RFID as a highly professional technology, in some companies, may also recruit professional RFID engineers. This paper describes the basic selection design, circuit framework, RFID antenna debugging, low-power card checking debugging and so on.
NFC (near field communication) chip selection:
The protocol supported by the chip, whether it supports low-power card checking, whether it can pass the financial certification, and the chip price
Chip support protocol:
ISO 14443a / B, ISO 15693, ISO 18092 and ISO 21481, etc
Iso14443a card: Mifare series, ultralight series, plus series, CPU card series, etc.
ISO14443B card: ID card, sr176, sri512, etc.
Iso15693: icode series of NXP, tag of Ti_ It HF-I, St LRI, etc.
Iso18092: including card reading mode, card mode and point-to-point communication mode.
Iso21481: compatible with iso15693 protocol on the basis of iso18092.
Lpcd function: chip low power detection card function. When the card is not close, the chip is in a low power consumption state. It only needs 10uA current to complete the card detection. When the card is close, the chip detects the card and wakes up the MCU to read the card.
Financial certification: pboc2.0/3.0 standard, EMV standard
The external circuit of NFC chip usually consists of the following parts: power supply circuit, communication interface circuit, antenna circuit and oscillation circuit;
Power supply circuit: mainly including analog power supply avdd, digital power supply dvdd, transmitter power supply tvdd, pin power supply PvdD, test pin power supply pvdd2;
a. If the transmission power needs to be increased, the voltage of tvdd can be increased. For example, the transmission power of tvdd with 5V power supply will be stronger than that of tvdd with 3V power supply;
b. The power supply current of the chip is usually tens to hundreds of Ma, and the main energy consumption is in the transmitter circuit. For example, the transmitting current of fm175xx antenna is 100mA, while that of rc663 antenna is 250mA. Therefore, when selecting power supply chip and inductor, enough margin should be reserved;
c. The 13.56MHz sine wave signal of the antenna of the card reader chip will interfere with the power supply. In order to reduce the conducted interference, a π type filter can be added to the power supply, but in order to reduce the redundancy of the circuit design, it is generally not added.
Generally, it supports SPI / I2C / UART, and is generally selected through external pin configuration. For the convenience of upgrade, it can be compatible with other designs;
The antenna circuit consists of four parts: EMC filter, matching circuit, antenna and receiving circuit. Take fm17550 as an example, as follows:
The low-pass filter composed of L1 and C1 is used to filter the derivative harmonics of 13.56MHz. The cut-off frequency of the filter should be designed above 14mhz. L1 inductors should not be placed close to each other to avoid mutual interference (mutual inductance effect). Filter circuit element matching formula: F = 1 / (2 π √ LC)
Used to adjust the transmitting load and resonant frequency. The power of RF circuit is affected by the internal resistance and external impedance of the chip. When the internal resistance and external impedance of the chip are the same, the transmission power efficiency is the highest. C2 is the load capacitance. The larger the antenna sensitivity, the smaller the value of C2. C3 is the resonant capacitor, which is directly related to the antenna inductance, so that the resonant frequency is 13.56MHz.
C4 filters out the DC signal, R2 and R3 form a voltage divider circuit, so that the amplitude of Rx receiver sine wave signal is between 1.5-3V.
It consists of R1 resistor (usually 1ohm or 0ohm) and printed PCB.
The larger the antenna is, the farther the card reading distance is. When the antenna area reaches 5cm x 5cm, the larger the antenna is, the longer the card reading distance is.
It is recommended to choose 0.5mm-1mm antenna line width. No more than 3 circles of antenna larger than 5cm x 5cm, no less than 4 circles of antenna smaller than 3cm x 3cm
In order to reduce the EMC radiation interference, it is necessary to draw an arc at the PCB routing corner.
In the antenna area and the edge of the antenna, it is forbidden to draw the signal, power supply and ground wire into a circle or semicircle. Large area metal objects and metal coating are not allowed in the antenna circle, so as to avoid causing eddy current effect of magnetic field and causing serious loss of capacity.
The winding mode of antenna PCB is relative, not in the same direction.
The precision of antenna circuit design elements should be controlled within 2%, otherwise it is easy to lead to antenna resonance frequency point deviation, resulting in serious degradation of card reading performance, and product consistency is difficult to guarantee
Relationship between antenna size and card reading distance