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0 Preface
RF cards, also known as contactless IC cards, are currently being used in a wide variety of applications. Chips, sensing antennas, and capacitors are integrated into the RF card. When reading and writing, the RF card is placed close to the card reader, and the electromagnetic wave emitted by the card reader antenna generates an induced current on the antenna in the RF card to provide energy for the integrated chip in the card. The chip is pre-stored with a unique identification number, which is encoded to modulate the current signal on the antenna and then transmitted back to the card reader in the form of electromagnetic waves [1]. Most RF cards encode the ID number (ID number) in the card into a Manchester code, which is then decoded by the microcontroller.
However, many current MCU decoding programs use the timing query or the edge state of the signal to be decoded. These decoding methods have higher requirements on the carrier frequency on the antenna, and the accuracy of the timing is relatively high. When the carrier slightly deviates from the specified The card will not be read correctly within the range. This paper introduces a new decoding technique. The offset of the carrier frequency has no effect on the decoding, and it does not need to detect the edge state of the signal, so that the card can be read more reliably and quickly.
1 card reading principle
Now, the decoding technology of Manchester code is introduced by taking a read-only RF card with EM4100 A6 chip in the card as an example. The basic principle is shown in Figure 1.
When the resonant signal of the decoder chip EM4095 is generated and there is a carrier signal with a constant voltage of about 125KHz, the carrier signal of the same frequency will also be generated on the antenna of the EM4100 through the induction of the electromagnetic field, and the EM4100 will identify the 64-bit Manchester-coded identification number in the card. Modulated onto this carrier signal, through the coupling of the signal, the EM4095 antenna also generates a modulated signal with a 64-bit ID number, and then through the EM4095 demodulation system, filter the carrier [2] [3], 64-bit data It is transmitted to the MCU, and the ID number is decoded by the MCU.
The EM4100 chip in the RF card has pre-stored un-rewritable 64-bit data. When it is output through the antenna, the format is shown in Figure 2 [2].
The beginning of all 64-bit data is a sync header consisting of 9 1s. Due to the format of data and even parity, 9 1s will not appear in the following data strings, which guarantees the uniqueness of the data header. The sync header is followed by 10 sets of 4-bit data Dxx. The first two sets of 4-bit data are the manufacturer numbers, and the last 8 sets of 4-bit data are 32-bit internal code numbers. Each set of 4-bit data is followed by an even parity bit Px. The last set of 4-bit data is the even parity bit PCx for each of the first 10 sets of data. The last bit S0 is the stop bit and the stop bit is always 0. When the card number is transmitted, the 64-bit data continues to appear end to end on the antenna through the carrier.
When reading the card, the 64-bit data is sequentially output in Manchester-encoded format, modulating the current of the carrier on the antenna, so that the receiving device recognizes the reception. The receiver EM4095 detects 64-bit data from the carrier and sends it to the microcontroller in the order shown in Figure 2. The 40-bit ID number is further extracted by the microcontroller. Since the signal sender EM4100 and the receiver EM4095 transmit information in a coupled manner, the actual modulation signal on the antenna is as shown in FIG. 3, so that the high current is used to represent logic 0 on the EM4100 carrier and on the EM4095 carrier. The low current represents a logic zero.
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