This project focuses on developing a portable radio frequency identification (RFID) system for reading and writing data. It utilizes the AT90 series microcontroller, known for its enhanced RISC architecture and built-in Flash memory, as the central control unit. The system stores data on an RF IC card, which communicates with the reader via non-contact data transmission. Detailed circuit design and program flow are also presented in this work. Due to its strong anti-interference capability and high reliability, RFID technology has seen rapid development in recent years and is now widely applied in various fields.
In recent years, automatic identification technologies have gained widespread use in sectors such as services, logistics, commerce, manufacturing, and material distribution. These systems aim to provide accurate information about individuals, animals, and goods, enhancing efficiency and reducing manual errors.
Currently, most RF cards operate at 125 kHz, but they consume significant power, requiring several hundred turns of coil. This leads to large physical dimensions and the need for additional capacitors, making it challenging to integrate them into compact plastic cards.
To address these limitations, this system uses a transponder operating at 13.56 MHz. Its coil consists of only about five turns, significantly reducing size and making it more portable. This opens up new market opportunities, such as using non-contact IC cards (RF cards) for public transportation tickets, access control systems, attendance tracking, and highway tolls. Additionally, it could be used in blind navigation systems by embedding the reader in walking sticks and placing small RF cards along pedestrian paths to signal important locations. While GPS-based navigation systems exist, their high cost and occasional inaccuracies limit their widespread adoption.
The system employs the AT90S8515 microcontroller, offering excellent cost-performance, which makes it ideal for research, practical applications, and generalization.
The communication protocol of this system is serial, asynchronous, and full-duplex. The international standard ISO14443 defines the working principles and parameters for 13.56 MHz RF cards. This standard is divided into four parts: physical characteristics, RF interface, initialization and collision handling, and data transmission protocol.
According to the ISO14443 standard, the RFID system operates at a baud rate of 106 Kb/s, with a carrier frequency of 13.56 MHz. Data from the reader to the RF card is modulated using 10% ASK, with asynchronous timing and non-return-to-zero (NRZ) encoding. A subcarrier at 847 kHz (1/16 of the carrier frequency) is used for data transmission from the RF card to the reader, employing phase shift keying (PSK) and NRZ encoding.
The RF card model used in this solution is the AT88PF256-13, which fully complies with ISO14443. It follows the specific packet format defined by the standard, as illustrated in Figure 1.
The frame format from the reader to the RF card does not include a protection bit (EGT), while the reverse direction includes EGT for data integrity.
The main control device in this system is the advanced AT90S8515 from the AVR series. It handles keyboard input, controls the modulation and demodulation units, couples signals to the transponder (PICC) through the antenna, and performs reading and writing operations. It also supports online programming and debugging via a serial download port.
Key features of the AT90S8515 include:
- 8 KB of Flash program memory
- 512 bytes of EEPROM
- 512 bytes of SRAM
- 32-bit general-purpose I/O ports
- 32 general-purpose working registers
- One 8-bit pre-scale timer/counter
- One pre-scaleable 16-bit timer/counter with capture and PWM functions
- Two external interrupt sources
- On-chip analog comparator
- Programmable UART
- Programmable watchdog timer
- SPI port and two software-selectable power-saving modes
3.1 Input and Output Unit
The system uses a 4x4 external keypad connected to the AT90S8515, including special function keys like SHIFT, KTR, KENT, and KESC, to handle input commands. A standard LCD is used for output display. The microcontroller processes the input command, converts it into the frame format specified by ISO14443, and sends it through the modulation and coupling unit. The RF card responds within its range, and the data is returned via the coupling and demodulation unit. After removing the frame header and trailer, the data is displayed on the LCD. The connection diagram for the input and output sections is shown in Figure 2.
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