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PCF2127A/PCF2129A CMOS Real-Time Clock/Calendars

NXP’s high-precision RTCs with time stamp, embedded quartz, and advanced power backup

Highly accurate RTCs with embedded quartz crystalNXP's highly accurate real-time clock/calendars (RTCs) integrate a temperature-compensated crystal oscillator (TCXO) that uses a 32.768-kHz quartz crystal for very high precision with very low-power consumption. Manufactured using the CMOS process, these RTCs support access via a I²C-bus or an SPI bus and offer special support features such as a backup battery switch-over circuit, programmable Watchdog timer, time stamp function, and more. The PCF2127A adds 512 bytes of general-purpose SRAM. The rich functionality and low overall power consumption of these RTCs let the system use smaller batteries and allow the microcontroller to stay in hibernation longer.

The backup battery switch-over circuit ensures a constant supply of power. In standard mode, the oscillator supply is switched over to the battery as soon as the supply voltage drops below the battery voltage. The smart mode is used to keep the RTC running while preserving battery backup time. With a lithium battery of up to 4.2 V connected as a backup source, the RTC continues to run even with a VDD as low as 3.3 or 2.5V. The battery is only switched on if the VDD drops below 2.2V. The uninterrupted voltage is also made available on a device pin and can be used to buffer external circuitry, including RAM memory.

  • Features/Applications
  • Specifications
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  • Time keeping with typical accuracy of ±3 ppm
  • Fully integrated quartz oscillator
  • Time stamp function
  • Battery switchover circuit
  • System reset generation
  • 512 bytes of RAM (PCF2127)
  • Factory calibrated
  • Clock operating down to 1.2V
  • Low supply current: 650nA at 3V
  • 1MHz fast-mode plus I²C-bus and 6MHz SPI interface
  • Very accurate time references
  • Accurate switching
  • Utility meters
  • Industrial applications
  • HVAC equipment
  • GPS, shorter cold startup time (TTFF)
  • Gaming machines
  • Alarm systems
  • Token for access control
  • Cellular infrastructure


Characteristics PCF2127A PCF2129A
Supply Voltage Range 1.8 - 4.2 V 1.8 - 4.2 V
Battery Supply Voltage Range 1.8 - 4.2 V 1.8 - 4.2 V
Watchdog & Countdown Timer Yes Watchdog only
System Reset Yes No
RAM 512 Byte No
Accuracy within Temperature Range -20 to +70°C min. +/- 5ppm +/- 5ppm

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Digi-Key P/N MFG P/N Description Datasheet
568-4961-5-ND PCF2127AT/1,512 IC REAL TIME CLK/CALENDAR 20SOIC Datasheet
568-4960-5-ND PCF2129AT/1,512 IC REAL TIME CLK/CALENDAR 20SOIC Datasheet