FID is the abbreviation of RadioFrequencyIdenTIficaTIon, which is radio frequency identification. Radio Frequency Identification (RFID) technology is an automatic identification technology that has emerged and matured since the 1980s. It uses radio frequency for non-contact two-way communication to achieve target recognition and data exchange. RFID is a non-contact automatic identification technology that automatically recognizes target objects and acquires relevant data through radio frequency signals, and does not require manual intervention for identification work. As a wireless version of the barcode, RFID technology has the advantages of waterproof, anti-magnetic, high temperature resistance, long service life, large reading distance, data encryption on the label, larger storage data capacity, and free storage information. Recognized by the world as one of the ten most important technologies of the century, it has broad application prospects in various industries such as production, retail, logistics and transportation. China's second-generation ID card uses RFID technology, and Wal-Mart, the world's largest retailer, has asked its largest 100 suppliers to adopt RFID technology since January 1, 2005.
1, RFID overview
A basic RFID system is shown in Figure 1. It consists of the following components: a tag consisting of a coupling component and a chip. Each tag has a unique electronic code that is attached to the object to identify the target object. Reader (Reader) ), a device that reads (and sometimes writes) tag information; an antenna (Antenna) that transmits RF signals between the tag and the reader.
There are three operating frequencies for electronic tags: low frequency (125 kHz), intermediate frequency (13.56 MHz), and high frequency (2.45 GHz, 5.8 GHz). The reader design in this paper is based on the IS015693 standard and works at 13.56MHz. The applicable electronic tags are passive. Passive tags obtain energy in an inductively coupled manner from the electromagnetic field generated by the reader. The reader first receives the command from the background computer, then encodes and modulates the command data according to the ISO standard and transmits it through the antenna. The electronic tag receiving command data in the working area of ​​the reader and the reader transmits the response information by changing the energy intensity, and the reader passes The antenna receives the response signal of the electronic tag, performs demodulation and decoding, and transmits it to the host computer for further processing.
2, the design of the reader
2.1 reader core control
The control core device used in the design of this reader is DSMS320F2812, which is a 32bit fixed-point DSP chip introduced by TI in 2003. The highest frequency is up to 150MHz, 128kbit Flash, 18kbit RAM, 16 channel 12bit ADC, support ANCIC/C++. Since the TMS320F2812 integrates a 16-channel 12-bit ADC, there is no need to expand the ADC, which makes the hardware circuit more compact. Making the DSP work It uses a bit-domain programming environment, the program structure is clearer, and the software development cycle is shortened.
2.2 reader hardware design
The hardware component of the reader, as shown in Figure 2, is a DSP system based on TMS320LF2812, which performs two-way communication with the electronic tag and the host computer. The DSP performs the encoding and decoding functions in the data exchange with the electronic tag.
The DSP generates pulse position coding and controls the output of the 13.56 MHz carrier frequency to achieve pulse position modulation. The power of the output signal of the modulation circuit is very weak, and the signal needs to be power amplified, and then filtered and tuned to be added to the antenna to increase the operating distance of the card. The power amplifier circuit adopts the NPN type RF power transistor MRF426, and the transmission power is 4W, and the operating frequency can reach 25MHz. The output is regulated by a potentiometer. The minimum power that can be adjusted is 0.5W and the maximum is 6W. The antenna coil exhibits an impedance z at an operating frequency of 13.56 MHz. To achieve power matching with a 50 Ω system, the system converts this impedance to 50 Ω through a passive matching circuit, and then powers the reader from the reader via a 50 Ω coaxial cable. The final stage is transmitted to the antenna matching circuit.
There are 4 antennas in the design process, which can be exchanged according to different distance requirements. In the ISO15693 protocol, the data from the electronic tag to the reader/writer is transmitted in a load-modulated manner (using subcarriers at the same time), that is, the Manchester-encoded signal is first loaded into the subcarrier (with ASK single subcarrier 423.75 kHz and FSK dual pair). The carrier is 423.75 kHz and 484.28 kHz), and then the signal is loaded onto the primary carrier 13.56 MHz. Therefore, in the receiving channel of the reader, a sideband is first taken out by the bandpass filter, amplified and sent to the demodulator, and the demodulator mixes the sideband signal with the local 13.56MHz carrier to obtain modulation. The IF signal on the carrier is then subjected to ASK or FSK detection to obtain the Manchester code waveform. The Manchester code waveform obtained here is not subjected to sampling decision as an analog signal, and is decoded and verified by the on-chip AD sampling, processing, and decision of the DSP to complete the reception process of the entire signal.
2.3 reader software design
In the ISO15693 standard, the data encoding from the reader to the electronic tag uses pulse position modulation, and the electronic tag supports two encoding modes, one is 1/256 mode, and the other is 1/4 mode. In the 1/256 mode, the value of one byte is represented by the position of the pulse, and the position of the pulse is somewhere in 256 consecutive time periods, and the time period is 256/f, (18.88 μs, the high and low levels are respectively 9.44μs), so a byte transfer requires 4.833ms. In 1/4 mode, the position of one pulse determines two bits (00, 01, 10 or 11) of one byte. As shown in Figure 3, four consecutive cycles determine one byte, and one byte is required to transmit 302.08. Ss.
The two encoding modes are basically the same. Firstly, according to the data to be encoded x, the time before and after the pulse is high (for the 1/256 mode, X318.88μs and (FF-x) 318.88μs respectively), and then sequentially called. The high level before the pulse generates a subroutine, a pulse generation subroutine, and a high level generation subroutine after the pulse. The timing of 18.88 μs should be as accurate as possible to avoid coding errors caused by the accumulation of deviations. This design uses TMS320F2812 as the processor, the highest frequency can reach 150MHz, the main frequency can be set to 135.6MHz, so there will be some convenience in the program design, make full use of the characteristics of TMS320F2812, improve the accuracy, but also need to pay attention The effect of jump instructions and pipelines on precise timing. This design square selects 1/4 mode coding, using 4 to 1 pulse position modulation mode, this position determines 2 bits at a time. Four consecutive pairs of bits form one byte, first transmitting the lowest pair of bits. For example, Figure 3 shows a VCD (Reader/Writer) transmission E1 = (11100001) b = 225.
2.4 Analysis of design results
The data received by the reader from the electronic tag is sent in frames. Each frame includes a frame header (SOF), data and end of frame (EOF), and the CRC check value of 2 bytes (16 bits) before the end of the frame. . The frame header waveform of the data received by the reader/writer is as shown in FIG. 4, and the frame non-waveform of the received data is opposite to the frame header waveform. The waveform of the data received by the reader is shown in Figure 5. The start part is the receive command, the second part is the frame header, the third part is the transmission data, and finally the end of the frame. The reader starts sampling after issuing a command to the electronic tag. If the SOF is received within a certain time, indicating that there is a return signal, the sampling is continued until EOF is received; otherwise, it returns immediately.
In the actual experiment, the read/write distance, signal strength and noise interference of the reader have a great influence on the accuracy of reading and writing. When the electronic tag is close to the antenna of the reader, the signal is relatively strong with respect to the interference signal, and the decision threshold is easy to select. The discrimination of the signal is relatively easy, the decoding is convenient, and the result is relatively accurate. However, when the electronic tag is far from the antenna of the reader but within the working range of the reader, the strength of the signal is equivalent to the noise, the decision threshold is difficult to select, the sampled signal needs to be filtered, and then adaptively selected. Determine the threshold to improve reading and writing distance and reading and writing accuracy.
2.5 anti-collision programming
Anti-collision programming is an important part of the reader program design. The purpose of the anti-collision sequence is to generate a VICCs directory determined by the VICC's unique ID (UID) in the VCD work domain. VCDs are dominant in communicating with one or more VICCs. It initiates card communication by issuing a directory request. When the reader enters the working state, all tags within its antenna coverage will be activated, waiting, ready to respond to the reader command operation, which causes label read and write conflicts. In order to solve this problem, the tag is internally designed with its own anti-collision mechanism, and only needs to use the relevant instruction set to assist in designing an anti-collision program.
Anti-collision program flow chart, as shown in Figure 6. When the active tag receives the reader SELECT command, it sends its own UID to the reader. At this time, if more than one tag simultaneously sends the UID, the reader determines that the collision occurs, and sends a FAIL command to the tag. After the tag modifies the relevant parameter value by the internal anti-collision algorithm, the qualified tag will send the UID again. The reader/writer, the reader determines the conflict, repeats the above operation until only one label meets the condition, then jumps out of the anti-collision program and enters the label subsequent processing program. At the same time, the remaining tags automatically modify their own relevant values ​​to prepare for the next read. If there is no qualified tag at this time, the reader sends a SUCCESS command, the tag modifies its own parameters, and waits for the reader to detect the command.
3, the conclusion
Based on the international standard ISO15693 based on RFID, the RFID reader is designed to work at 13.56MHz. It can read and write tags in all directions. It is equipped with communication interfaces such as input and output IO, RS232, RS485 and CAN bus. With two antennas, the maximum read/write distance can reach 1.5m-1.8m, and the multi-card recognition capability reaches 45 sheets per second, which can effectively meet the needs of various RFID applications. The access control system based on the reader has been applied in practice, and the actual effect is good.
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