Introduction to software and hardware design of read/write modules in RFID systems

Radio Frequency Identification (RFID) technology (Radio Frequency IdenTIficaTIon) technology emerged in the 1980s and has grown rapidly in recent years. It is an automatic identification technology that uses RF communication methods to achieve non-contact, fast, real-time and accurate acquisition and processing of information. The combination of RFID technology and Internet technology can realize the tracking and information sharing of items in the world. It is recognized by the world as one of the top ten most important technologies in the 21st century and has broad application prospects. In recent years, the RFID industry has grown rapidly in China and has penetrated into all aspects of people's lives and work. RFID market analysis and forecast shows that China's RFID demand is growing rapidly. In October 2006, the country listed radio frequency identification (RFID) technology as a major application project of 863. The Ministry of Science and Technology allocated a large amount of funds to support the breakthrough and independent innovation of China's RFID technology.

Readers play a pivotal role in the entire RFID system. The frequency of the reader determines the working frequency band of the RFID system, and its power directly affects the distance of the RFID. People use computer application software to process the writing or reading of data information carried by radio frequency tags. Due to the non-contact nature of tags, people must use the readers between the application system and tags to implement data reading and writing. This makes the reader play a key role in the entire communication process.

The reader can be simplified into two basic functional modules: the control part and the radio part. The control part is the MCU control circuit, and its function is that the intelligent unit issues a command to perform necessary processing on the signal recovered by the radio frequency part, and puts the result into the storage unit. The radio frequency part is composed of a reader chip and peripheral circuits, including a transmitter and a receiver, and its functions include modulating the transmitted signal, transmitting the data to the electronic tag, and receiving and demodulating the high frequency signal from the electronic tag.

2.1 reader chip S6700

The S6700 is a representative multi-protocol transceiver chip (RI-R6C-001A) from TI, which operates at 13.56MHz. It supports three protocols, namely Tag- it protocol and ISO/IEC 15693-2. ISO/IEC 14443-2 (TYPE A); its typical power supply is +5V, typical transmit power is 200mW, ensuring voltage between 3 and 5.5V; in SSOP20 package, internal emission regulator and receiver demodulator are integrated. Manchester coding, there are three power management functions: IDLE, POWER DOWN, and FULL POWER. Supported modulation methods are 100% and 10% to 30% ASK (Amplitude Shift) Keying), the two modulation modes can be switched by the application software, and the modulation depth is realized by changing the resistance value connected to the R_MOD terminal.

The transmit channel first decodes the data of DIN and SCLOCK, and after deep modulation, it is output through the power amplifier and low-pass filter. The receiving channel detects the received data to obtain a signal of 423/484/848 kHz, and outputs DOUT and M_ERR signals after decoding.

In the process of reading and writing, the interface of the reader/receiver communication physical layer protocol specified by ISO/IEO15693-2 is implemented by S6700. The CPU is connected to the S6700 through the synchronous serial interface (SPI). The communication interface between the CPU and the S6700 is provided. There are four lines: clock line (SCLOCK), data input line (DIN), data output line (DOUT), and error detection line (M_ERR). The clock line is bidirectional, latching data on the rising edge of the clock. In addition to the data output function during data reception, DOUT is also used to characterize the internal FIFO of the chip.

2.2 Overall design of the module

The control part MCU selects ATMEL128 chip of ATMEL company, which is a very powerful single-chip microcomputer in AVR. The chip has high performance, low power consumption AVR 8-bit microprocessor, advanced RISC structure, and 128KB system. Online programmable Flash.

The designed reader circuit consists of three parts: S6700 typical application circuit and peripheral auxiliary circuit, CPU interface circuit and antenna equivalent circuit. The corresponding ends of the S6700 are connected to the PE5, PA0, PA1, and PB5 pins of the ATmega128. The modulated signal is output on the TX_OUT terminal of the S6700, and the signal is resonantly amplified through an LC network, and then the selection of the passband is matched with the impedance through a T-shaped network (double L network), and finally output to the 50 Ω antenna. Since the same transceiver antenna is used, a 2.2kΩ protection resistor should be connected to the receiving end of the chip to avoid damage to the chip receiving pin caused by excessive signal voltage. R4, L4, C8, and C9 form a series resonant circuit with a matching impedance of 50Ω. The adjustable capacitor C9 is used to accurately adjust the circuit resonance point at 13.56MHz. Because the S6700 is directly connected to the external antenna module, considering the addition of the coaxial cable, the stability of the interface circuit will be degraded. By adjusting C9, 13.56 MHz can be obtained and the matching impedance of 50 Ω can be satisfied. This design facilitates the reader to correctly send and receive information.

The reader and the electronic tag communicate in a half-duplex mode, that is, a question-and-answer mode. It is usually spoken by the reader (Reader talks first). Referring to the communication process of the reader/writer shown in FIG. 3, it is necessary to comply with the communication protocol between the controller and the transceiver S6700 and the ISO15693-3 specification followed by the transceiver S6700 and the electronic tag. This is mainly discussed in conjunction with the communication protocols of these two parts.

3.1 Request Command Structure

The controller must send a command to the transceiver to ensure proper timing. The structure of a typical command is: start bit S1, 8-bit command, data (domain), end bit ES1.

(1) Start bit S1, end bit ES1 waveform.

(2) Command byte: specifies the relevant parameters when the transceiver communicates with the electronic tag.

(3) Data: The data field content is specified by ISO15693-3, depending on the content of the command.

The command parameters are the communication rules between the far coupler (VCD) and the far-coupled IC card (VICC), including what support protocol, pulse position coding, modulation depth, AM or FSK (frequency shift keying), for example: 2EH, which means normal mode, supports radio frequency protocol 15693 (1 out of 4), AM modulation mode, modulation depth 100%, and high data rate for return data. Only the order in which the command byte is sent is the high order first, that is, MSB FIRST. The other data and flag bits are sent in the lower order first, that is, LSB FIRST.

The communication with the command 2E (00101110) and the data 4E (01001110) as an example is as shown.