Design Considerations for HF-RFID Reader Systems

By Jack Shandle

Contributed By Electronic Products

Of the many RFID technology variants, one of the most popular operates at 13.56 MHz and is known generically as HF-RFID. Common applications include access control, contactless payments, bank and identity cards and product authentication – all of which have specific implementation requirements like readability distance that can make designing a system challenging. These standard interference avoidance criteria and physical constraints have limited the maximum read distance of all HF-RFID implementations to about 1.5 meters.

HF-RFID tags are primarily implemented as passive devices that rely on the RFID reader to initiate contact with the tag and to supply transmission power through inductive coupling. Designing a HF-RFID reader involves relatively few components, primarily the RFID reader/transceiver chip, a microcontroller, an antenna, and an interface to the IT system that processes the information received from the RFID reader. Figure 1 shows many of the system components.

RFID Reader Chips

Figure 1: RFID reader chips simplify the design of tag reading systems but specific implementations require choosing from a variety of interfaces and MCU options. (Courtesy: Texas instruments)

In addition to the primary components mentioned above, fixed reader systems must also include a means for powering the system and a system power controller. For example, an AC/DC isolated supply and AC/DC converter if the reader is powered from a wall socket. A portable reader changes the configuration a bit by requiring a battery and power management block. Local status reporting or feedback devices such as LED drivers and/or buzzer drivers (audio amplifiers) are also frequently included in designs.

The RFID reader chips central to any design are provided by several manufacturers. Three manufacturers’ chips include NXP Semiconductor’s 13.56 MHz MFRC50001T, Texas Instruments’ TRF7960 and Melexis’ MLX12115EFRCT.

Standards Compliance

Depending on the application, several strategic choices must be made early in the design cycle. The first and most important involves the protocol that will run on the reader. This is typically one of two ISO/IEC standards – 15693 or 14443.

ISO/IEC 15693 is used for so-called “vicinity cards” that have a maximum read distance of 1 to 1.5 meters. Typical 15693 applications include asset tracking visibility, pharmaceutical supply chain authentication, library management, event and venue ticketing, and laundry and textile rental tracking.

ISO/IEC 14443 is used in “proximity cards” that operate a distances shorter than 1 meter. Many 13.56 MHz applications involve RFID tags that are embedded in smart cards – primarily bank cards, memory cards and building access cards. For security oriented applications such as contactless payments, the read distance is a few centimeters. There are two types of 14443 cards: Type A and Type B. They both use the same transmission protocol but have different modulation methods, coding schemes and protocol initialization procedures. ISO/IEC 15693 and 14443 bit rates range from 106 kbps to 848 kbps.

The application determines which standard will be supported, of course, and in some instances chipmakers offer reader chips that support multiple standards. Handling the standards-oriented considerations mentioned above is made easier by using a chip manufacturer’s development kit.

Design and Development Tools

Development kits offer designers assistance in developing a system based on a specific manufacturer’s products. NXP Semiconductor’s MFEV700/HAB, 122 is an evaluation kit that includes an RFID reader module, five sample tags, an antenna and software. Two additional popular development kits are Texas Instruments’ (TI) TRF7960EVM and Melexis’s DEMO90121LR. Options determine the cost of a development kit and should be considered depending on the design team’s present and future needs.

A microcontroller (MCU) is essential for the finished design and manufacturers that have MCU product lines will often include one of their own chips on the development board. A complete development kit will also include a PCB antenna, an SMA port for an external antenna and sample tags for designing and testing a complete system.

Other primary design considerations center on added features that have been integrated into the chip and its options for interfacing with other systems. Reader chips that integrate LDO regulators and clock output, for example, reduce the design’s electronic bill of materials (eBoM). Similarly, if the reader chip integrates hardware blocks for encoding, decoding and data framing that are consistent with the ISO/IEC 15693 and 14443A and B standards, software development will be much easier.

Interface options between the RFID reader IC and the MCU tend to be limited to SPI and parallel. However, a wider variety of interface chips can be used for connectivity with the host system. These are typically determined by the application and the amount of data being transferred. Processor-to-system interfaces include Ethernet, RS-485, RS-232, and UART.

The MCU’s level of integration also is an important consideration because it simplifies design and reduces system cost. The processor’s power consumption should also be considered, particularly in the case of a battery powered RFID reader design.

Of the several MCU families that TI offers, the ultra-low-power MSP430 family is best suited for HF-RFID designs. Even within the MSP430F2xx line, the choice of a particular chip depends on the Flash/RAM requirements of the software end of the complete solution. TI’s MSP430F2370IRHAT is a 16-bit RISC that offers 32-Kbit of Flash memory and a low-cost design.

In its development kit, Melexis uses a member of Atmel Corp’s ATMega8 family of MCUs in its demo boards. The ATMEGA8HVA-4CKU is a good choice for a design.

Antennas are another critical component of HF RFID readers. For example, DLP Design Inc. has two antennas appropriate for 13.56 MHz designs – the DLP-FANT, which is a ferrite antenna and the DLP-RFID-ANT, which is a round antenna with coax. Feig Electronic Inc. offers three HF antennas, the 1451.008.00 loop antenna; the 1663.000.00 MR pad antenna; and the 1967.000.00 PR pad antenna.

For design teams that wish to use an HF RFID module instead of starting with a reader chip and other components, both Skyetek Inc and Feig offer reader modules. Among these are Skyetek’s SM-M4-00-HF-CP, which supports ISO14443-A, ISO14443-B and ISO15693. Skyetek also has made several development kits available for its HF RFID modules including the DK-M4-00-HF, which is targeted at contactless card payment applications. Feig’s 2873.000.00 supports all three HF-RFID standards as well.

While RFID reader chips simplify the design of tag reading systems, there are many considerations to weigh depending on the specific implementation. Industry standards and packaged development kits are available help ease developers’ pain.

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