"[Abstract] this paper introduces a contactless IC card (H4001) reading mechanism based on P4095 chip, and proposes a decoding method of Manchester code. Key words: contactless IC card, Manchester code, decoding
After more than 20 years of development, integrated circuit card has been widely used in finance, telecommunications, insurance, commerce, national defense, public utilities and other fields. According to the connection mode of external interface equipment, IC card can be divided into contact IC card and non-contact IC card (also known as RF card). Contact IC card means that when the IC card communicates with the outside world, the electrode contact of the chip must be directly connected with the IC card reading and writing equipment; When the contactless IC card is used, it does not need to be directly connected with the IC card reader and writer equipment, but realizes the data communication with the IC card reader and writer equipment by means of radio wave or electromagnetic induction. In the environment of high card swiping speed, bad card environment and serious pollution, contactless IC card has its unique advantages. We use the p4095 chip produced by EM company (EM micronicmarin SA, 2074 Marin Switzerland) to make the IC card reading head. On this basis, we write the card reading program of EM company h4001 contactless IC card. 1 working principle
The working principle of card reading is shown in Figure 1. When the IC card (h4001) is close to the card reader, the two circuits in Figure 1 resonate at 125kHz. The IC card obtains energy from it and continuously sends signals to the outside. P4095 receives the signal and sends it to the single chip microcomputer to analyze and process the information of the card.
2 p4095 chip introduces that the chip integrates phase synchronization logic system, which can obtain the same carrier frequency as the coil resonance frequency; No external crystal oscillator is required; The carrier frequency is 100 to 150kHz; Support a variety of IC card transmission protocols, such as h400x, p4150, p4069, etc; The package form is so16 or psop216. The RF card reader made of p4095 can complete the following functions: (1) carrier frequency drive coil（ 2) Amplitude modulation of the magnetic field of a writable card（ 3) Demodulate the amplitude of the modulation signal caused by the card on the coil（ 4) It communicates with the microprocessor through a simple connection. The operation of the chip is controlled by SHD and mod. When the SHD is high, it enters the sleep mode. After the chip is powered, the SHD must be high to properly initialize the chip, and then the SHD is connected to the low level, so that the chip can send out the RF field, and the demodulation module sends the signal read on the coil to demod__ Out output. Mod grounding causes the chip to enter the read-only state. The application diagram is shown in Figure 2 below. 3 h4001 principle h4001 is a CMOS integrated circuit applied in different frequency wireless transmission. The circuit is driven by a coil placed in a magnetic field, from which a clock pulse is obtained. The switch modulation current can send back 64 bit information contained in the laser programmable logic device. The serial output data includes 9 start bits, 40 information bits, 14 check bits and 1 stop bit. The 64 bit information is modulated by Manchester code. Logic "1" generates a falling edge in a bit cycle, and logic "0" generates a rising edge in a bit cycle. The 64 bit Manchester code is shown in Figure 3. Once sent, it will be sent continuously. Figure 4 shows the relationship between the logic level and the Manchester code. It can be seen from the figure that the 64 bit signal is output continuously and circularly.
4 the decoding method of Manchester code according to the characteristics of Manchester code, we know that there are two kinds of high level duration in Manchester Code: half bit period and one bit period. The high level lasts for one bit cycle because the high level of the first half of the bit cycle belongs to the previous bit logic "0", and the high level of the second half of the bit cycle belongs to the latter bit logic "1". According to this feature, we analyze the waveform shown in Fig. 5. First, we find a down jump and then an up jump to confirm whether the signal (Manchester code) is received or not. Then enter a delay stage T (the delay t is greater than half a bit cycle and less than one bit cycle). Then read in the signal, which must be "1". Then the "1" must be a bit in the data sequence (if it is not "1", start all over again). The next step is to find the next hop of this bit "1". After finding it, delay T and continue to read the next bit of data. The Manchester code sequence can be read out correctly by repeating the operation of looking for jump and delay. Since h4001 outputs its information in electromagnetic field continuously and circularly, in order to ensure that 64 bit information on the card is fully read, we read 128 bits for each card.
According to the characteristics of h4001, we know that its data sequence starts with 9 consecutive "1", followed by data and check bits. Therefore, as long as nine consecutive "1s" are found in the above 128 bit information, and the subsequent data bits and row and column check bits are analyzed, the information of this card can be determined. 5 research results during the test, the program compiled according to the above principle can correctly read the information on the card every time. After measurement, the program card reading distance is 9.5cm ~ 11.5cm. The card reading time is 0.086s（ Measurement method: put the card on the coil, start the program, and stop when the signal is correctly read.) 6 procedure flow chart The program flow chart is shown in Figure 6. The procedure is omitted.
Reference 1 sun Hanfang, Xu Aiqing. Principle and application of MCS-51 and 96 series single chip microcomputer. Beijing: Beijing University of Aeronautics and Astronautics Press, 1988, reading the full text, technology zone
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