[1:0]: Interrupt Sense Control 0 Bit 1 and Bit 0
The External Interrupt 0 is activated by the external pin INT0 if the SREG I-flag and the corresponding interrupt
mask are set. The level and edges on the external INT0 pin that activate the interrupt are defined in Table 9-2. The
value on the INT0 pin is sampled before detecting edges. If edge or toggle interrupt is selected, pulses that last lon-
ger than one clock period will generate an interrupt. Shorter pulses are not guaranteed to generate an interrupt. If
low level interrupt is selected, the low level must be held until the completion of the currently executing instruction
to generate an interrupt.
9.3.2 GIMSK – General Interrupt Mask Register
Bits 7, 4:0 – Res: Reserved Bits
These bits are reserved bits in the ATtiny25/45/85 and will always read as zero.
Bit 6 – INT0: External Interrupt Request 0 Enable
When the INT0 bit is set (one) and the I-bit in the Status Register (SREG) is set (one), the external pin interrupt is
enabled. The Interrupt Sense Control0 bits 1/0 (ISC01 and ISC00) in the MCU Control Register (MCUCR) define
whether the external interrupt is activated on rising and/or falling edge of the INT0 pin or level sensed. Activity on
the pin will cause an interrupt request even if INT0 is configured as an output. The corresponding interrupt of Exter-
nal Interrupt Request 0 is executed from the INT0 Interrupt Vector.
Bit 5 – PCIE: Pin Change Interrupt Enable
When the PCIE bit is set (one) and the I-bit in the Status Register (SREG) is set (one), pin change interrupt is
enabled. Any change on any enabled PCINT[5:0] pin will cause an interrupt. The corresponding interrupt of Pin
Change Interrupt Request is executed from the PCI Interrupt Vector. PCINT[5:0] pins are enabled individually by
the PCMSK0 Register.
Bit 76543210
0x35 BODS PUD SE SM1 SM0 BODSE ISC01 ISC00 MCUCR
Read/Write R R/W R/W R/W R/W R R/W R/W
Initial Value00000000
Table 9-2. Interrupt 0 Sense Control
ISC01 ISC00 Description
0 0 The low level of INT0 generates an interrupt request.
0 1 Any logical change on INT0 generates an interrupt request.
1 0 The falling edge of INT0 generates an interrupt request.
1 1 The rising edge of INT0 generates an interrupt request.
Bit 76543210
0x3B INT0 PCIE – – – – – GIMSK
Read/Write RR/WR/WRRRRR
Initial Value00000000
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9.3.3 GIFR – General Interrupt Flag Register
Bits 7, 4:0 – Res: Reserved Bits
These bits are reserved bits in the ATtiny25/45/85 and will always read as zero.
Bit 6 – INTF0: External Interrupt Flag 0
When an edge or logic change on the INT0 pin triggers an interrupt request, INTF0 becomes set (one). If the I-bit in
SREG and the INT0 bit in GIMSK are set (one), the MCU will jump to the corresponding Interrupt Vector. The flag
is cleared when the interrupt routine is executed. Alternatively, the flag can be cleared by writing a logical one to it.
This flag is always cleared when INT0 is configured as a level interrupt.
Bit 5 – PCIF: Pin Change Interrupt Flag
When a logic change on any PCINT[5:0] pin triggers an interrupt request, PCIF becomes set (one). If the I-bit in
SREG and the PCIE bit in GIMSK are set (one), the MCU will jump to the corresponding Interrupt Vector. The flag
is cleared when the interrupt routine is executed. Alternatively, the flag can be cleared by writing a logical one to it.
9.3.4 PCMSK – Pin Change Mask Register
Bits 7:6 – Res: Reserved Bits
These bits are reserved bits in the ATtiny25/45/85 and will always read as zero.
Bits 5:0 – PCINT[5:0]: Pin Change Enable Mask 5:0
Each PCINT[5:0] bit selects whether pin change interrupt is enabled on the corresponding I/O pin. If PCINT[5:0] is
set and the PCIE bit in GIMSK is set, pin change interrupt is enabled on the corresponding I/O pin. If PCINT[5:0] is
cleared, pin change interrupt on the corresponding I/O pin is disabled.
Bit 76543210
0x3A INTF0PCIF–––––GIFR
Read/Write RR/WR/WRRRRR
Initial Value00000000
Bit 76543210
0x15 PCINT5 PCINT4 PCINT3 PCINT2 PCINT1 PCINT0 PCMSK
Read/Write R R R/W R/W R/W R/W R/W R/W
Initial Value00000000
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10. I/O Ports
10.1 Introduction
All AVR ports have true Read-Modify-Write functionality when used as general digital I/O ports. This means that
the direction of one port pin can be changed without unintentionally changing the direction of any other pin with the
SBI and CBI instructions. The same applies when changing drive value (if configured as output) or enabling/dis-
abling of pull-up resistors (if configured as input). Each output buffer has symmetrical drive characteristics with
both high sink and source capability. The pin driver is strong enough to drive LED displays directly. All port pins
have individually selectable pull-up resistors with a supply-voltage invariant resistance. All I/O pins have protection
diodes to both VCC and Ground as indicated in Figure 10-1. Refer to “Electrical Characteristics” on page 161 for a
complete list of parameters.
Figure 10-1. I/O Pin Equivalent Schematic
All registers and bit references in this section are written in general form. A lower case “x” represents the number-
ing letter for the port, and a lower case “n” represents the bit number. However, when using the register or bit
defines in a program, the precise form must be used. For example, PORTB3 for bit no. 3 in Port B, here docu-
mented generally as PORTxn. The physical I/O Registers and bit locations are listed in “Register Description” on
page 64.
Three I/O memory address locations are allocated for each port, one each for the Data Register – PORTx, Data
Direction Register – DDRx, and the Port Input Pins – PINx. The Port Input Pins I/O location is read only, while the
Data Register and the Data Direction Register are read/write. However, writing a logic one to a bit in the PINx Reg-
ister, will result in a toggle in the corresponding bit in the Data Register. In addition, the Pull-up Disable – PUD bit
in MCUCR disables the pull-up function for all pins in all ports when set.
Using the I/O port as General Digital I/O is described in “Ports as General Digital I/O” on page 53. Most port pins
are multiplexed with alternate functions for the peripheral features on the device. How each alternate function inter-
feres with the port pin is described in “Alternate Port Functions” on page 57. Refer to the individual module sections
for a full description of the alternate functions.
Note that enabling the alternate function of some of the port pins does not affect the use of the other pins in the port
as general digital I/O.
10.2 Ports as General Digital I/O
The ports are bi-directional I/O ports with optional internal pull-ups. Figure 10-2 shows a functional description of
one I/O-port pin, here generically called Pxn.
Logic
Rpu
See Figure
"General Digital I/O" for
Details
Pxn
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Figure 10-2. General Digital I/O(1)
Note: 1. WRx, WPx, WDx, RRx, RPx, and RDx are common to all pins within the same port. clkI/O, SLEEP, and PUD are
common to all ports.
10.2.1 Configuring the Pin
Each port pin consists of three register bits: DDxn, PORTxn, and PINxn. As shown in “Register Description” on
page 64, the DDxn bits are accessed at the DDRx I/O address, the PORTxn bits at the PORTx I/O address, and
the PINxn bits at the PINx I/O address.
The DDxn bit in the DDRx Register selects the direction of this pin. If DDxn is written logic one, Pxn is configured
as an output pin. If DDxn is written logic zero, Pxn is configured as an input pin.
If PORTxn is written logic one when the pin is configured as an input pin, the pull-up resistor is activated. To switch
the pull-up resistor off, PORTxn has to be written logic zero or the pin has to be configured as an output pin. The
port pins are tri-stated when reset condition becomes active, even if no clocks are running.
If PORTxn is written logic one when the pin is configured as an output pin, the port pin is driven high (one). If
PORTxn is written logic zero when the pin is configured as an output pin, the port pin is driven low (zero).
10.2.2 Toggling the Pin
Writing a logic one to PINxn toggles the value of PORTxn, independent on the value of DDRxn. Note that the SBI
instruction can be used to toggle one single bit in a port.
10.2.3 Switching Between Input and Output
When switching between tri-state ({DDxn, PORTxn} = 0b00) and output high ({DDxn, PORTxn} = 0b11), an inter-
mediate state with either pull-up enabled {DDxn, PORTxn} = 0b01) or output low ({DDxn, PORTxn} = 0b10) must
occur. Normally, the pull-up enabled state is fully acceptable, as a high-impedant environment will not notice the
clk
RPx
RRx
RDx
WDx
PUD
SYNCHRONIZER
WDx: WRITE DDRx
WRx: WRITE PORTx
RRx: READ PORTx REGISTER
RPx: READ PORTx PIN
PUD: PULLUP DISABLE
clkI/O: I/O CLOCK
RDx: READ DDRx
D
L
Q
Q
RESET
RESET
Q
Q
D
Q
QD
CLR
PORTxn
Q
QD
CLR
DDxn
PINxn
DATA BU S
SLEEP
SLEEP: SLEEP CONTROL
Pxn
I/O
WPx
0
1
WRx
WPx: WRITE PINx REGISTER
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difference between a strong high driver and a pull-up. If this is not the case, the PUD bit in the MCUCR Register
can be set to disable all pull-ups in all ports.
Switching between input with pull-up and output low generates the same problem. The user must use either the tri-
state ({DDxn, PORTxn} = 0b00) or the output high state ({DDxn, PORTxn} = 0b10) as an intermediate step.
Table 10-1 summarizes the control signals for the pin value.
10.2.4 Reading the Pin Value
Independent of the setting of Data Direction bit DDxn, the port pin can be read through the PINxn Register bit. As
shown in Figure 10-2, the PINxn Register bit and the preceding latch constitute a synchronizer. This is needed to
avoid metastability if the physical pin changes value near the edge of the internal clock, but it also introduces a
delay. Figure 10-3 shows a timing diagram of the synchronization when reading an externally applied pin value.
The maximum and minimum propagation delays are denoted tpd,max and tpd,min respectively.
Figure 10-3. Synchronization when Reading an Externally Applied Pin value
Consider the clock period starting shortly after the first falling edge of the system clock. The latch is closed when
the clock is low, and goes transparent when the clock is high, as indicated by the shaded region of the “SYNC
LATCH” signal. The signal value is latched when the system clock goes low. It is clocked into the PINxn Register at
the succeeding positive clock edge. As indicated by the two arrows tpd,max and tpd,min, a single signal transition
on the pin will be delayed between ½ and 1½ system clock period depending upon the time of assertion.
When reading back a software assigned pin value, a nop instruction must be inserted as indicated in Figure 10-4.
The out instruction sets the “SYNC LATCH” signal at the positive edge of the clock. In this case, the delay tpd
through the synchronizer is one system clock period.
Table 10-1. Port Pin Configurations
DDxn PORTxn PUD
(in MCUCR) I/O Pull-up Comment
0 0 X Input No Tri-state (Hi-Z)
0 1 0 Input Yes Pxn will source current if ext. pulled low.
0 1 1 Input No Tri-state (Hi-Z)
1 0 X Output No Output Low (Sink)
1 1 X Output No Output High (Source)
XXX in r17, PINx
0x00 0xFF
INSTRUCTIONS
SYNC LATCH
PINxn
r17
XXX
SYSTEM CLK
t
pd, max
t
pd, min
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Figure 10-4. Synchronization when Reading a Software Assigned Pin Value
The following code example shows how to set port B pins 0 and 1 high, 2 and 3 low, and define the port pins from
4 to 5 as input with a pull-up assigned to port pin 4. The resulting pin values are read back again, but as previously
discussed, a nop instruction is included to be able to read back the value recently assigned to some of the pins.
Note: 1. For the assembly program, two temporary registers are used to minimize the time from pull-ups are set on pins 0, 1
and 4, until the direction bits are correctly set, defining bit 2 and 3 as low and redefining bits 0 and 1 as strong high
drivers.
Assembly Code Example(1)
...
; Define pull-ups and set outputs high
; Define directions for port pins
ldi r16,(1<<PB4)|(1<<PB1)|(1<<PB0)
ldi r17,(1<<DDB3)|(1<<DDB2)|(1<<DDB1)|(1<<DDB0)
out PORTB,r16
out DDRB,r17
; Insert nop for synchronization
nop
; Read port pins
in r16,PINB
...
C Code Example
unsigned char i;
...
/* Define pull-ups and set outputs high */
/* Define directions for port pins */
PORTB = (1<<PB4)|(1<<PB1)|(1<<PB0);
DDRB = (1<<DDB3)|(1<<DDB2)|(1<<DDB1)|(1<<DDB0);
/* Insert nop for synchronization*/
_NOP();
/* Read port pins */
i = PINB;
...
out PORTx, r16 nop in r17, PINx
0xFF
0x00 0xFF
SYSTEM CLK
r16
INSTRUCTIONS
SYNC LATCH
PINxn
r17
tpd
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10.2.5 Digital Input Enable and Sleep Modes
As shown in Figure 10-2, the digital input signal can be clamped to ground at the input of the schmitt-trigger. The
signal denoted SLEEP in the figure, is set by the MCU Sleep Controller in Power-down mode to avoid high power
consumption if some input signals are left floating, or have an analog signal level close to VCC/2.
SLEEP is overridden for port pins enabled as external interrupt pins. If the external interrupt request is not enabled,
SLEEP is active also for these pins. SLEEP is also overridden by various other alternate functions as described in
“Alternate Port Functions” on page 57.
If a logic high level (“one”) is present on an asynchronous external interrupt pin configured as “Interrupt on Rising
Edge, Falling Edge, or Any Logic Change on Pin” while the external interrupt is not enabled, the corresponding
External Interrupt Flag will be set when resuming from the above mentioned Sleep mode, as the clamping in these
sleep mode produces the requested logic change.
10.2.6 Unconnected Pins
If some pins are unused, it is recommended to ensure that these pins have a defined level. Even though most of
the digital inputs are disabled in the deep sleep modes as described above, floating inputs should be avoided to
reduce current consumption in all other modes where the digital inputs are enabled (Reset, Active mode and Idle
mode).
The simplest method to ensure a defined level of an unused pin, is to enable the internal pull-up. In this case, the
pull-up will be disabled during reset. If low power consumption during reset is important, it is recommended to use
an external pull-up or pulldown. Connecting unused pins directly to VCC or GND is not recommended, since this
may cause excessive currents if the pin is accidentally configured as an output.
10.3 Alternate Port Functions
Most port pins have alternate functions in addition to being general digital I/Os. Figure 10-5 shows how the port pin
control signals from the simplified Figure 10-2 can be overridden by alternate functions. The overriding signals may
not be present in all port pins, but the figure serves as a generic description applicable to all port pins in the AVR
microcontroller family.
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Figure 10-5. Alternate Port Functions(1)
Note: 1. WRx, WPx, WDx, RRx, RPx, and RDx are common to all pins within the same port. clkI/O, SLEEP, and PUD are
common to all ports. All other signals are unique for each pin.
clk
RPx
RRx
WRx
RDx
WDx
PUD
SYNCHRONIZER
WDx: WRITE DDRx
WRx: WRITE PORTx
RRx: READ PORTx REGISTER
RPx: READ PORTx PIN
PUD: PULLUP DISABLE
clkI/O: I/O CLOCK
RDx: READ DDRx
D
L
Q
Q
SET
CLR
0
1
0
1
0
1
DIxn
AIOxn
DIEOExn
PVOVxn
PVOExn
DDOVxn
DDOExn
PUOExn
PUOVxn
PUOExn: Pxn PULL-UP OVERRIDE ENABLE
PUOVxn: Pxn PULL-UP OVERRIDE VALUE
DDOExn: Pxn DATA DIRECTION OVERRIDE ENABLE
DDOVxn: Pxn DATA DIRECTION OVERRIDE VALUE
PVOExn: Pxn PORT VALUE OVERRIDE ENABLE
PVOVxn: Pxn PORT VALUE OVERRIDE VALUE
DIxn: DIGITAL INPUT PIN n ON PORTx
AIOxn: ANALOG INPUT/OUTPUT PIN n ON PORTx
RESET
RESET
Q
QD
CLR
Q
QD
CLR
Q
Q
D
CLR
PINxn
PORTxn
DDxn
DATA BU S
0
1
DIEOVxn
SLEEP
DIEOExn: Pxn DIGITAL INPUT-ENABLE OVERRIDE ENABLE
DIEOVxn: Pxn DIGITAL INPUT-ENABLE OVERRIDE VALUE
SLEEP: SLEEP CONTROL
Pxn
I/O
0
1
PTOExn
PTOExn: Pxn, PORT TOGGLE OVERRIDE ENABLE
WPx: WRITE PINx
WPx
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Table 10-2 summarizes the function of the overriding signals. The pin and port indexes from Figure 10-5 are not
shown in the succeeding tables. The overriding signals are generated internally in the modules having the alternate
function.
The following subsections shortly describe the alternate functions for each port, and relate the overriding signals to
the alternate function. Refer to the alternate function description for further details.
Table 10-2. Generic Description of Overriding Signals for Alternate Functions
Signal Name Full Name Description
PUOE Pull-up Override
Enable
If this signal is set, the pull-up enable is controlled by the PUOV
signal. If this signal is cleared, the pull-up is enabled when
{DDxn, PORTxn, PUD} = 0b010.
PUOV Pull-up Override
Value
If PUOE is set, the pull-up is enabled/disabled when PUOV is
set/cleared, regardless of the setting of the DDxn, PORTxn,
and PUD Register bits.
DDOE Data Direction
Override Enable
If this signal is set, the Output Driver Enable is controlled by the
DDOV signal. If this signal is cleared, the Output driver is
enabled by the DDxn Register bit.
DDOV Data Direction
Override Value
If DDOE is set, the Output Driver is enabled/disabled when
DDOV is set/cleared, regardless of the setting of the DDxn
Register bit.
PVOE Port Value
Override Enable
If this signal is set and the Output Driver is enabled, the port
value is controlled by the PVOV signal. If PVOE is cleared, and
the Output Driver is enabled, the port Value is controlled by the
PORTxn Register bit.
PVOV Port Value
Override Value If PVOE is set, the port value is set to PVOV, regardless of the
setting of the PORTxn Register bit.
PTOE Port Toggle
Override Enable If PTOE is set, the PORTxn Register bit is inverted.
DIEOE Digital Input
Enable Override
Enable
If this bit is set, the Digital Input Enable is controlled by the
DIEOV signal. If this signal is cleared, the Digital Input Enable
is determined by MCU state (Normal mode, sleep mode).
DIEOV Digital Input
Enable Override
Value
If DIEOE is set, the Digital Input is enabled/disabled when
DIEOV is set/cleared, regardless of the MCU state (Normal
mode, sleep mode).
DI Digital Input
This is the Digital Input to alternate functions. In the figure, the
signal is connected to the output of the schmitt-trigger but
before the synchronizer. Unless the Digital Input is used as a
clock source, the module with the alternate function will use its
own synchronizer.
AIO Analog
Input/Output
This is the Analog Input/Output to/from alternate functions. The
signal is connected directly to the pad, and can be used bi-
directionally.
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10.3.1 Alternate Functions of Port B
The Port B pins with alternate function are shown in Table 10-3.
Port B, Bit 5 – RESET/dW/ADC0/PCINT5
• RESET: External Reset input is active low and enabled by unprogramming (“1”) the RSTDISBL Fuse. Pullup is
activated and output driver and digital input are deactivated when the pin is used as the RESET pin.
dW: When the debugWIRE Enable (DWEN) Fuse is programmed and Lock bits are unprogrammed, the
debugWIRE system within the target device is activated. The RESET port pin is configured as a wire-AND
(open-drain) bi-directional I/O pin with pull-up enabled and becomes the communication gateway between
target and emulator.
ADC0: Analog to Digital Converter, Channel 0.
PCINT5: Pin Change Interrupt source 5.
Table 10-3. Port B Pins Alternate Functions
Port Pin Alternate Function
PB5
RESET:Reset Pin
dW: debugWIRE I/O
ADC0: ADC Input Channel 0
PCINT5:Pin Change Interrupt, Source 5
PB4
XTAL2: Crystal Oscillator Output
CLKO: System Clock Output
ADC2: ADC Input Channel 2
OC1B: Timer/Counter1 Compare Match B Output
PCINT4:Pin Change Interrupt 0, Source 4
PB3
XTAL1: Crystal Oscillator Input
CLKI: External Clock Input
ADC3: ADC Input Channel 3
OC1B: Complementary Timer/Counter1 Compare Match B Output
PCINT3:Pin Change Interrupt 0, Source 3
PB2
SCK: Serial Clock Input
ADC1: ADC Input Channel 1
T0: Timer/Counter0 Clock Source
USCK: USI Clock (Three Wire Mode)
SCL : USI Clock (Two Wire Mode)
INT0: External Interrupt 0 Input
PCINT2:Pin Change Interrupt 0, Source 2
PB1
MISO: SPI Master Data Input / Slave Data Output
AIN1: Analog Comparator, Negative Input
OC0B: Timer/Counter0 Compare Match B Output
OC1A: Timer/Counter1 Compare Match A Output
DO: USI Data Output (Three Wire Mode)
PCINT1:Pin Change Interrupt 0, Source 1
PB0
MOSI:: SPI Master Data Output / Slave Data Input
AIN0: Analog Comparator, Positive Input
OC0A: Timer/Counter0 Compare Match A output
OC1A: Complementary Timer/Counter1 Compare Match A Output
DI: USI Data Input (Three Wire Mode)
SDA: USI Data Input (Two Wire Mode)
AREF: External Analog Reference
PCINT0:Pin Change Interrupt 0, Source 0
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Port B, Bit 4 – XTAL2/CLKO/ADC2/OC1B/PCINT4
XTAL2: Chip Clock Oscillator pin 2. Used as clock pin for all chip clock sources except internal calibrateble RC
Oscillator and external clock. When used as a clock pin, the pin can not be used as an I/O pin. When using
internal calibratable RC Oscillator or External clock as a Chip clock sources, PB4 serves as an ordinary I/O pin.
CLKO: The devided system clock can be output on the pin PB4. The divided system clock will be output if the
CKOUT Fuse is programmed, regardless of the PORTB4 and DDB4 settings. It will also be output during reset.
ADC2: Analog to Digital Converter, Channel 2.
OC1B: Output Compare Match output: The PB4 pin can serve as an external output for the Timer/Counter1
Compare Match B when configured as an output (DDB4 set). The OC1B pin is also the output pin for the PWM
mode timer function.
PCINT4: Pin Change Interrupt source 4.
Port B, Bit 3 – XTAL1/CLKI/ADC3/OC1B/PCINT3
XTAL1: Chip Clock Oscillator pin 1. Used for all chip clock sources except internal calibrateble RC oscillator.
When used as a clock pin, the pin can not be used as an I/O pin.
CLKI: Clock Input from an external clock source, see “External Clock” on page 26.
ADC3: Analog to Digital Converter, Channel 3.
•OC1B: Inverted Output Compare Match output: The PB3 pin can serve as an external output for the
Timer/Counter1 Compare Match B when configured as an output (DDB3 set). The OC1B pin is also the inverted
output pin for the PWM mode timer function.
PCINT3: Pin Change Interrupt source 3.
Port B, Bit 2 – SCK/ADC1/T0/USCK/SCL/INT0/PCINT2
SCK: Master Clock output, Slave Clock input pin for SPI channel. When the SPI is enabled as a Slave, this pin
is configured as an input regardless of the setting of DDB2. When the SPI is enabled as a Master, the data
direction of this pin is controlled by DDPB2. When the pin is forced by the SPI to be an input, the pull-up can still
be controlled by the PORTB2 bit.
ADC1: Analog to Digital Converter, Channel 1.
T0: Timer/Counter0 counter source.
USCK: Three-wire mode Universal Serial Interface Clock.
SCL: Two-wire mode Serial Clock for USI Two-wire mode.
INT0: External Interrupt source 0.
PCINT2: Pin Change Interrupt source 2.
Port B, Bit 1 – MISO/AIN1/OC0B/OC1A/DO/PCINT1
MISO: Master Data input, Slave Data output pin for SPI channel. When the SPI is enabled as a Master, this pin
is configured as an input regardless of the setting of DDB1. When the SPI is enabled as a Slave, the data
direction of this pin is controlled by DDB1. When the pin is forced by the SPI to be an input, the pull-up can still
be controlled by the PORTB1 bit.
AIN1: Analog Comparator Negative Input. Configure the port pin as input with the internal pull-up switched off to
avoid the digital port function from interfering with the function of the Analog Comparator.
OC0B: Output Compare Match output. The PB1 pin can serve as an external output for the Timer/Counter0
Compare Match B. The PB1 pin has to be configured as an output (DDB1 set (one)) to serve this function. The
OC0B pin is also the output pin for the PWM mode timer function.
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OC1A: Output Compare Match output: The PB1 pin can serve as an external output for the Timer/Counter1
Compare Match B when configured as an output (DDB1 set). The OC1A pin is also the output pin for the PWM
mode timer function.
DO: Three-wire mode Universal Serial Interface Data output. Three-wire mode Data output overrides PORTB1
value and it is driven to the port when data direction bit DDB1 is set (one). PORTB1 still enables the pull-up, if
the direction is input and PORTB1 is set (one).
PCINT1: Pin Change Interrupt source 1.
Port B, Bit 0 – MOSI/AIN0/OC0A/OC1A/DI/SDA/AREF/PCINT0
MOSI: SPI Master Data output, Slave Data input for SPI channel. When the SPI is enabled as a Slave, this pin
is configured as an input regardless of the setting of DDB0. When the SPI is enabled as a Master, the data
direction of this pin is controlled by DDB0. When the pin is forced by the SPI to be an input, the pull-up can still
be controlled by the PORTB0 bit.
AIN0: Analog Comparator Positive Input. Configure the port pin as input with the internal pull-up switched off to
avoid the digital port function from interfering with the function of the Analog Comparator.
OC0A: Output Compare Match output. The PB0 pin can serve as an external output for the Timer/Counter0
Compare Match A when configured as an output (DDB0 set (one)). The OC0A pin is also the output pin for the
PWM mode timer function.
•OC1A
: Inverted Output Compare Match output: The PB0 pin can serve as an external output for the
Timer/Counter1 Compare Match B when configured as an output (DDB0 set). The OC1A pin is also the inverted
output pin for the PWM mode timer function.
SDA: Two-wire mode Serial Interface Data.
AREF: External Analog Reference for ADC. Pullup and output driver are disabled on PB0 when the pin is used
as an external reference or Internal Voltage Reference with external capacitor at the AREF pin.
DI: Data Input in USI Three-wire mode. USI Three-wire mode does not override normal port functions, so pin
must be configure as an input for DI function.
PCINT0: Pin Change Interrupt source 0.
Table 10-4 and Table 10-5 relate the alternate functions of Port B to the overriding signals shown in Figure 10-5 on
page 58.
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Note: 1. 1 when the Fuse is “0” (Programmed).
Table 10-4. Overriding Signals for Alternate Functions in PB[5:3]
Signal
Name PB5/RESET/
ADC0/PCINT5 PB4/ADC2/XTAL2/
OC1B/PCINT4 PB3/ADC3/XTAL1/
OC1B/PCINT3
PUOE RSTDISBL(1) • DWEN(1) 00
PUOV100
DDOE RSTDISBL(1) • DWEN(1) 00
DDOV debugWire Transmit 0 0
PVOE 0 OC1B Enable OC1B Enable
PVOV 0 OC1B OC1B
PTOE000
DIEOE RSTDISBL(1) + (PCINT5 •
PCIE + ADC0D) PCINT4 • PCIE + ADC2D PCINT3 • PCIE + ADC3D
DIEOV ADC0D ADC2D ADC3D
DI PCINT5 Input PCINT4 Input PCINT3 Input
AIO RESET Input, ADC0 Input ADC2 Input ADC3 Input
Table 10-5. Overriding Signals for Alternate Functions in PB[2:0]
Signal
Name PB2/SCK/ADC1/T0/
USCK/SCL/INT0/PCINT2 PB1/MISO/DO/AIN1/
OC1A/OC0B/PCINT1
PB0/MOSI/DI/SDA/AIN0/AR
EF/OC1A/OC0A/
PCINT0
PUOE USI_TWO_WIRE 0 USI_TWO_WIRE
PUOV000
DDOE USI_TWO_WIRE 0 USI_TWO_WIRE
DDOV (USI_SCL_HOLD +
PORTB2) • DDB2 0(SDA
+ PORTB0) • DDB0
PVOE USI_TWO_WIRE • DDB2 OC0B Enable + OC1A
Enable +
USI_THREE_WIRE
OC0A Enable + OC1A
Enable + (USI_TWO_WIRE
• DDB0)
PVOV 0 OC0B + OC1A + DO OC0A + OC1A
PTOE USITC 0 0
DIEOE PCINT2 • PCIE + ADC1D +
USISIE PCINT1 • PCIE + AIN1D PCINT0 • PCIE + AIN0D +
USISIE
DIEOV ADC1D AIN1D AIN0D
DI T0/USCK/SCL/INT0/
PCINT2 Input PCINT1 Input DI/SDA/PCINT0 Input
AIO ADC1 Input Analog Comparator
Negative Input Analog Comparator Positive
Input
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10.4 Register Description
10.4.1 MCUCR – MCU Control Register
Bit 6 – PUD: Pull-up Disable
When this bit is written to one, the pull-ups in the I/O ports are disabled even if the DDxn and PORTxn Registers
are configured to enable the pull-ups ({DDxn, PORTxn} = 0b01). See “Configuring the Pin” on page 54 for more
details about this feature.
10.4.2 PORTB – Port B Data Register
10.4.3 DDRB – Port B Data Direction Register
10.4.4 PINB – Port B Input Pins Address
Bit 7 6 5 4 3 2 1 0
0x35 BODS PUD SE SM1 SM0 BODSE ISC01 ISC00 MCUCR
Read/Write R R/W R/W R/W R/W R R/W R/W
Initial Value 0 0 0 0 0 0 0 0
Bit 76543210
0x18 PORTB5 PORTB4 PORTB3 PORTB2 PORTB1 PORTB0 PORTB
Read/Write R R R/W R/W R/W R/W R/W R/W
Initial Value00000000
Bit 76543210
0x17 DDB5 DDB4 DDB3 DDB2 DDB1 DDB0 DDRB
Read/Write R R R/W R/W R/W R/W R/W R/W
Initial Value00000000
Bit 76543210
0x16 PINB5 PINB4 PINB3 PINB2 PINB1 PINB0 PINB
Read/Write R R R/W R/W R/W R/W R/W R/W
Initial Value 0 0 N/A N/A N/A N/A N/A N/A
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11. 8-bit Timer/Counter0 with PWM
11.1 Features
Two Independent Output Compare Units
Double Buffered Output Compare Registers
Clear Timer on Compare Match (Auto Reload)
Glitch Free, Phase Correct Pulse Width Modulator (PWM)
Variable PWM Period
Frequency Generator
Three Independent Interrupt Sources (TOV0, OCF0A, and OCF0B)
11.2 Overview
Timer/Counter0 is a general purpose 8-bit Timer/Counter module, with two independent Output Compare Units,
and with PWM support. It allows accurate program execution timing (event management) and wave generation.
A simplified block diagram of the 8-bit Timer/Counter is shown in Figure 11-1. For the actual placement of I/O pins,
refer to “Pinout ATtiny25/45/85” on page 2. CPU accessible I/O Registers, including I/O bits and I/O pins, are
shown in bold. The device-specific I/O Register and bit locations are listed in the “Register Description” on page 77.
Figure 11-1. 8-bit Timer/Counter Block Diagram
11.2.1 Registers
The Timer/Counter (TCNT0) and Output Compare Registers (OCR0A and OCR0B) are 8-bit registers. Interrupt
request (abbreviated to Int.Req. in the figure) signals are all visible in the Timer Interrupt Flag Register (TIFR). All
Clock Select
Timer/Counter
DATA BUS
OCRnA
OCRnB
=
=
TCNTn
Waveform
Generation
Waveform
Generation
OCnA
OCnB
=
Fixed
TOP
Value
Control Logic
= 0
TOP BOTTOM
Count
Clear
Direction
TOVn
(Int.Req.)
OCnA
(Int.Req.)
OCnB
(Int.Req.)
TCCRnA TCCRnB
Tn
Edge
Detector
( From Prescaler )
clkTn
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interrupts are individually masked with the Timer Interrupt Mask Register (TIMSK). TIFR and TIMSK are not shown
in the figure.
The Timer/Counter can be clocked internally, via the prescaler, or by an external clock source on the T0 pin. The
Clock Select logic block controls which clock source and edge the Timer/Counter uses to increment (or decrement)
its value. The Timer/Counter is inactive when no clock source is selected. The output from the Clock Select logic is
referred to as the timer clock (clkT0).
The double buffered Output Compare Registers (OCR0A and OCR0B) is compared with the Timer/Counter value
at all times. The result of the compare can be used by the Waveform Generator to generate a PWM or variable fre-
quency output on the Output Compare pins (OC0A and OC0B). See “Output Compare Unit” on page 69. for details.
The Compare Match event will also set the Compare Flag (OCF0A or OCF0B) which can be used to generate an
Output Compare interrupt request.
11.2.2 Definitions
Many register and bit references in this section are written in general form. A lower case “n” replaces the
Timer/Counter number, in this case 0. A lower case “x” replaces the Output Compare Unit, in this case Compare
Unit A or Compare Unit B. However, when using the register or bit defines in a program, the precise form must be
used, i.e., TCNT0 for accessing Timer/Counter0 counter value and so on.
The definitions in Table 11-1 are also used extensively throughout the document.
11.3 Timer/Counter0 Prescaler and Clock Sources
The Timer/Counter can be clocked by an internal or an external clock source. The clock source is selected by the
Clock Select logic which is controlled by the Clock Select (c) bits located in the Timer/Counter0 Control Register
(TCCR0B).
11.3.1 Internal Clock Source with Prescaler
Timer/Counter0 can be clocked directly by the system clock (by setting the CS0[2:0] = 1). This provides the fastest
operation, with a maximum timer/counter clock frequency equal to system clock frequency (fCLK_I/O). Alternatively,
one of four taps from the prescaler can be used as a clock source. The prescaled clock has a frequency of either
fCLK_I/O/8, fCLK_I/O/64, fCLK_I/O/256, or fCLK_I/O/1024.
11.3.2 Prescaler Reset
The prescaler is free running, i.e. it operates independently of the Clock Select logic of Timer/Counter0. Since the
prescaler is not affected by the timer/counter’s clock select, the state of the prescaler will have implications for situ-
ations where a prescaled clock is used. One example of a prescaling artifact is when the timer/counter is enabled
and clocked by the prescaler (6 > CS0[2:0] > 1). The number of system clock cycles from when the timer is
enabled to the first count occurs can be from 1 to N+1 system clock cycles, where N equals the prescaler divisor
(8, 64, 256, or 1024).
It is possible to use the Prescaler Reset for synchronizing the Timer/Counter to program execution.
Table 11-1. Definitions
Constant Description
BOTTOM The counter reaches BOTTOM when it becomes 0x00
MAX The counter reaches its MAXimum when it becomes 0xFF (decimal 255)
TOP The counter reaches the TOP when it becomes equal to the highest value in the count
sequence. The TOP value can be assigned to be the fixed value 0xFF (MAX) or the
value stored in the OCR0A Register. The assignment depends on the mode of operation
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11.3.3 External Clock Source
An external clock source applied to the T0 pin can be used as timer/counter clock (clkT0). The T0 pin is sampled
once every system clock cycle by the pin synchronization logic. The synchronized (sampled) signal is then passed
through the edge detector. Figure 11-2 shows a functional equivalent block diagram of the T0 synchronization and
edge detector logic. The registers are clocked at the positive edge of the internal system clock (clkI/O). The latch is
transparent in the high period of the internal system clock.
The edge detector generates one clkT0 pulse for each positive (CS0[2:0] = 7) or negative (CS0[2:0] = 6) edge it
detects.
Figure 11-2. T0 Pin Sampling
The synchronization and edge detector logic introduces a delay of 2.5 to 3.5 system clock cycles from an edge has
been applied to the T0 pin to the counter is updated.
Enabling and disabling of the clock input must be done when T0 has been stable for at least one system clock
cycle, otherwise it is a risk that a false timer/counter clock pulse is generated.
Each half period of the external clock applied must be longer than one system clock cycle to ensure correct sam-
pling. The external clock must be guaranteed to have less than half the system clock frequency (fExtClk < fclk_I/O/2)
given a 50/50% duty cycle. Since the edge detector uses sampling, the maximum frequency of an external clock it
can detect is half the sampling frequency (following the Nyquist sampling theorem). However, due to variation of
the system clock frequency and duty cycle caused by oscillator source (crystal, resonator, and capacitors) toler-
ances, it is recommended that maximum frequency of an external clock source is less than fclk_I/O/2.5.
An external clock source can not be prescaled.
Tn_sync
(To Clock
Select Logic)
Edge DetectorSynchronization
DQDQ
LE
DQ
Tn
clk
I/O
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Figure 11-3. Timer/Counter0 Prescaler
The synchronization logic on the input pins (T0) in Figure 11-3 is shown in Figure 11-2 on page 67.
11.4 Counter Unit
The main part of the 8-bit Timer/Counter is the programmable bi-directional counter unit. Figure 11-4 shows a
block diagram of the counter and its surroundings.
Figure 11-4. Counter Unit Block Diagram
Signal description (internal signals):
count Increment or decrement TCNT0 by 1.
direction Select between increment and decrement.
clear Clear TCNT0 (set all bits to zero).
clkTnTimer/Counter clock, referred to as clkT0 in the following.
top Signalize that TCNT0 has reached maximum value.
bottom Signalize that TCNT0 has reached minimum value (zero).
Depending of the mode of operation used, the counter is cleared, incremented, or decremented at each timer clock
(clkT0). clkT0 can be generated from an external or internal clock source, selected by the Clock Select bits
(CS0[2:0]). When no clock source is selected (CS0[2:0] = 0) the timer is stopped. However, the TCNT0 value can
PSR10
Clear
clk
T0
T0
clk
I/O
Synchronization
DATA BUS
TCNTn Control Logic
count
TOVn
(Int.Req.)
Clock Select
top
Tn
Edge
Detector
( From Prescaler )
clk
Tn
bottom
direction
clear
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be accessed by the CPU, regardless of whether clkT0 is present or not. A CPU write overrides (has priority over) all
counter clear or count operations.
The counting sequence is determined by the setting of the WGM01 and WGM00 bits located in the Timer/Counter
Control Register (TCCR0A) and the WGM02 bit located in the Timer/Counter Control Register B (TCCR0B). There
are close connections between how the counter behaves (counts) and how waveforms are generated on the Out-
put Compare output OC0A. For more details about advanced counting sequences and waveform generation, see
“Modes of Operation” on page 71.
The Timer/Counter Overflow Flag (TOV0) is set according to the mode of operation selected by the WGM0[1:0]
bits. TOV0 can be used for generating a CPU interrupt.
11.5 Output Compare Unit
The 8-bit comparator continuously compares TCNT0 with the Output Compare Registers (OCR0A and OCR0B).
Whenever TCNT0 equals OCR0A or OCR0B, the comparator signals a match. A match will set the Output Com-
pare Flag (OCF0A or OCF0B) at the next timer clock cycle. If the corresponding interrupt is enabled, the Output
Compare Flag generates an Output Compare interrupt. The Output Compare Flag is automatically cleared when
the interrupt is executed. Alternatively, the flag can be cleared by software by writing a logical one to its I/O bit loca-
tion. The Waveform Generator uses the match signal to generate an output according to operating mode set by the
WGM0[2:0] bits and Compare Output mode (COM0x[1:0]) bits. The max and bottom signals are used by the Wave-
form Generator for handling the special cases of the extreme values in some modes of operation (See “Modes of
Operation” on page 71.).
Figure 11-5 shows a block diagram of the Output Compare unit.
Figure 11-5. Output Compare Unit, Block Diagram
The OCR0x Registers are double buffered when using any of the Pulse Width Modulation (PWM) modes. For the
normal and Clear Timer on Compare (CTC) modes of operation, the double buffering is disabled. The double buff-
ering synchronizes the update of the OCR0x Compare Registers to either top or bottom of the counting sequence.
The synchronization prevents the occurrence of odd-length, non-symmetrical PWM pulses, thereby making the
output glitch-free.
OCFnx (Int.Req.)
= (8-bit Comparator )
OCRnx
OCnx
DATA BUS
TCNTn
WGMn[1:0]
Waveform Generat or
top
FOCn
COMnX[1:0]
bottom
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The OCR0x Register access may seem complex, but this is not case. When the double buffering is enabled, the
CPU has access to the OCR0x Buffer Register, and if double buffering is disabled the CPU will access the OCR0x
directly.
11.5.1 Force Output Compare
In non-PWM waveform generation modes, the match output of the comparator can be forced by writing a one to
the Force Output Compare (FOC0x) bit. Forcing Compare Match will not set the OCF0x Flag or reload/clear the
timer, but the OC0x pin will be updated as if a real Compare Match had occurred (the COM0x[1:0] bits settings
define whether the OC0x pin is set, cleared or toggled).
11.5.2 Compare Match Blocking by TCNT0 Write
All CPU write operations to the TCNT0 Register will block any Compare Match that occur in the next timer clock
cycle, even when the timer is stopped. This feature allows OCR0x to be initialized to the same value as TCNT0
without triggering an interrupt when the Timer/Counter clock is enabled.
11.5.3 Using the Output Compare Unit
Since writing TCNT0 in any mode of operation will block all Compare Matches for one timer clock cycle, there are
risks involved when changing TCNT0 when using the Output Compare Unit, independently of whether the
Timer/Counter is running or not. If the value written to TCNT0 equals the OCR0x value, the Compare Match will be
missed, resulting in incorrect waveform generation. Similarly, do not write the TCNT0 value equal to BOTTOM
when the counter is down-counting.
The setup of the OC0x should be performed before setting the Data Direction Register for the port pin to output.
The easiest way of setting the OC0x value is to use the Force Output Compare (FOC0x) strobe bits in Normal
mode. The OC0x Registers keep their values even when changing between Waveform Generation modes.
Be aware that the COM0x[1:0] bits are not double buffered together with the compare value. Changing the
COM0x[1:0] bits will take effect immediately.
11.6 Compare Match Output Unit
The Compare Output mode (COM0x[1:0]) bits have two functions. The Waveform Generator uses the COM0x[1:0]
bits for defining the Output Compare (OC0x) state at the next Compare Match. Also, the COM0x[1:0] bits control
the OC0x pin output source. Figure 11-6 shows a simplified schematic of the logic affected by the COM0x[1:0] bit
setting. The I/O Registers, I/O bits, and I/O pins in the figure are shown in bold. Only the parts of the general I/O
Port Control Registers (DDR and PORT) that are affected by the COM0x[1:0] bits are shown. When referring to the
OC0x state, the reference is for the internal OC0x Register, not the OC0x pin. If a system reset occur, the OC0x
Register is reset to “0”.
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Figure 11-6. Compare Match Output Unit, Schematic
The general I/O port function is overridden by the Output Compare (OC0x) from the Waveform Generator if either
of the COM0x[1:0] bits are set. However, the OC0x pin direction (input or output) is still controlled by the Data
Direction Register (DDR) for the port pin. The Data Direction Register bit for the OC0x pin (DDR_OC0x) must be
set as output before the OC0x value is visible on the pin. The port override function is independent of the Wave-
form Generation mode.
The design of the Output Compare pin logic allows initialization of the OC0x state before the output is enabled.
Note that some COM0x[1:0] bit settings are reserved for certain modes of operation. See “Register Description” on
page 77.
11.6.1 Compare Output Mode and Waveform Generation
The Waveform Generator uses the COM0x[1:0] bits differently in Normal, CTC, and PWM modes. For all modes,
setting the COM0x[1:0] = 0 tells the Waveform Generator that no action on the OC0x Register is to be performed
on the next Compare Match. For compare output actions in the non-PWM modes refer to Table 11-2 on page 78.
For fast PWM mode, refer to Table 11-3 on page 78, and for phase correct PWM refer to Table 11-4 on page 78.
A change of the COM0x[1:0] bits state will have effect at the first Compare Match after the bits are written. For non-
PWM modes, the action can be forced to have immediate effect by using the FOC0x strobe bits.
11.7 Modes of Operation
The mode of operation, i.e., the behavior of the Timer/Counter and the Output Compare pins, is defined by the
combination of the Waveform Generation mode (WGM0[2:0]) and Compare Output mode (COM0x[1:0]) bits. The
Compare Output mode bits do not affect the counting sequence, while the Waveform Generation mode bits do.
The COM0x[1:0] bits control whether the PWM output generated should be inverted or not (inverted or non-
inverted PWM). For non-PWM modes the COM0x[1:0] bits control whether the output should be set, cleared, or
toggled at a Compare Match (See “Compare Match Output Unit” on page 70.).
PORT
DDR
DQ
DQ
OCn
Pin
OCnx
DQ
Waveform
Generator
COMnx1
COMnx0
0
1
DATA BU S
FOCn
clk
I/O
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For detailed timing information refer to Figure 11-10, Figure 11-11, Figure 11-12 and Figure 11-13 in “Timer/Coun-
ter Timing Diagrams” on page 76.
11.7.1 Normal Mode
The simplest mode of operation is the Normal mode (WGM0[2:0] = 0). In this mode the counting direction is always
up (incrementing), and no counter clear is performed. The counter simply overruns when it passes its maximum 8-
bit value (TOP = 0xFF) and then restarts from the bottom (0x00). In normal operation the Timer/Counter Overflow
Flag (TOV0) will be set in the same timer clock cycle as the TCNT0 becomes zero. The TOV0 Flag in this case
behaves like a ninth bit, except that it is only set, not cleared. However, combined with the timer overflow interrupt
that automatically clears the TOV0 Flag, the timer resolution can be increased by software. There are no special
cases to consider in the Normal mode, a new counter value can be written anytime.
The Output Compare Unit can be used to generate interrupts at some given time. Using the Output Compare to
generate waveforms in Normal mode is not recommended, since this will occupy too much of the CPU time.
11.7.2 Clear Timer on Compare Match (CTC) Mode
In Clear Timer on Compare or CTC mode (WGM0[2:0] = 2), the OCR0A Register is used to manipulate the counter
resolution. In CTC mode the counter is cleared to zero when the counter value (TCNT0) matches the OCR0A. The
OCR0A defines the top value for the counter, hence also its resolution. This mode allows greater control of the
Compare Match output frequency. It also simplifies the operation of counting external events.
The timing diagram for the CTC mode is shown in Figure 11-7. The counter value (TCNT0) increases until a Com-
pare Match occurs between TCNT0 and OCR0A, and then counter (TCNT0) is cleared.
Figure 11-7. CTC Mode, Timing Diagram
An interrupt can be generated each time the counter value reaches the TOP value by using the OCF0A Flag. If the
interrupt is enabled, the interrupt handler routine can be used for updating the TOP value. However, changing TOP
to a value close to BOTTOM when the counter is running with none or a low prescaler value must be done with
care since the CTC mode does not have the double buffering feature. If the new value written to OCR0A is lower
than the current value of TCNT0, the counter will miss the Compare Match. The counter will then have to count to
its maximum value (0xFF) and wrap around starting at 0x00 before the Compare Match can occur.
For generating a waveform output in CTC mode, the OC0A output can be set to toggle its logical level on each
Compare Match by setting the Compare Output mode bits to toggle mode (COM0A[1:0] = 1). The OC0A value will
not be visible on the port pin unless the data direction for the pin is set to output. The waveform generated will have
a maximum frequency of fOC0 = fclk_I/O/2 when OCR0A is set to zero (0x00). The waveform frequency is defined by
the following equation:
TCNTn
OCn
(Toggle)
OCnx Interrupt Flag Set
1 4
Per i o d
2 3
(COMnx[1:0] = 1)
fOCnx
fclk_I/O
2 N 1 OCRnx+
--------------------------------------------------=
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The N variable represents the prescale factor (1, 8, 64, 256, or 1024).
As for the Normal mode of operation, the TOV0 Flag is set in the same timer clock cycle that the counter counts
from MAX to 0x00.
11.7.3 Fast PWM Mode
The fast Pulse Width Modulation or fast PWM mode (WGM0[2:0] = 3 or 7) provides a high frequency PWM wave-
form generation option. The fast PWM differs from the other PWM option by its single-slope operation. The counter
counts from BOTTOM to TOP then restarts from BOTTOM. TOP is defined as 0xFF when WGM0[2:0] = 3, and
OCR0A when WGM0[2:0] = 7.
In non-inverting Compare Output mode, the Output Compare (OC0x) is cleared on the Compare Match between
TCNT0 and OCR0x, and set at BOTTOM. In inverting Compare Output mode, the output is set on Compare Match
and cleared at BOTTOM.
Due to the single-slope operation, the operating frequency of the fast PWM mode can be twice as high as the
phase correct PWM mode that use dual-slope operation. This high frequency makes the fast PWM mode well
suited for power regulation, rectification, and DAC applications. High frequency allows physically small sized exter-
nal components (coils, capacitors), and therefore reduces total system cost.
In fast PWM mode, the counter is incremented until the counter value matches the TOP value. The counter is then
cleared at the following timer clock cycle. The timing diagram for the fast PWM mode is shown in Figure 11-8. The
TCNT0 value is in the timing diagram shown as a histogram for illustrating the single-slope operation. The diagram
includes non-inverted and inverted PWM outputs. The small horizontal line marks on the TCNT0 slopes represent
Compare Matches between OCR0x and TCNT0.
Figure 11-8. Fast PWM Mode, Timing Diagram
The Timer/Counter Overflow Flag (TOV0) is set each time the counter reaches TOP. If the interrupt is enabled, the
interrupt handler routine can be used for updating the compare value.
In fast PWM mode, the compare unit allows generation of PWM waveforms on the OC0x pins. Setting the
COM0x[1:0] bits to two will produce a non-inverted PWM and an inverted PWM output can be generated by setting
the COM0x[1:0] to three: Setting the COM0A[1:0] bits to one allowes the AC0A pin to toggle on Compare Matches
if the WGM02 bit is set. This option is not available for the OC0B pin (See Table 11-3 on page 78). The actual
OC0x value will only be visible on the port pin if the data direction for the port pin is set as output. The PWM wave-
TCNTn
OCRnx Update and
TOVn Int er rupt Flag Set
1
Per i o d
2 3
OCn
OCn
(COMnx[1:0] = 2)
(COMnx[1:0] = 3)
OCRnx Interrupt Flag Set
4 5 6 7
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form is generated by setting (or clearing) the OC0x Register at the Compare Match between OCR0x and TCNT0,
and clearing (or setting) the OC0x Register at the timer clock cycle the counter is cleared (changes from TOP to
BOTTOM).
The PWM frequency for the output can be calculated by the following equation:
The N variable represents the prescale factor (1, 8, 64, 256, or 1024).
The extreme values for the OCR0A Register represents special cases when generating a PWM waveform output in
the fast PWM mode. If the OCR0A is set equal to BOTTOM, the output will be a narrow spike for each MAX+1
timer clock cycle. Setting the OCR0A equal to MAX will result in a constantly high or low output (depending on the
polarity of the output set by the COM0A[1:0] bits.)
A frequency (with 50% duty cycle) waveform output in fast PWM mode can be achieved by setting OC0x to toggle
its logical level on each Compare Match (COM0x[1:0] = 1). The waveform generated will have a maximum fre-
quency of fOC0 = fclk_I/O/2 when OCR0A is set to zero. This feature is similar to the OC0A toggle in CTC mode,
except the double buffer feature of the Output Compare unit is enabled in the fast PWM mode.
11.7.4 Phase Correct PWM Mode
The phase correct PWM mode (WGM0[2:0] = 1 or 5) provides a high resolution phase correct PWM waveform gen-
eration option. The phase correct PWM mode is based on a dual-slope operation. The counter counts repeatedly
from BOTTOM to TOP and then from TOP to BOTTOM. TOP is defined as 0xFF when WGM0[2:0] = 1, and
OCR0A when WGM0[2:0] = 5. In non-inverting Compare Output mode, the Output Compare (OC0x) is cleared on
the Compare Match between TCNT0 and OCR0x while upcounting, and set on the Compare Match while down-
counting. In inverting Output Compare mode, the operation is inverted. The dual-slope operation has lower maxi-
mum operation frequency than single slope operation. However, due to the symmetric feature of the dual-slope
PWM modes, these modes are preferred for motor control applications.
In phase correct PWM mode the counter is incremented until the counter value matches TOP. When the counter
reaches TOP, it changes the count direction. The TCNT0 value will be equal to TOP for one timer clock cycle. The
timing diagram for the phase correct PWM mode is shown on Figure 11-9. The TCNT0 value is in the timing dia-
gram shown as a histogram for illustrating the dual-slope operation. The diagram includes non-inverted and
inverted PWM outputs. The small horizontal line marks on the TCNT0 slopes represent Compare Matches
between OCR0x and TCNT0.
fOCnxPWM
fclk_I/O
N 256
------------------=
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Figure 11-9. Phase Correct PWM Mode, Timing Diagram
The Timer/Counter Overflow Flag (TOV0) is set each time the counter reaches BOTTOM. The Interrupt Flag can
be used to generate an interrupt each time the counter reaches the BOTTOM value.
In phase correct PWM mode, the compare unit allows generation of PWM waveforms on the OC0x pins. Setting
the COM0x[1:0] bits to two will produce a non-inverted PWM. An inverted PWM output can be generated by setting
the COM0x[1:0] to three: Setting the COM0A0 bits to one allows the OC0A pin to toggle on Compare Matches if
the WGM02 bit is set. This option is not available for the OC0B pin (See Table 11-4 on page 78). The actual OC0x
value will only be visible on the port pin if the data direction for the port pin is set as output. The PWM waveform is
generated by clearing (or setting) the OC0x Register at the Compare Match between OCR0x and TCNT0 when the
counter increments, and setting (or clearing) the OC0x Register at Compare Match between OCR0x and TCNT0
when the counter decrements. The PWM frequency for the output when using phase correct PWM can be calcu-
lated by the following equation:
The N variable represents the prescale factor (1, 8, 64, 256, or 1024).
The extreme values for the OCR0A Register represent special cases when generating a PWM waveform output in
the phase correct PWM mode. If the OCR0A is set equal to BOTTOM, the output will be continuously low and if set
equal to MAX the output will be continuously high for non-inverted PWM mode. For inverted PWM the output will
have the opposite logic values.
At the very start of period 2 in Figure 11-9 OCn has a transition from high to low even though there is no Compare
Match. The point of this transition is to guaratee symmetry around BOTTOM. There are two cases that give a tran-
sition without Compare Match, as follows:
OCR0A changes its value from MAX, like in Figure 11-9. When the OCR0A value is MAX the OCn pin value is
the same as the result of a down-counting Compare Match. To ensure symmetry around BOTTOM the OCn
value at MAX must correspond to the result of an up-counting Compare Match.
TOVn Interrupt Flag Set
OCnx Interrupt Flag Set
1 2 3
TCNTn
Period
OCn
OCn
(COMnx[1:0] = 2)
(COMnx[1:0] = 3)
OCRnx Update
fOCnxPCPWM
fclk_I/O
N510
------------------=
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The timer starts counting from a value higher than the one in OCR0A, and for that reason misses the Compare
Match and hence the OCn change that would have happened on the way up.
11.8 Timer/Counter Timing Diagrams
The Timer/Counter is a synchronous design and the timer clock (clkT0) is therefore shown as a clock enable signal
in the following figures. The figures include information on when Interrupt Flags are set. Figure 11-10 contains tim-
ing data for basic Timer/Counter operation. The figure shows the count sequence close to the MAX value in all
modes other than phase correct PWM mode.
Figure 11-10. Timer/Counter Timing Diagram, no Prescaling
Figure 11-11 shows the same timing data, but with the prescaler enabled.
Figure 11-11. Timer/Counter Timing Diagram, with Prescaler (fclk_I/O/8)
Figure 11-12 shows the setting of OCF0B in all modes and OCF0A in all modes except CTC mode and PWM
mode, where OCR0A is TOP.
Figure 11-12. Timer/Counter Timing Diagram, Setting of OCF0x, with Prescaler (fclk_I/O/8)
clkTn
(clk
I/O
/1)
TOVn
clkI/O
TCNTn MAX - 1 MAX BOTTOM BOTTOM + 1
TOVn
TCNTn MAX - 1 MAX BOTTOM BOTTOM + 1
clkI/O
clkTn
(clk
I/O
/8)
OCFnx
OCRnx
TCNTn
OCRnx Value
OCRnx - 1 OCRnx OCRnx + 1 OCRnx + 2
clk
I/O
clk
Tn
(clk
I/O
/8)
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Figure 11-13 shows the setting of OCF0A and the clearing of TCNT0 in CTC mode and fast PWM mode where
OCR0A is TOP.
Figure 11-13. Timer/Counter Timing Diagram, Clear Timer on Compare Match mode, with Prescaler (fclk_I/O/8)
11.9 Register Description
11.9.1 GTCCR – General Timer/Counter Control Register
Bit 7 – TSM: Timer/Counter Synchronization Mode
Writing the TSM bit to one activates the Timer/Counter Synchronization Mode. In this mode, the value written to
PSR0 is kept, hence keeping the Prescaler Reset signal asserted. This ensures that the timer/counter is halted and
can be configured without the risk of advancing during configuration. When the TSM bit is written to zero, the PSR0
bit is cleared by hardware, and the timer/counter start counting.
Bit 0 – PSR0: Prescaler Reset Timer/Counter0
When this bit is one, the Timer/Counter0 prescaler will be Reset. This bit is normally cleared immediately by hard-
ware, except if the TSM bit is set.
11.9.2 TCCR0A – Timer/Counter Control Register A
Bits 7:6 – COM0A[1:0]: Compare Match Output A Mode
Bits 5:4 – COM0B[1:0]: Compare Match Output B Mode
The COM0A[1:0] and COM0B[1:0] bits control the behaviour of Output Compare pins OC0A and OC0B, respec-
tively. If any of the COM0A[1:0] bits are set, the OC0A output overrides the normal port functionality of the I/O pin it
is connected to. Similarly, if any of the COM0B[1:0] bits are set, the OC0B output overrides the normal port func-
tionality of the I/O pin it is connected to. However, note that the Data Direction Register (DDR) bit corresponding to
the OC0A and OC0B pins must be set in order to enable the output driver.
OCFnx
OCRnx
TCNTn
(CTC)
TOP
TOP - 1 TOP BOTTOM BOTTOM + 1
clkI/O
clkTn
(clk
I/O
/8)
Bit 7 6 5 4 3 2 1 0
0x2C TSM PWM1B COM1B1 COM1B0 FOC1B FOC1A PSR1 PSR0 GTCCR
Read/Write R/W R R R R R R R/W
Initial Value 0 0 0 0 0 0 0 0
Bit 7 6 5 4 3 210
0x2A COM0A1COM0A0COM0B1COM0B0––WGM01 WGM00 TCCR0A
Read/Write R/W R/W R/W R/W R R R/W R/W
Initial Value 0 0 0 0 0 0 0 0
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When OC0A/OC0B is connected to the I/O pin, the function of the COM0A[1:0]/COM0B[1:0] bits depend on the
WGM0[2:0] bit setting. Table 11-2 shows the COM0x[1:0] bit functionality when the WGM0[2:0] bits are set to a
normal or CTC mode (non-PWM).
Table 11-3 shows the COM0x[1:0] bit functionality when the WGM0[2:0] bits are set to fast PWM mode.
Note: 1. A special case occurs when OCR0A or OCR0B equals TOP and COM0A1/COM0B1 is set. In this case, the com-
pare match is ignored, but the set or clear is done at BOTTOM. See “Fast PWM Mode” on page 73 for more details.
Table 11-4 shows the COM0x[1:0] bit functionality when the WGM0[2:0] bits are set to phase correct PWM mode.
Note: 1. A special case occurs when OCR0A or OCR0B equals TOP and COM0A1/COM0B1 is set. In this case, the Com-
pare Match is ignored, but the set or clear is done at TOP. See “Phase Correct PWM Mode” on page 74 for more
details.
Bits 3:2 – Res: Reserved Bits
These bits are reserved bits in the ATtiny25/45/85 and will always read as zero.
Table 11-2. Compare Output Mode, non-PWM Mode
COM0A1
COM0B1 COM0A0
COM0B0 Description
0 0 Normal port operation, OC0A/OC0B disconnected.
0 1 Toggle OC0A/OC0B on Compare Match
1 0 Clear OC0A/OC0B on Compare Match
1 1 Set OC0A/OC0B on Compare Match
Table 11-3. Compare Output Mode, Fast PWM Mode(1)
COM0A1
COM0B1 COM0A0
COM0B0 Description
0 0 Normal port operation, OC0A/OC0B disconnected.
01Reserved
10
Clear OC0A/OC0B on Compare Match, set OC0A/OC0B at BOTTOM
(non-inverting mode)
11
Set OC0A/OC0B on Compare Match, clear OC0A/OC0B at BOTTOM
(inverting mode)
Table 11-4. Compare Output Mode, Phase Correct PWM Mode(1)
COM0A1
COM0B1 COM0A0
COM0B0 Description
0 0 Normal port operation, OC0A/OC0B disconnected.
01Reserved
10
Clear OC0A/OC0B on Compare Match when up-counting.
Set OC0A/OC0B on Compare Match when down-counting.
11
Set OC0A/OC0B on Compare Match when up-counting.
Clear OC0A/OC0B on Compare Match when down-counting.
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Bits 1:0 – WGM0[1:0]: Waveform Generation Mode
Combined with the WGM02 bit found in the TCCR0B Register, these bits control the counting sequence of the
counter, the source for maximum (TOP) counter value, and what type of waveform generation to be used, see
Table 11-5. Modes of operation supported by the Timer/Counter unit are: Normal mode (counter), Clear Timer on
Compare Match (CTC) mode, and two types of Pulse Width Modulation (PWM) modes (see “Modes of Operation”
on page 71).
Notes: 1. MAX = 0xFF
2. BOTTOM = 0x00
11.9.3 TCCR0B – Timer/Counter Control Register B
Bit 7 – FOC0A: Force Output Compare A
The FOC0A bit is only active when the WGM bits specify a non-PWM mode.
However, for ensuring compatibility with future devices, this bit must be set to zero when TCCR0B is written when
operating in PWM mode. When writing a logical one to the FOC0A bit, an immediate Compare Match is forced on
the Waveform Generation unit. The OC0A output is changed according to its COM0A[1:0] bits setting. Note that
the FOC0A bit is implemented as a strobe. Therefore it is the value present in the COM0A[1:0] bits that determines
the effect of the forced compare.
A FOC0A strobe will not generate any interrupt, nor will it clear the timer in CTC mode using OCR0A as TOP.
The FOC0A bit is always read as zero.
Bit 6 – FOC0B: Force Output Compare B
The FOC0B bit is only active when the WGM bits specify a non-PWM mode.
However, for ensuring compatibility with future devices, this bit must be set to zero when TCCR0B is written when
operating in PWM mode. When writing a logical one to the FOC0B bit, an immediate Compare Match is forced on
the Waveform Generation unit. The OC0B output is changed according to its COM0B[1:0] bits setting. Note that
the FOC0B bit is implemented as a strobe. Therefore it is the value present in the COM0B[1:0] bits that determines
the effect of the forced compare.
A FOC0B strobe will not generate any interrupt, nor will it clear the timer in CTC mode using OCR0B as TOP.
Table 11-5. Waveform Generation Mode Bit Description
Mode WGM
02 WGM
01 WGM
00 Timer/Counter Mode
of Operation TOP Update of
OCRx at TOV Flag
Set on
0 0 0 0 Normal 0xFF Immediate MAX(1)
1 0 0 1 PWM, Phase Correct 0xFF TOP BOTTOM(2)
2 0 1 0 CTC OCRA Immediate MAX(1)
3 0 1 1 Fast PWM 0xFF BOTTOM(2) MAX(1)
4 1 0 0 Reserved
5 1 0 1 PWM, Phase Correct OCRA TOP BOTTOM(2)
6 1 1 0 Reserved
7 1 1 1 Fast PWM OCRA BOTTOM(2) TOP
Bit 7 6 5 4 3 2 1 0
0x33 FOC0A FOC0B WGM02 CS02 CS01 CS00 TCCR0B
Read/Write W W R R R/W R/W R/W R/W
Initial Value 0 0 0 0 0 0 0 0
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The FOC0B bit is always read as zero.
Bits 5:4 – Res: Reserved Bits
These bits are reserved bits in the ATtiny25/45/85 and will always read as zero.
Bit 3 – WGM02: Waveform Generation Mode
See the description in the “TCCR0A – Timer/Counter Control Register A” on page 77.
Bits 2:0 – CS0[2:0]: Clock Select
The three Clock Select bits select the clock source to be used by the Timer/Counter.
If external pin modes are used for the Timer/Counter0, transitions on the T0 pin will clock the counter even if the
pin is configured as an output. This feature allows software control of the counting.
11.9.4 TCNT0 – Timer/Counter Register
The Timer/Counter Register gives direct access, both for read and write operations, to the Timer/Counter unit 8-bit
counter. Writing to the TCNT0 Register blocks (removes) the Compare Match on the following timer clock. Modify-
ing the counter (TCNT0) while the counter is running, introduces a risk of missing a Compare Match between
TCNT0 and the OCR0x Registers.
11.9.5 OCR0A – Output Compare Register A
The Output Compare Register A contains an 8-bit value that is continuously compared with the counter value
(TCNT0). A match can be used to generate an Output Compare interrupt, or to generate a waveform output on the
OC0A pin.
Table 11-6. Clock Select Bit Description
CS02 CS01 CS00 Description
0 0 0 No clock source (Timer/Counter stopped)
001clk
I/O/(No prescaling)
010clk
I/O/8 (From prescaler)
011clk
I/O/64 (From prescaler)
100clk
I/O/256 (From prescaler)
101clk
I/O/1024 (From prescaler)
1 1 0 External clock source on T0 pin. Clock on falling edge.
1 1 1 External clock source on T0 pin. Clock on rising edge.
Bit 76543210
0x32 TCNT0[7:0] TCNT0
Read/Write R/W R/W R/W R/W R/W R/W R/W R/W
Initial Value00000000
Bit 76543210
0x29 OCR0A[7:0] OCR0A
Read/Write R/W R/W R/W R/W R/W R/W R/W R/W
Initial Value00000000
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11.9.6 OCR0B – Output Compare Register B
The Output Compare Register B contains an 8-bit value that is continuously compared with the counter value
(TCNT0). A match can be used to generate an Output Compare interrupt, or to generate a waveform output on the
OC0B pin.
11.9.7 TIMSK – Timer/Counter Interrupt Mask Register
Bits 7, 0 – Res: Reserved Bits
These bits are reserved bits and will always read as zero.
Bit 4 – OCIE0A: Timer/Counter0 Output Compare Match A Interrupt Enable
When the OCIE0A bit is written to one, and the I-bit in the Status Register is set, the Timer/Counter0 Compare
Match A interrupt is enabled. The corresponding interrupt is executed if a Compare Match in Timer/Counter0
occurs, i.e., when the OCF0A bit is set in the Timer/Counter 0 Interrupt Flag Register – TIFR0.
Bit 3 – OCIE0B: Timer/Counter Output Compare Match B Interrupt Enable
When the OCIE0B bit is written to one, and the I-bit in the Status Register is set, the Timer/Counter Compare
Match B interrupt is enabled. The corresponding interrupt is executed if a Compare Match in Timer/Counter occurs,
i.e., when the OCF0B bit is set in the Timer/Counter Interrupt Flag Register – TIFR0.
Bit 1 – TOIE0: Timer/Counter0 Overflow Interrupt Enable
When the TOIE0 bit is written to one, and the I-bit in the Status Register is set, the Timer/Counter0 Overflow inter-
rupt is enabled. The corresponding interrupt is executed if an overflow in Timer/Counter0 occurs, i.e., when the
TOV0 bit is set in the Timer/Counter 0 Interrupt Flag Register – TIFR0.
11.9.8 TIFR – Timer/Counter Interrupt Flag Register
Bits 7, 0 – Res: Reserved Bits
These bits are reserved bits and will always read as zero.
Bit 4 – OCF0A: Output Compare Flag 0 A
The OCF0A bit is set when a Compare Match occurs between the Timer/Counter0 and the data in OCR0A – Out-
put Compare Register0. OCF0A is cleared by hardware when executing the corresponding interrupt handling
vector. Alternatively, OCF0A is cleared by writing a logic one to the flag. When the I-bit in SREG, OCIE0A
(Timer/Counter0 Compare Match Interrupt Enable), and OCF0A are set, the Timer/Counter0 Compare Match Inter-
rupt is executed.
Bit 76543210
0x28 OCR0B[7:0] OCR0B
Read/Write R/W R/W R/W R/W R/W R/W R/W R/W
Initial Value00000000
Bit 76543210
0x39 OCIE1A OCIE1B OCIE0A OCIE0B TOIE1 TOIE0 TIMSK
Read/Write R R/W R/W R/W R/W R/W R/W R
Initial Value 0 0 0 0 0 0 0 0
Bit 76543210
0x38 OCF1A OCF1B OCF0A OCF0B TOV1 TOV0 –TIFR
Read/Write R R/W R/W R/W R/W R/W R/W R
Initial Value00000000
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Bit 3 – OCF0B: Output Compare Flag 0 B
The OCF0B bit is set when a Compare Match occurs between the Timer/Counter and the data in OCR0B – Output
Compare Register0 B. OCF0B is cleared by hardware when executing the corresponding interrupt handling vector.
Alternatively, OCF0B is cleared by writing a logic one to the flag. When the I-bit in SREG, OCIE0B (Timer/Counter
Compare B Match Interrupt Enable), and OCF0B are set, the Timer/Counter Compare Match Interrupt is executed.
Bit 1 – TOV0: Timer/Counter0 Overflow Flag
The bit TOV0 is set when an overflow occurs in Timer/Counter0. TOV0 is cleared by hardware when executing the
corresponding interrupt handling vector. Alternatively, TOV0 is cleared by writing a logic one to the flag. When the
SREG I-bit, TOIE0 (Timer/Counter0 Overflow Interrupt Enable), and TOV0 are set, the Timer/Counter0 Overflow
interrupt is executed.
The setting of this flag is dependent of the WGM0[2:0] bit setting. Refer to Table 11-5, “Waveform Generation
Mode Bit Description” on page 79.
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12. 8-bit Timer/Counter1
The Timer/Counter1 is a general purpose 8-bit Timer/Counter module that has a separate prescaling selection
from the separate prescaler.
12.1 Timer/Counter1 Prescaler
Figure 12-1 shows the Timer/Counter1 prescaler that supports two clocking modes, a synchronous clocking mode
and an asynchronous clocking mode. The synchronous clocking mode uses the system clock (CK) as the clock
timebase and asynchronous mode uses the fast peripheral clock (PCK) as the clock time base. The PCKE bit from
the PLLCSR register enables the asynchronous mode when it is set (‘1’).
Figure 12-1. Timer/Counter1 Prescaler
In the asynchronous clocking mode the clock selections are from PCK to PCK/16384 and stop, and in the synchro-
nous clocking mode the clock selections are from CK to CK/16384 and stop. The clock options are described in
Table 12-5 on page 89 and the Timer/Counter1 Control Register, TCCR1. Setting the PSR1 bit in GTCCR register
resets the prescaler. The PCKE bit in the PLLCSR register enables the asynchronous mode. The frequency of the
fast peripheral clock is 64 MHz (or 32 MHz in Low Speed Mode).
12.2 Counter and Compare Units
The Timer/Counter1 general operation is described in the asynchronous mode and the operation in the synchro-
nous mode is mentioned only if there are differences between these two modes. Figure 12-2 shows Timer/Counter
1 synchronization register block diagram and synchronization delays in between registers. Note that all clock gat-
ing details are not shown in the figure. The Timer/Counter1 register values go through the internal synchronization
registers, which cause the input synchronization delay, before affecting the counter operation. The registers
TCCR1, GTCCR, OCR1A, OCR1B, and OCR1C can be read back right after writing the register. The read back
values are delayed for the Timer/Counter1 (TCNT1) register and flags (OCF1A, OCF1B, and TOV1), because of
the input and output synchronization.
The Timer/Counter1 features a high resolution and a high accuracy usage with the lower prescaling opportunities.
It can also support two accurate, high speed, 8-bit Pulse Width Modulators using clock speeds up to 64 MHz (or 32
MHz in Low Speed Mode). In this mode, Timer/Counter1 and the output compare registers serve as dual stand-
alone PWMs with non-overlapping non-inverted and inverted outputs. Refer to page 86 for a detailed description
on this function. Similarly, the high prescaling opportunities make this unit useful for lower speed functions or exact
timing functions with infrequent actions.
TIMER/COUNTER1 COUNT ENABLE
PSR1
CS10
CS11
CS12
PCK 64/32 MHz
0
CS13
14-BIT
T/C PRESCALER
T1CK/2
T1CK
T1CK/4
T1CK/8
T1CK/16
T1CK/32
T1CK/64
T1CK/128
T1CK/256
T1CK/512
T1CK/1024
T1CK/2048
T1CK/4096
T1CK/8192
T1CK/16384
CK
PCKE
T1CK
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Figure 12-2. Timer/Counter 1 Synchronization Register Block Diagram.
Timer/Counter1 and the prescaler allow running the CPU from any clock source while the prescaler is operating on
the fast 64 MHz (or 32 MHz in Low Speed Mode) PCK clock in the asynchronous mode.
Note that the system clock frequency must be lower than one third of the PCK frequency. The synchronization
mechanism of the asynchronous Timer/Counter1 needs at least two edges of the PCK when the system clock is
high. If the frequency of the system clock is too high, it is a risk that data or control values are lost.
The following Figure 12-3 shows the block diagram for Timer/Counter1.
8-BIT DATABUS
OCR1A OCR1A_SI
TCNT_SO
OCR1B OCR1B_SI
OCR1C OCR1C_SI
TCCR1 TCCR1_SI
GTCCR GTCCR_SI
TCNT1 TCNT1_SI
OCF1A OCF1A_SI
OCF1B OCF1B_SI
TOV1 TOV1_SI TOV1_SO
OCF1B_SO
OCF1A_SO
TCNT1
S
AS
A
PCKE
CK
PCK
IO-registers Input synchronization
registers
Timer/Counter1 Output synchronization
registers
SYNC
MODE
ASYNC
MODE
1 CK Delay
~1 CK Delay1 PCK Delay No Delay
TCNT1
OCF1A
OCF1B
TOV1
1/2 CK Delay 1 CK Delay 1/2 CK Delay
1..2 PCK Delay
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Figure 12-3. Timer/Counter1 Block Diagram
Three status flags (overflow and compare matches) are found in the Timer/Counter Interrupt Flag Register - TIFR.
Control signals are found in the Timer/Counter Control Registers TCCR1 and GTCCR. The interrupt enable/dis-
able settings are found in the Timer/Counter Interrupt Mask Register - TIMSK.
The Timer/Counter1 contains three Output Compare Registers, OCR1A, OCR1B, and OCR1C as the data source
to be compared with the Timer/Counter1 contents. In normal mode the Output Compare functions are operational
with all three output compare registers. OCR1A determines action on the OC1A pin (PB1), and it can generate
Timer1 OC1A interrupt in normal mode and in PWM mode. Likewise, OCR1B determines action on the OC1B pin
(PB4) and it can generate Timer1 OC1B interrupt in normal mode and in PWM mode. OCR1C holds the
Timer/Counter maximum value, i.e. the clear on compare match value. In the normal mode an overflow interrupt
(TOV1) is generated when Timer/Counter1 counts from $FF to $00, while in the PWM mode the overflow interrupt
is generated when Timer/Counter1 counts either from $FF to $00 or from OCR1C to $00. The inverted PWM out-
puts OC1A and OC1B are not connected in normal mode.
In PWM mode, OCR1A and OCR1B provide the data values against which the Timer Counter value is compared.
Upon compare match the PWM outputs (OC1A, OC1A, OC1B, OC1B) are generated. In PWM mode, the Timer
Counter counts up to the value specified in the output compare register OCR1C and starts again from $00. This
feature allows limiting the counter “full” value to a specified value, lower than $FF. Together with the many pres-
caler options, flexible PWM frequency selection is provided. Table 12-3 on page 88 lists clock selection and
OCR1C values to obtain PWM frequencies from 20 kHz to 250 kHz in 10 kHz steps and from 250 kHz to 500 kHz
in 50 kHz steps. Higher PWM frequencies can be obtained at the expense of resolution.
8-BIT DATABUS
TIMER INT. FLAG
REGISTER (TIFR)
TIMER/COUNTER1
8-BIT COMPARATOR
T/C1 OUTPUT
COMPARE REGISTER
TIMER INT. MASK
REGISTER (TIMSK)
TIMER/COUNTER1
(TCNT1)
T/C CLEAR
T/C1 CONTROL
LOGIC
TOV1
OCF1B OCF1B
TOV1
TOIE0
TOIE1
OCIE1B
OCIE1A
OCF1A
OCF1A
CK
PCK
T/C1 OVER-
FLOW IRQ
T/C1 COMPARE
MATCH B IRQ
OC1A
(PB1)
T/C1 COMPARE
MATCH A IRQ
T/C CONTROL
REGISTER 1 (TCCR1)
COM1B1
PWM1A
PWM1B
COM1B0
FOC1A
FOC1B
(OCR1A) (OCR1B) (OCR1C)
8-BIT COMPARATOR
T/C1 OUTPUT
COMPARE REGISTER
TOV0
COM1A1
COM1A0
8-BIT COMPARATOR
T/C1 OUTPUT
COMPARE REGISTER
GLOBAL T/C CONTROL
REGISTER (GTCCR)
CS12
PSR1
CS11
CS10
CS13
CTC1
OC1A
(PB0)
OC1B
(PB4)
OC1B
(PB3)
DEAD TIME GENERATOR DEAD TIME GENERATOR
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12.2.1 Timer/Counter1 Initialization for Asynchronous Mode
To set Timer/Counter1 in asynchronous mode first enable PLL and then wait 100 µs for PLL to stabilize. Next, poll
the PLOCK bit until it is set and then set the PCKE bit.
12.2.2 Timer/Counter1 in PWM Mode
When the PWM mode is selected, Timer/Counter1 and the Output Compare Register C - OCR1C form a dual 8-bit,
free-running and glitch-free PWM generator with outputs on the PB1(OC1A) and PB4(OC1B) pins and inverted
outputs on pins PB0(OC1A) and PB3(OC1B). As default non-overlapping times for complementary output pairs are
zero, but they can be inserted using a Dead Time Generator (see description on page 100).
Figure 12-4. The PWM Output Pair
When the counter value match the contents of OCR1A or OCR1B, the OC1A and OC1B outputs are set or cleared
according to the COM1A1/COM1A0 or COM1B1/COM1B0 bits in the Timer/Counter1 Control Register A - TCCR1,
as shown in Table 12-1.
Timer/Counter1 acts as an up-counter, counting from $00 up to the value specified in the output compare register
OCR1C, and starting from $00 up again. A compare match with OC1C will set an overflow interrupt flag (TOV1)
after a synchronization delay following the compare event.
Note that in PWM mode, writing to the Output Compare Registers OCR1A or OCR1B, the data value is first trans-
ferred to a temporary location. The value is latched into OCR1A or OCR1B when the Timer/Counter reaches
OCR1C. This prevents the occurrence of odd-length PWM pulses (glitches) in the event of an unsynchronized
OCR1A or OCR1B. See Figure 12-5 for an example.
Table 12-1. Compare Mode Select in PWM Mode
COM1x1 COM1x0 Effect on Output Compare Pins
00
OC1x not connected.
OC1x not connected.
01
OC1x cleared on compare match. Set whenTCNT1 = $00.
OC1x set on compare match. Cleared when TCNT1 = $00.
10
OC1x cleared on compare match. Set when TCNT1 = $00.
OC1x not connected.
11
OC1x Set on compare match. Cleared when TCNT1= $00.
OC1x not connected.
PWM1x
PWM1x
x = A or B
t
non-overlap
=0
t
non-overlap
=0
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Figure 12-5. Effects of Unsynchronized OCR Latching
During the time between the write and the latch operation, a read from OCR1A or OCR1B will read the contents of
the temporary location. This means that the most recently written value always will read out of OCR1A or OCR1B.
When OCR1A or OCR1B contain $00 or the top value, as specified in OCR1C register, the output PB1(OC1A) or
PB4(OC1B) is held low or high according to the settings of COM1A1/COM1A0. This is shown in Table 12-2.
In PWM mode, the Timer Overflow Flag - TOV1 is set when the TCNT1 counts to the OCR1C value and the
TCNT1 is reset to $00. The Timer Overflow Interrupt1 is executed when TOV1 is set provided that Timer Overflow
Interrupt and global interrupts are enabled. This also applies to the Timer Output Compare flags and interrupts.
The frequency of the PWM will be Timer Clock 1 Frequency divided by (OCR1C value + 1). See the following
equation:
Resolution shows how many bits are required to express the value in the OCR1C register and can be calculated
using the following equation:
Table 12-2. PWM Outputs OCR1x = $00 or OCR1C, x = A or B
COM1x1 COM1x0 OCR1x Output OC1x Output OC1x
0 1 $00 L H
0 1 OCR1C H L
1 0 $00 L Not connected.
1 0 OCR1C H Not connected.
1 1 $00 H Not connected.
1 1 OCR1C L Not connected.
PWM Output OC1x
PWM Output OC1x
Unsynchronized OC1x Latch
Synchronized OC1x Latch
Counter Value
Compare Value
Counter Value
Compare Value
Compare Value changes
Glitch
Compare Value changes
fPWM
fTCK1
OCR1C + 1
------------------------------------=
R2OCR1C 1+()log=
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Table 12-3. Timer/Counter1 Clock Prescale Select in the Asynchronous Mode
PWM Frequency Clock Selection CS1[3:0] OCR1C RESOLUTION
20 kHz PCK/16 0101 199 7.6
30 kHz PCK/16 0101 132 7.1
40 kHz PCK/8 0100 199 7.6
50 kHz PCK/8 0100 159 7.3
60 kHz PCK/8 0100 132 7.1
70 kHz PCK/4 0011 228 7.8
80 kHz PCK/4 0011 199 7.6
90 kHz PCK/4 0011 177 7.5
100 kHz PCK/4 0011 159 7.3
110 kHz PCK/4 0011 144 7.2
120 kHz PCK/4 0011 132 7.1
130 kHz PCK/2 0010 245 7.9
140 kHz PCK/2 0010 228 7.8
150 kHz PCK/2 0010 212 7.7
160 kHz PCK/2 0010 199 7.6
170 kHz PCK/2 0010 187 7.6
180 kHz PCK/2 0010 177 7.5
190 kHz PCK/2 0010 167 7.4
200 kHz PCK/2 0010 159 7.3
250 kHz PCK 0001 255 8.0
300 kHz PCK 0001 212 7.7
350 kHz PCK 0001 182 7.5
400 kHz PCK 0001 159 7.3
450 kHz PCK 0001 141 7.1
500 kHz PCK 0001 127 7.0
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12.3 Register Description
12.3.1 TCCR1 – Timer/Counter1 Control Register
Bit 7 – CTC1 : Clear Timer/Counter on Compare Match
When the CTC1 control bit is set (one), Timer/Counter1 is reset to $00 in the CPU clock cycle after a compare
match with OCR1C register value. If the control bit is cleared, Timer/Counter1 continues counting and is unaffected
by a compare match.
Bit 6 – PWM1A: Pulse Width Modulator A Enable
When set (one) this bit enables PWM mode based on comparator OCR1A in Timer/Counter1 and the counter
value is reset to $00 in the CPU clock cycle after a compare match with OCR1C register value.
Bits 5:4 – COM1A[1:0]: Comparator A Output Mode, Bits 1 and 0
The COM1A1 and COM1A0 control bits determine any output pin action following a compare match with compare
register A in Timer/Counter1. Since the output pin action is an alternative function to an I/O port, the corresponding
direction control bit must be set (one) in order to control an output pin.
In Normal mode, the COM1A1 and COM1A0 control bits determine the output pin actions that affect pin PB1
(OC1A) as described in Table 12-4. Note that OC1A is not connected in normal mode.
In PWM mode, these bits have different functions. Refer to Table 12-1 on page 86 for a detailed description.
Bits 3:0 - CS1[3:0]: Clock Select Bits 3, 2, 1, and 0
The Clock Select bits 3, 2, 1, and 0 define the prescaling source of Timer/Counter1.
Bit 7 6 5 4 3 2 1 0
0x30 CTC1 PWM1A COM1A1 COM1A0 CS13 CS12 CS11 CS10 TCCR1
Read/Write R/W R/W R/W R/W R/W R/W R/W R/W
Initial value 0 0 0 0 0 0 0 0
Table 12-4. Comparator A Mode Select in Normal Mode
COM1A1 COM1A0 Description
0 0 Timer/Counter Comparator A disconnected from output pin OC1A.
0 1 Toggle the OC1A output line.
1 0 Clear the OC1A output line.
1 1 Set the OC1A output line
Table 12-5. Timer/Counter1 Prescale Select
CS13 CS12 CS11 CS10 Asynchronous
Clocking Mode
Synchronous
Clocking Mode
0 0 0 0 T/C1 stopped T/C1 stopped
0001PCK CK
0 0 1 0 PCK/2 CK/2
0 0 1 1 PCK/4 CK/4
0 1 0 0 PCK/8 CK/8
0 1 0 1 PCK/16 CK/16
0 1 1 0 PCK/32 CK/32
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The Stop condition provides a Timer Enable/Disable function.
12.3.2 GTCCR – General Timer/Counter1 Control Register
Bit 6 – PWM1B: Pulse Width Modulator B Enable
When set (one) this bit enables PWM mode based on comparator OCR1B in Timer/Counter1 and the counter
value is reset to $00 in the CPU clock cycle after a compare match with OCR1C register value.
Bits 5:4 – COM1B[1:0]: Comparator B Output Mode, Bits 1 and 0
The COM1B1 and COM1B0 control bits determine any output pin action following a compare match with compare
register B in Timer/Counter1. Since the output pin action is an alternative function to an I/O port, the corresponding
direction control bit must be set (one) in order to control an output pin.
In Normal mode, the COM1B1 and COM1B0 control bits determine the output pin actions that affect pin PB4
(OC1B) as described in Table 12-6. Note that OC1B is not connected in normal mode.
In PWM mode, these bits have different functions. Refer to Table 12-1 on page 86 for a detailed description.
Bit 3 – FOC1B: Force Output Compare Match 1B
Writing a logical one to this bit forces a change in the compare match output pin PB4 (OC1B) according to the val-
ues already set in COM1B1 and COM1B0. If COM1B1 and COM1B0 written in the same cycle as FOC1B, the new
settings will be used. The Force Output Compare bit can be used to change the output pin value regardless of the
timer value. The automatic action programmed in COM1B1 and COM1B0 takes place as if a compare match had
0 1 1 1 PCK/64 CK/64
1 0 0 0 PCK/128 CK/128
1 0 0 1 PCK/256 CK/256
1 0 1 0 PCK/512 CK/512
1 0 1 1 PCK/1024 CK/1024
1 1 0 0 PCK/2048 CK/2048
1 1 0 1 PCK/4096 CK/4096
1 1 1 0 PCK/8192 CK/8192
1 1 1 1 PCK/16384 CK/16384
Table 12-5. Timer/Counter1 Prescale Select (Continued)
CS13 CS12 CS11 CS10 Asynchronous
Clocking Mode
Synchronous
Clocking Mode
Bit 7 6 5 4 3 2 1 0
0x2C TSM PWM1B COM1B1 COM1B0 FOC1B FOC1A PSR1 PSR0 GTCCR
Read/Write R/W R/W R/W R/W W W R/W R/W
Initial value 0 0 0 0 0 0 0 0
Table 12-6. Comparator B Mode Select in Normal Mode
COM1B1 COM1B0 Description
0 0 Timer/Counter Comparator B disconnected from output pin OC1B.
0 1 Toggle the OC1B output line.
1 0 Clear the OC1B output line.
1 1 Set the OC1B output line
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occurred, but no interrupt is generated. The FOC1B bit always reads as zero. FOC1B is not in use if PWM1B bit is
set.
Bit 2 – FOC1A: Force Output Compare Match 1A
Writing a logical one to this bit forces a change in the compare match output pin PB1 (OC1A) according to the val-
ues already set in COM1A1 and COM1A0. If COM1A1 and COM1A0 written in the same cycle as FOC1A, the new
settings will be used. The Force Output Compare bit can be used to change the output pin value regardless of the
timer value. The automatic action programmed in COM1A1 and COM1A0 takes place as if a compare match had
occurred, but no interrupt is generated. The FOC1A bit always reads as zero. FOC1A is not in use if PWM1A bit is
set.
Bit 1 – PSR1 : Prescaler Reset Timer/Counter1
When this bit is set (one), the Timer/Counter prescaler (TCNT1 is unaffected) will be reset. The bit will be cleared
by hardware after the operation is performed. Writing a zero to this bit will have no effect. This bit will always read
as zero.
12.3.3 TCNT1 – Timer/Counter1
This 8-bit register contains the value of Timer/Counter1.
Timer/Counter1 is realized as an up counter with read and write access. Due to synchronization of the CPU,
Timer/Counter1 data written into Timer/Counter1 is delayed by one and half CPU clock cycles in synchronous
mode and at most one CPU clock cycles for asynchronous mode.
12.3.4 OCR1A –Timer/Counter1 Output Compare RegisterA
The output compare register A is an 8-bit read/write register.
The Timer/Counter Output Compare Register A contains data to be continuously compared with Timer/Counter1.
Actions on compare matches are specified in TCCR1. A compare match does only occur if Timer/Counter1 counts
to the OCR1A value. A software write that sets TCNT1 and OCR1A to the same value does not generate a com-
pare match.
A compare match will set the compare interrupt flag OCF1A after a synchronization delay following the compare
event.
12.3.5 OCR1B – Timer/Counter1 Output Compare RegisterB
The output compare register B is an 8-bit read/write register.
The Timer/Counter Output Compare Register B contains data to be continuously compared with Timer/Counter1.
Actions on compare matches are specified in TCCR1. A compare match does only occur if Timer/Counter1 counts
Bit 76543210
0x2F MSB LSB TCNT1
Read/Write R/W R/W R/W R/W R/W R/W R/W R/W
Initial value 0 0 0 0 0 0 0 0
Bit 76543210
0x2E MSB LSB OCR1A
Read/Write R/W R/W R/W R/W R/W R/W R/W R/W
Initial value 0 0 0 0 0 0 0 0
Bit 76543210
0x2B MSB LSB OCR1B
Read/Write R/W R/W R/W R/W R/W R/W R/W R/W
Initial value 0 0 0 0 0 0 0 0
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to the OCR1B value. A software write that sets TCNT1 and OCR1B to the same value does not generate a com-
pare match.
A compare match will set the compare interrupt flag OCF1B after a synchronization delay following the compare
event.
12.3.6 OCR1C – Timer/Counter1 Output Compare RegisterC
The output compare register C is an 8-bit read/write register.
The Timer/Counter Output Compare Register C contains data to be continuously compared with Timer/Counter1.
A compare match does only occur if Timer/Counter1 counts to the OCR1C value. A software write that sets TCNT1
and OCR1C to the same value does not generate a compare match. If the CTC1 bit in TCCR1 is set, a compare
match will clear TCNT1.
This register has the same function in normal mode and PWM mode.
12.3.7 TIMSK – Timer/Counter Interrupt Mask Register
Bit 7 – Res: Reserved Bit
This bit is a reserved bit in the ATtiny25/45/85 and always reads as zero.
Bit 6 – OCIE1A: Timer/Counter1 Output Compare Interrupt Enable
When the OCIE1A bit is set (one) and the I-bit in the Status Register is set (one), the Timer/Counter1 Compare
MatchA, interrupt is enabled. The corresponding interrupt at vector $003 is executed if a compare matchA occurs.
The Compare Flag in Timer/Counter1 is set (one) in the Timer/Counter Interrupt Flag Register.
Bit 5 – OCIE1B: Timer/Counter1 Output Compare Interrupt Enable
When the OCIE1B bit is set (one) and the I-bit in the Status Register is set (one), the Timer/Counter1 Compare
MatchB, interrupt is enabled. The corresponding interrupt at vector $009 is executed if a compare matchB occurs.
The Compare Flag in Timer/Counter1 is set (one) in the Timer/Counter Interrupt Flag Register.
Bit 2 – TOIE1: Timer/Counter1 Overflow Interrupt Enable
When the TOIE1 bit is set (one) and the I-bit in the Status Register is set (one), the Timer/Counter1 Overflow inter-
rupt is enabled. The corresponding interrupt (at vector $004) is executed if an overflow in Timer/Counter1 occurs.
The Overflow Flag (Timer1) is set (one) in the Timer/Counter Interrupt Flag Register - TIFR.
Bit 0 – Res: Reserved Bit
This bit is a reserved bit in the ATtiny25/45/85 and always reads as zero.
Bit 76543210
0x2D MSB LSB OCR1C
Read/Write R/W R/W R/W R/W R/W R/W R/W R/W
Initial value 1 1 1 1 1 1 1 1
Bit 7 6 5 4 3 2 1 0
0x39 OCIE1A OCIE1B OCIE0A OCIE0B TOIE1 TOIE0 – TIMSK
Read/Write R R/W R/W R/W R/W R/W R/W R
Initial value 0 0 0 0 0 0 0 0
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12.3.8 TIFR – Timer/Counter Interrupt Flag Register
Bit 7 – Res: Reserved Bit
This bit is a reserved bit in the ATtiny25/45/85 and always reads as zero.
Bit 6 – OCF1A: Output Compare Flag 1A
The OCF1A bit is set (one) when compare match occurs between Timer/Counter1 and the data value in OCR1A -
Output Compare Register 1A. OCF1A is cleared by hardware when executing the corresponding interrupt handling
vector. Alternatively, OCF1A is cleared, after synchronization clock cycle, by writing a logic one to the flag. When
the I-bit in SREG, OCIE1A, and OCF1A are set (one), the Timer/Counter1 A compare match interrupt is executed.
Bit 5 – OCF1B: Output Compare Flag 1B
The OCF1B bit is set (one) when compare match occurs between Timer/Counter1 and the data value in OCR1B -
Output Compare Register 1A. OCF1B is cleared by hardware when executing the corresponding interrupt handling
vector. Alternatively, OCF1B is cleared, after synchronization clock cycle, by writing a logic one to the flag. When
the I-bit in SREG, OCIE1B, and OCF1B are set (one), the Timer/Counter1 B compare match interrupt is executed.
Bit 2 – TOV1: Timer/Counter1 Overflow Flag
In normal mode (PWM1A=0 and PWM1B=0) the bit TOV1 is set (one) when an overflow occurs in Timer/Counter1.
The bit TOV1 is cleared by hardware when executing the corresponding interrupt handling vector. Alternatively,
TOV1 is cleared, after synchronization clock cycle, by writing a logical one to the flag.
In PWM mode (either PWM1A=1 or PWM1B=1) the bit TOV1 is set (one) when compare match occurs between
Timer/Counter1 and data value in OCR1C - Output Compare Register 1C.
When the SREG I-bit, and TOIE1 (Timer/Counter1 Overflow Interrupt Enable), and TOV1 are set (one), the
Timer/Counter1 Overflow interrupt is executed.
Bit 0 – Res: Reserved Bit
This bit is a reserved bit in the ATtiny25/45/85 and always reads as zero.
Bit 7 6 5 4 3 2 1 0
0x38 –OCF1AOCF1B
OCF0A OCF0B TOV1 TOV0 – TIFR
Read/Write R R/W R/W R/W R/W R/W R/W R
Initial value 0 0 0 0 0 0 0 0
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12.3.9 PLLCSR – PLL Control and Status Register
Bit 7 – LSM: Low Speed Mode
The high speed mode is enabled as default and the fast peripheral clock is 64 MHz, but the low speed mode can
be set by writing the LSM bit to one. Then the fast peripheral clock is scaled down to 32 MHz. The low speed mode
must be set, if the supply voltage is below 2.7 volts, because the Timer/Counter1 is not running fast enough on low
voltage levels. It is highly recommended that Timer/Counter1 is stopped whenever the LSM bit is changed.
Note, that LSM can not be set if PLLCLK is used as system clock.
Bit 6:3 – Res : Reserved Bits
These bits are reserved bits in the ATtiny25/45/85 and always read as zero.
Bit 2 – PCKE: PCK Enable
The PCKE bit change the Timer/Counter1 clock source. When it is set, the asynchronous clock mode is enabled
and fast 64 MHz (or 32 MHz in Low Speed Mode) PCK clock is used as Timer/Counter1 clock source. If this bit is
cleared, the synchronous clock mode is enabled, and system clock CK is used as Timer/Counter1 clock source.
This bit can be set only if PLLE bit is set. It is safe to set this bit only when the PLL is locked i.e the PLOCK bit is 1.
The bit PCKE can only be set, if the PLL has been enabled earlier.
Bit 1 – PLLE: PLL Enable
When the PLLE is set, the PLL is started and if needed internal RC-oscillator is started as a PLL reference clock. If
PLL is selected as a system clock source the value for this bit is always 1.
Bit 0 – PLOCK: PLL Lock Detector
When the PLOCK bit is set, the PLL is locked to the reference clock. The PLOCK bit should be ignored during ini-
tial PLL lock-in sequence when PLL frequency overshoots and undershoots, before reaching steady state. The
steady state is obtained within 100 µs. After PLL lock-in it is recommended to check the PLOCK bit before enabling
PCK for Timer/Counter1.
Bit 76543210
0x27 LSM - - - - PCKE PLLE PLOCK PLLCSR
Read/Write R/W R R R R R/W R/W R
Initial value 0 0 0 0 0 0 0/1 0
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13. 8-bit Timer/Counter1 in ATtiny15 Mode
The ATtiny15 compatibility mode is selected by writing the code “0011” to the CKSEL fuses (if any other code is
written, the Timer/Counter1 is working in normal mode). When selected the ATtiny15 compatibility mode provides
an ATtiny15 backward compatible prescaler and Timer/Counter. Furthermore, the clocking system has same clock
frequencies as in ATtiny15.
13.1 Timer/Counter1 Prescaler
Figure 13-1 shows an ATtiny15 compatible prescaler. It has two prescaler units, a 10-bit prescaler for the system
clock (CK) and a 3-bit prescaler for the fast peripheral clock (PCK). The clocking system of the Timer/Counter1 is
always synchronous in the ATtiny15 compatibility mode, because the same RC Oscillator is used as a PLL clock
source (generates the input clock for the prescaler) and the AVR core.
Figure 13-1. Timer/Counter1 Prescaler
The same clock selections as in ATtiny15 can be chosen for Timer/Counter1 from the output multiplexer, because
the frequency of the fast peripheral clock is 25.6 MHz and the prescaler is similar in the ATtiny15 compatibility
mode. The clock selections are PCK, PCK/2, PCK/4, PCK/8, CK, CK/2, CK/4, CK/8, CK/16, CK/32, CK/64,
CK/128, CK/256, CK/512, CK/1024 and stop.
13.2 Counter and Compare Units
Figure 13-2 shows Timer/Counter 1 synchronization register block diagram and synchronization delays in between
registers. Note that all clock gating details are not shown in the figure. The Timer/Counter1 register values go
through the internal synchronization registers, which cause the input synchronization delay, before affecting the
counter operation. The registers TCCR1, GTCCR, OCR1A and OCR1C can be read back right after writing the
register. The read back values are delayed for the Timer/Counter1 (TCNT1) register and flags (OCF1A and TOV1),
because of the input and output synchronization.
The Timer/Counter1 features a high resolution and a high accuracy usage with the lower prescaling opportunities.
It can also support an accurate, high speed, 8-bit Pulse Width Modulator (PWM) using clock speeds up to 25.6
MHz. In this mode, Timer/Counter1 and the Output Compare Registers serve as a stand-alone PWM. Refer to
“Timer/Counter1 in PWM Mode” on page 97 for a detailed description on this function. Similarly, the high prescal-
ing opportunities make this unit useful for lower speed functions or exact timing functions with infrequent actions.
TIMER/COUNTER1 COUNT ENABLE
PSR1
CS10
CS11
CS12
PCK (25.6 MHz)
0
CS13
3-BIT T/C PRESCALER
PCK/2
PCK
PCK/4
PCK/8
CK/2
CK/4
CK/8
CK/16
CK/32
CK/64
CK/128
CK/256
CK/512
CK/1024
10-BIT T/C PRESCALER
CK (1.6 MHz)
CK
CLEAR
CLEAR
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Figure 13-2. Timer/Counter 1 Synchronization Register Block Diagram.
Timer/Counter1 and the prescaler allow running the CPU from any clock source while the prescaler is operating on
the fast 25.6 MHz PCK clock in the asynchronous mode.
The following Figure 13-3 shows the block diagram for Timer/Counter1.
8-BIT DATABUS
OCR1A OCR1A_SI
TCNT_SO
OCR1C OCR1C_SI
TCCR1 TCCR1_SI
GTCCR GTCCR_SI
TCNT1 TCNT1_SI
OCF1A OCF1A_SI
TOV1 TOV1_SI TOV1_SO
OCF1A_SO
TCNT1
S
AS
A
PCKE
CK
PCK
IO-registers Input synchronization
registers
Timer/Counter1 Output synchronization
registers
SYNC
MODE
ASYNC
MODE
1 PCK Delay No Delay~1 CK Delay
1PCK Delay No Delay
TCNT1
OCF1A
TOV1
1..2 PCK Delay
~1 CK Delay1..2 PCK Delay
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Figure 13-3. Timer/Counter1 Block Diagram
Two status flags (overflow and compare match) are found in the Timer/Counter Interrupt Flag Register - TIFR.
Control signals are found in the Timer/Counter Control Registers TCCR1 and GTCCR. The interrupt enable/dis-
able settings are found in the Timer/Counter Interrupt Mask Register - TIMSK.
The Timer/Counter1 contains two Output Compare Registers, OCR1A and OCR1C as the data source to be com-
pared with the Timer/Counter1 contents. In normal mode the Output Compare functions are operational with
OCR1A only. OCR1A determines action on the OC1A pin (PB1), and it can generate Timer1 OC1A interrupt in nor-
mal mode and in PWM mode. OCR1C holds the Timer/Counter maximum value, i.e. the clear on compare match
value. In the normal mode an overflow interrupt (TOV1) is generated when Timer/Counter1 counts from $FF to
$00, while in the PWM mode the overflow interrupt is generated when the Timer/Counter1 counts either from $FF
to $00 or from OCR1C to $00.
In PWM mode, OCR1A provides the data values against which the Timer Counter value is compared. Upon com-
pare match the PWM outputs (OC1A) is generated. In PWM mode, the Timer Counter counts up to the value
specified in the output compare register OCR1C and starts again from $00. This feature allows limiting the counter
“full” value to a specified value, lower than $FF. Together with the many prescaler options, flexible PWM frequency
selection is provided. Table 12-3 on page 88 lists clock selection and OCR1C values to obtain PWM frequencies
from 20 kHz to 250 kHz in 10 kHz steps and from 250 kHz to 500 kHz in 50 kHz steps. Higher PWM frequencies
can be obtained at the expense of resolution.
13.2.1 Timer/Counter1 in PWM Mode
When the PWM mode is selected, Timer/Counter1 and the Output Compare Register A - OCR1A form an 8-bit,
free-running and glitch-free PWM generator with output on the PB1(OC1A).
8-BIT DATABUS
TIMER INT. FLAG
REGISTER (TIFR)
TIMER/COUNTER1
8-BIT COMPARATOR
T/C1 OUTPUT
COMPARE REGISTER
TIMER INT. MASK
REGISTER (TIMSK)
TIMER/COUNTER1
(TCNT1)
T/C CLEAR
T/C1 CONTROL
LOGIC
TOV1
TOV1
TOIE0
TOIE1
OCIE1A
OCF1A
OCF1A
CK
PCK
T/C1 OVER-
FLOW IRQ
OC1A
(PB1)
T/C1 COMPARE
MATCH A IRQ
GLOBAL T/C CONTROL
REGISTER 2 (GTCCR)
PWM1A
FOC1A
(OCR1A) (OCR1C)
8-BIT COMPARATOR
T/C1 OUTPUT
COMPARE REGISTER
TOV0
COM1A1
COM1A0
T/C CONTROL
REGISTER 1 (TCCR1)
CS12
PSR1
CS11
CS10
CS13
CTC1
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When the counter value match the content of OCR1A, the OC1A and output is set or cleared according to the
COM1A1/COM1A0 bits in the Timer/Counter1 Control Register A - TCCR1, as shown in Table 13-1.
Timer/Counter1 acts as an up-counter, counting from $00 up to the value specified in the output compare register
OCR1C, and starting from $00 up again. A compare match with OCR1C will set an overflow interrupt flag (TOV1)
after a synchronization delay following the compare event.
Note that in PWM mode, writing to the Output Compare Register OCR1A, the data value is first transferred to a
temporary location. The value is latched into OCR1A when the Timer/Counter reaches OCR1C. This prevents the
occurrence of odd-length PWM pulses (glitches) in the event of an unsynchronized OCR1A. See Figure 13-4 for an
e xample.
Figure 13-4. Effects of Unsynchronized OCR Latching
During the time between the write and the latch operation, a read from OCR1A will read the contents of the tempo-
rary location. This means that the most recently written value always will read out of OCR1A.
When OCR1A contains $00 or the top value, as specified in OCR1C register, the output PB1(OC1A) is held low or
high according to the settings of COM1A1/COM1A0. This is shown in Table 13-2.
Table 13-1. Compare Mode Select in PWM Mode
COM1A1 COM1A0 Effect on Output Compare Pin
0 0 OC1A not connected.
0 1 OC1A not connected.
1 0 OC1A cleared on compare match. Set when TCNT1 = $00.
1 1 OC1A set on compare match. Cleared when TCNT1 = $00.
Table 13-2. PWM Outputs OCR1A = $00 or OCR1C
COM1A1 COM1A0 OCR1A Output OC1A
01$00L
0 1 OCR1C H
10$00L
PWM Output OC1A
PWM Output OC1A
Unsynchronized OC1A Latch
Synchronized OC1A Latch
Counter Value
Compare Value
Counter Value
Compare Value
Compare Value changes
Glitch
Compare Value changes
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In PWM mode, the Timer Overflow Flag - TOV1 is set when the TCNT1 counts to the OCR1C value and the
TCNT1 is reset to $00. The Timer Overflow Interrupt1 is executed when TOV1 is set provided that Timer Overflow
Interrupt and global interrupts are enabled. This also applies to the Timer Output Compare flags and interrupts.
The PWM frequency can be derived from the timer/counter clock frequency using the following equation:
The duty cycle of the PWM waveform can be calculated using the following equation:
...where TPCK is the period of the fast peripheral clock (1/25.6 MHz = 39.1 ns).
Resolution indicates how many bits are required to express the value in the OCR1C register. It can be calculated
using the following equation:
1 0 OCR1C H
11$00H
1 1 OCR1C L
Table 13-3. Timer/Counter1 Clock Prescale Select in the Asynchronous Mode
PWM Frequency Clock Selection CS1[3:0] OCR1C RESOLUTION
20 kHz PCK/16 0101 199 7.6
30 kHz PCK/16 0101 132 7.1
40 kHz PCK/8 0100 199 7.6
50 kHz PCK/8 0100 159 7.3
60 kHz PCK/8 0100 132 7.1
70 kHz PCK/4 0011 228 7.8
80 kHz PCK/4 0011 199 7.6
90 kHz PCK/4 0011 177 7.5
100 kHz PCK/4 0011 159 7.3
110 kHz PCK/4 0011 144 7.2
120 kHz PCK/4 0011 132 7.1
130 kHz PCK/2 0010 245 7.9
140 kHz PCK/2 0010 228 7.8
150 kHz PCK/2 0010 212 7.7
Table 13-2. PWM Outputs OCR1A = $00 or OCR1C
COM1A1 COM1A0 OCR1A Output OC1A
ffTCK1
OCR1C + 1
------------------------------------=
DOCR1A 1+TTCK1 TPCK
OCR1C 1+TTCK1
----------------------------------------------------------------------------=
R2OCR1C 1+()log=
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13.3 Register Description
13.3.1 TCCR1 – Timer/Counter1 Control Register
Bit 7 – CTC1 : Clear Timer/Counter on Compare Match
When the CTC1 control bit is set (one), Timer/Counter1 is reset to $00 in the CPU clock cycle after a compare
match with OCR1A register. If the control bit is cleared, Timer/Counter1 continues counting and is unaffected by a
compare match.
Bit 6 – PWM1A: Pulse Width Modulator A Enable
When set (one) this bit enables PWM mode based on comparator OCR1A in Timer/Counter1 and the counter
value is reset to $00 in the CPU clock cycle after a compare match with OCR1C register value.
Bits 5:4 – COM1A[1:0]: Comparator A Output Mode, Bits 1 and 0
The COM1A1 and COM1A0 control bits determine any output pin action following a compare match with compare
register A in Timer/Counter1. Output pin actions affect pin PB1 (OC1A). Since this is an alternative function to an
I/O port, the corresponding direction control bit must be set (one) in order to control an output pin.
In PWM mode, these bits have different functions. Refer to Table 13-1 on page 98 for a detailed description.
160 kHz PCK/2 0010 199 7.6
170 kHz PCK/2 0010 187 7.6
180 kHz PCK/2 0010 177 7.5
190 kHz PCK/2 0010 167 7.4
200 kHz PCK/2 0010 159 7.3
250 kHz PCK 0001 255 8.0
300 kHz PCK 0001 212 7.7
350 kHz PCK 0001 182 7.5
400 kHz PCK 0001 159 7.3
450 kHz PCK 0001 141 7.1
500 kHz PCK 0001 127 7.0
Table 13-3. Timer/Counter1 Clock Prescale Select in the Asynchronous Mode (Continued)
PWM Frequency Clock Selection CS1[3:0] OCR1C RESOLUTION
Bit 7 6 5 4 3 2 1 0
0x30 CTC1 PWM1A COM1A1 COM1A0 CS13 CS12 CS11 CS10 TCCR1A
Read/Write R/W R/W R/W R/W R/W R/W R/W R/W
Initial value 0 0 0 0 0 0 0 0
Table 13-4. Comparator A Mode Select
COM1A1 COM1A0 Description
0 0 Timer/Counter Comparator A disconnected from output pin OC1A.
0 1 Toggle the OC1A output line.
1 0 Clear the OC1A output line.
1 1 Set the OC1A output line
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Bits 3:0 – CS1[3:0]: Clock Select Bits 3, 2, 1, and 0
The Clock Select bits 3, 2, 1, and 0 define the prescaling source of Timer/Counter1.
The Stop condition provides a Timer Enable/Disable function.
13.3.2 GTCCR – General Timer/Counter1 Control Register
Bit 2 – FOC1A: Force Output Compare Match 1A
Writing a logical one to this bit forces a change in the compare match output pin PB1 (OC1A) according to the val-
ues already set in COM1A1 and COM1A0. If COM1A1 and COM1A0 written in the same cycle as FOC1A, the new
settings will be used. The Force Output Compare bit can be used to change the output pin value regardless of the
timer value. The automatic action programmed in COM1A1 and COM1A0 takes place as if a compare match had
occurred, but no interrupt is generated. The FOC1A bit always reads as zero. FOC1A is not in use if PWM1A bit is
set.
Bit 1 – PSR1 : Prescaler Reset Timer/Counter1
When this bit is set (one), the Timer/Counter prescaler (TCNT1 is unaffected) will be reset. The bit will be cleared
by hardware after the operation is performed. Writing a zero to this bit will have no effect. This bit will always read
as zero.
Table 13-5. Timer/Counter1 Prescale Select
CS13 CS12 CS11 CS10 T/C1 Clock
0 0 0 0 T/C1 stopped
0001PCK
0010PCK/2
0011PCK/4
0100PCK/8
0101CK
0110CK/2
0111CK/4
1000CK/8
1001CK/16
1010CK/32
1011CK/64
1100CK/128
1101CK/256
1110CK/512
1111CK/1024
Bit 7 6 5 4 3 2 1 0
0x2C TSM PWM1B COM1B1 COM1B0 FOC1B FOC1A PSR1 PSR0 GTCCR
Read/Write R/W R/W R/W R/W W W R/W R/W
Initial value 0 0 0 0 0 0 0 0
AtmeL
102
ATtiny25/45/85 [DATASHEET]
2586Q–AVR–08/2013
13.3.3 TCNT1 – Timer/Counter1
This 8-bit register contains the value of Timer/Counter1.
Timer/Counter1 is realized as an up counter with read and write access. Due to synchronization of the CPU,
Timer/Counter1 data written into Timer/Counter1 is delayed by one CPU clock cycle in synchronous mode and at
most two CPU clock cycles for asynchronous mode.
13.3.4 OCR1A – Timer/Counter1 Output Compare RegisterA
The output compare register A is an 8-bit read/write register.
The Timer/Counter Output Compare Register A contains data to be continuously compared with Timer/Counter1.
Actions on compare matches are specified in TCCR1. A compare match does only occur if Timer/Counter1 counts
to the OCR1A value. A software write that sets TCNT1 and OCR1A to the same value does not generate a com-
pare match.
A compare match will set the compare interrupt flag OCF1A after a synchronization delay following the compare
event.
13.3.5 OCR1C – Timer/Counter1 Output Compare Register C
The Output Compare Register B - OCR1B from ATtiny15 is replaced with the output compare register C - OCR1C
that is an 8-bit read/write register. This register has the same function as the Output Compare Register B in
ATtiny15.
The Timer/Counter Output Compare Register C contains data to be continuously compared with Timer/Counter1.
A compare match does only occur if Timer/Counter1 counts to the OCR1C value. A software write that sets TCNT1
and OCR1C to the same value does not generate a compare match. If the CTC1 bit in TCCR1 is set, a compare
match will clear TCNT1.
13.3.6 TIMSK – Timer/Counter Interrupt Mask Register
Bit 7 – Res: Reserved Bit
This bit is a reserved bit in the ATtiny25/45/85 and always reads as zero.
Bit 76543210
0x2F MSB LSB TCNT1
Read/Write R/W R/W R/W R/W R/W R/W R/W R/W
Initial value 0 0 0 0 0 0 0 0
Bit 76543210
0x2E MSB LSB OCR1A
Read/Write R/W R/W R/W R/W R/W R/W R/W R/W
Initial value 0 0 0 0 0 0 0 0
Bit 76543210
0x2D MSB LSB OCR1C
Read/Write R/W R/W R/W R/W R/W R/W R/W R/W
Initial value 1 1 1 1 1 1 1 1
Bit 7 6 5 4 3 2 1 0
0x39 –OCIE1A
OCIE1B OCIE0A OCIE0B TOIE1 TOIE0 – TIMSK
Read/Write R R/W R/W R/W R/W R/W R/W