This topic attempts to implement a portable radio frequency identification reading and writing system. The AT90 series MCU with enhanced RISC structure and built-in Flash is used as the main control device. The data is stored in the RF IC card, communicates with the reader through non-contact data transmission, and the corresponding circuit principle and program flow are given. Due to its anti-interference and high operational reliability, radio frequency identification systems have developed rapidly in recent years and are widely used.
1 IntroductionIn recent years, automatic identification methods have been rapidly popularized and promoted in the service field, cargo sales and logistics, commerce, manufacturing companies and material circulation. The purpose of automatic identification is to provide information about individuals, animals, goods and goods.
At present, the operating frequency of the RF card on the market is 125 kHz, and the power consumption is very large. Therefore, the coil of the transponder must be several hundred åŒ, and the resulting large winding interface and the need for additional capacitors are difficult to package in small plastic cards. Inside.
This scheme adopts a transponder system with a working frequency of 13.56MHz. Its transponder coil is composed of about 5 turns, which is greatly reduced in size, easy to carry, and has a new market, such as using a non-contact IC card (RF card) as a public. Traffic tickets are also applicable to access control systems such as access control, attendance, conference attendance, and highways, gas stations, parking lots, and public transportation. At the same time, it can also be considered for the blind navigation system. The reader is installed in the crutches. The RF card is only one dollar coin size and can be conveniently installed on the blind road to remind important buildings and intersections. Although there are GPS blind navigation systems nowadays, due to their high cost and certain errors, the promotion is limited. This system adopts AT90S8515 single-chip microcomputer with high cost performance as the main control device, which has stronger research, practicality and generalization.
2 communication protocolThe communication mode of this system is serial, asynchronous and full duplex. The international standard ISO14443 specifies the working principle and working parameters of the 13.56MHz RF card. The ISO14443 standard consists of four parts: the first part is the physical characteristics; the second part is the RF interface; the third part is the initialization and anti-collision; the fourth part is the transmission. protocol.
The standard specifies that the RFID system has a baud rate of 106 Kb/s, a carrier frequency of 13.56 MHz, data modulation from the reader to the RF card to the carrier frequency, and uses 10% ASK, asynchronous timing and non-return to zero (NRZ). The coding method; the subcarrier is its 16-divided frequency, which is 847 kHz. The data from the RF card to the reader is modulated on the subcarrier, and the communication uses the 847 kHz subcarrier phase shift modulation (PSK) and non-return to zero (NRZ). Encoding.
The RF card model used in this solution is AT88PF256-13, which is fully compliant with ISO14443, and specifies the specific transport packet format according to ISO14443, as shown in Figure 1.
There is no protection bit (EGT) in the frame format from the reader to the RF card, and the protection bit (EGT) is included in the frame format from the RF card to the reader.
3 hardware designThe main control device is the more advanced AT90S-8515 in the AVR series. It completes reading the keyboard and display, controls its modulation and demodulation unit, and couples the signal to the transponder (PICC) through the antenna to complete the reading and writing of the transponder. operating. In addition, online programming and debugging is also possible through the serial download port.
The AT90S8515 has the following features:
8k byte Flash program memory;
512 bytes: EEPROM;
512-byte SRAM;
32-bit (4&TImes; 8 bit) general purpose I/O port;
32 general working registers;
1 Preseale 8-bit timer, counter;
1 pre-scalable, 16-bit timer with counter, capture and 8/9/10-bit PWM function, counter;
2 external interrupt sources;
On-chip analog comparator;
Programmable UART;
Programmable watchdog timer;
SPI port and 2 power saving modes that can be selected by software.
3.1 input, output unit
AT90S8515 external 4x4 keypad, which contains special function keys SHIFT, KTR, KENT, KESC, complete the input function, and then connect a standard LCD to complete the output function. The main control device obtains the operation command word through the keyboard, converts it into the frame format specified by the ISO14443 standard, and transmits it through the modulation and coupling (antenna) unit. The transponder returns data within its response range, through the coupling (antenna) and the demodulation unit. Reach the master device, remove the frame header and the end of the frame, and display it through the standard LCD. The connection circuit of the input and output sections is shown in Figure 2.
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