Realize the module design of emulating magnetic card with RFID card

In the 1980s of the 20th century, with magnetic cards and barcodes as information carriers, various identifications were gradually applied to the fields of service, logistics and enterprise management in China. Radio frequency identification (RFID) card has outstanding advantages such as non-contact, convenient operation, reliability, high recognition rate and long service life. It has been widely used in many fields since the 90s of the 20th century. At present, there are still a large number of application systems based on magnetic cards and barcodes being put into use or will be put into use. Many of these systems are designed to consider the magnetic card or barcode decoding part as an independent module. Based on this, put forward the following assumptions: If you can use cheap read-only RFID cards instead of magnetic cards or bar codes, and let the RFID card reading module directly replace the original magnetic card movement or barcode decoding module, then the entire system does not need to make any changes, you can Put into operation or continue to use. In this way, whether the transformation of old systems or the transition to new systems using RFID cards, can increase efficiency and reduce waste, there is still market and positive significance in the current. The module introduced in this article only considers read-only RFID cards instead of magnetic cards that do not need to modify information (in other cases, the card reader program is different).

1 module composition and working principle

The function of this simulation module is to decode and read the information of the read-only RFID card (usually a simple serial number, and simulate the magnetic card movement to output the data of the American banker S association (ABA) magnetic card format, and its block diagram is shown in the figure. 1.

Figure 1 Block diagram of the simulation module

The key part of the module is the antenna, ATMEL's radio frequency card reading and writing base station chip U2270B and one-chip computer AT89C2051. During work, the base station chip U2270B uses an antenna (generally using a copper enameled wire to make a diameter of 3 cm, and the coil can be wound with 100 turns and the inductance is 1.35 mH) to provide energy for the RFID card with a modulated RF signal of about 125 kHz (power supply). At the same time, it can receive information from H4001 and output it in Manchester. The AT89C2051 obtains the 64-bit information of the H4001 card from the U2270B, decodes it according to the Manchester encoding rules, verifies the data, and obtains the 32-bit binary number that represents the 10-digit decimal sequence number and converts it to the corresponding ABA magnetic card format data. , Then from the /CLD, CLK, DATA 3 signal line simulation magnetic card movement output data.

2 RFID Card H4001 and Manchester Code

In the module shown in Fig. 1, the supporting RFID card is EM Microelectronic's H4001. The card is a passive low-frequency RFID card. The typical operating frequency is 125 kHz. The energy required for operation is through the electromagnetic coupling unit or antenna. Non-contact mode of transmission. When sufficient energy is obtained, H4001 will continue to send its own serial number and other 64-bit information to the outside.

Manchester encoding and signal modulation are required for data transmission. The rule is as follows: In the middle of each clock cycle (corresponding to 1-bit data), the level transitions from high to low when the data bit is "0", and from low to high when the data bit is "1". Change; The other way to represent this module is just the opposite, the waveform shown in Figure 2.

Figure 2 Manchester encoded waveforms of a typical data sequence

For the H4001 with Manchester encoding, the data rate is RF/64. Assume that the base station operates with a radio frequency of 125 kHz, which corresponds to the period of a 1-bit clock (CLOCK) (T) Îœs.

3 Base station chip U2270B and module circuit

RF card readers need to complete the data modulation, transmission and RF reception and data demodulation tasks from the RF card base station chip. The U2270B produced by ATMEL is a low-cost, full-featured low-frequency (100-150 kHz) RF card base station chip that uses Man-chester encoding and bi-phase encoding and can be used to read H4001 RFID cards. References [3] and [5], for the H4001 RFID card, the circuit of the RFID reader module designed with U2270B is shown in Figure 3.

Figure 3 U2270B-based REID card reader module circuit diagram

By adjusting the resistance of the 15th pin (RF end) of the U2270B, the internal oscillation frequency can be fixed at 125 kHz. Calculated as follows:

When reading the card, the signal modulated by the 125 kHz carrier transmitted by the RFID card is received by the base station antenna and fed to the 4th pin (Input) of the U2270B. At the output of the second pin, the demodulated Manchester code is output. The data signal is sent to the microprocessor. It should be noted that the RF card H4001 returns a Manchester-encoded data stream. The base station U2270B is responsible for signal reception, rectification, and demodulation. The decoding work is done by the microcontroller AT89C2051.

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