Using SLRC400 chip to realize the design of reader recognition radio frequency card

How to prevent the occurrence of major accidents in coal mine production has always been a focus of attention. This article uses SLRC400 chips to form an RFID system to monitor personnel safety. Equipped with radio frequency cards for all underground personnel. When the underground personnel pass through the monitoring point, the reader recognizes the radio frequency card and transmits the radio frequency card number and location information to the upper computer through the data bus for processing. Once an accident occurs, the trapped personnel can be quickly inquired. So as to minimize casualties.

How to prevent the occurrence of major accidents in coal mine production has always been a focus of attention. This article uses SLRC400 chips to form an RFID system to monitor personnel safety. Equipped with radio frequency cards for all underground personnel. When the underground personnel pass through the monitoring point, the reader recognizes the radio frequency card and transmits the radio frequency card number and location information to the upper computer through the data bus for processing. Once an accident occurs, the trapped personnel can be quickly inquired. So as to minimize casualties.

System hardware structure

The structure of this system is shown in Figure 1. Among them, the core component of the radio frequency identification system uses a reader composed of SLRC400 chips to monitor the location of underground employees.

Using SLRC400 chip to realize the design of reader recognition radio frequency card

Figure 1 System structure block diagram

Electrical characteristics of SLRC400

SLRC400 is a non-contact IC card reader working at 13.56MHz. It supports the S015693 protocol and can drive the antenna to transmit to a longer distance (1.5 meters) in a passive situation. Its main features are: the digital part has a CRC check function; it has a parallel interface, which can be directly connected to any 8-bit microprocessor, which provides greater flexibility for reader and terminal circuit design; highly integrated demodulation and coding simulation Circuit; flexible interrupt processing; programmable timer; unique serial data; user-programmable startup structure: independent power supply for digital, analog and transmission parts; external RS-485 and other chips can be connected.

The peripheral circuit design of SLRC400

The hardware design of the system includes the hardware circuit design of the reader composed of SLRC400 and the design of the CAN bus communication part. Among them, the SLRC400 peripheral circuit includes EMC low-pass filter circuit, receiving circuit, antenna matching circuit and antenna.

EMC low-pass filter circuit

The operating frequency of SLRC400 is 13.56MHz. As the clock signal of SLRC400 is generated by the oscillation of quartz crystal, it is also the basis for driving the antenna at 13.56MHz carrier frequency. This not only causes the emission of 13.56MHz energy, but also emits higher harmonics. . International EMC regulations stipulate the amplitude of the emitted energy in a wide frequency range. Therefore, in order to meet this requirement, a suitable filter is added.

Receiving circuit

The internal receiving circuit of SLRC400 works when the radio frequency card enters the range of the reader. When the input is connected to the pin RX, the internally generated VMID is used. In order to provide a stable reference voltage, the grounding capacitor C3 is connected to VMID. The receiving part of the reader needs to add a voltage divider between RX and VMID.

The problem of antenna coil inductance selection and impedance matching

It is impractical to accurately calculate the inductance value of the antenna coil, but it can be estimated with the coil inductance value formula. The actual capacitance and inductance of the antenna depends on many parameters, such as the structure of the antenna (the type of PCB), the thickness of the wire, the distance between the winding coils, the shielding layer, and the metal or ferrite in the surrounding environment.

The size of the capacitance value will seriously affect the performance of the reader. Software or hardware can be used to determine the capacitance value. A simple method is: SLRC400 has a SIG0uT pin. When the reader issues a command, it can be observed through an oscilloscope. The output signal of this pin constantly changes the distance between the card and the reader and the C2 value, the oscilloscope will output different waveforms, and the best performance of the reader can be determined according to the different waveforms.

Software implementation of the system

System software design

The system software structure is shown as in Fig. 2. The server, client and database adopt Windows 2000 Advanced Sever, Windows 2000 operating system and SQL Sever 2000 respectively. There are many softwares for developing databases, but VC++ has become the preferred tool for this design with its WYSIWYG interface design, efficient code execution and extremely fast compilation speed. Among them, the internal microcontroller of the radio frequency identification system uses the C51 high-level language, and the SLRC400 uses its standard program. In addition, the system also includes the application program design of the reader’s other circuits.

Using SLRC400 chip to realize the design of reader recognition radio frequency card

Figure 2 System software structure diagram

SLRC400 application algorithm

The binary search algorithm is composed of a set of commands and response rules stipulated between a reader and multiple radio frequency cards. The purpose is to select any card from multiple cards to realize data communication. In order to select one of a group of radio frequency cards, the reader sends a card reading command to consciously guide the data collision during the transmission of the radio frequency card serial number to the reader, that is, the reader judges whether there is a collision. The algorithm has three key elements: selecting a baseband code that is easy to identify collisions; using the unique characteristics of the serial number of the radio frequency card; designing a set of effective command rules to achieve card selection efficiently and quickly.

The instruction rules used in this system are: Inventory Request-request (serial number): request a response from the reader; Select (SNR)-select (serial number): use a (pre-determined) serial number as a parameter to send to the radio frequency Card. If the serial number of a radio frequency card in the field is the same as this parameter, the radio frequency card is selected and responds to other commands, while the radio frequency cards with other serial numbers only respond to the Inventory Request command; Stay quiet-quiet state: cancel A pre-selected radio frequency card, the radio frequency card enters a quiet state (inactive), and does not respond to the received Inventory Request command. In order to reactivate the radio frequency card, you can first move the radio frequency card out of the range of the reader antenna and then enter to reset, or receive the select and Reset to Ready commands.

What determines the reliability of the binary search algorithm system function is that all radio frequency cards need to be accurately synchronized, so that the occurrence of collisions can be judged on a bit-by-bit basis. In order to prevent many RF cards from colliding, the Inventory Request command needs to be repeated.

SLRC400 application programming

The behavior of the read-write chip SLRC400 is determined by executing the internal state of 9 specific commands. The statements or data required to execute the command are exchanged through the FIFO buffer. Start up command resets and initializes; IDLE switches SLRC400 to inactive state; Transmit transmits data from FIFO buffer to radio frequency card; Receive command activates the receiving circuit; Transceive transmits data from FIFO buffer to radio frequency card; WriteE2 command puts the slave buffer The data obtained by the processor is written to the EEPROM; ReadE2 puts the data read from the EEPROM in the FIFO buffer; LoadConfig reads the data from the EEPROM and initializes the register; CalcCRC activates the coprocessor. All registers are initialized before execution, and then the reader cyclically sends the Inventory Request command at a certain time interval to monitor whether there is a radio frequency card within the reading distance. If so, the radio frequency card responds to the Inventory Request command and sends the card number and CRC check value to the reader. If there is a communication error or no collision, use Transmit and WriteE2 to send to the microprocessor through the data bus, and then send the Stay quiet command to make the RF card just now enter a quiet state. If there is a collision, call the anti-collision program, and use the binary search algorithm to narrow the search range until a radio frequency response remains.

Concluding remarks

This system can realize the identification of personnel at a distance of 1.5 meters, and transmit the radio frequency card number and location information to the data center station through the data bus. With this system, the location of underground employees can be grasped in time. However, the recognition distance of this system is relatively short, and how to improve the recognition distance of the reader is the focus of future work.

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