Design of RFID receiving system based on MSC1211 single chip microcomputer

1 Introduction

Radio Frequency Identification (RFID) is a non-contact automatic identification technology that uses radio frequency signals and their spatial coupling and transmission characteristics for non-contact two-way communication. Data exchange is not through the contact of current but through electric and magnetic fields, that is, through wireless communication, to achieve automatic identification of stationary or moving objects. At present, my country's research on RFID technology is still in the primary stage of low-frequency RFID. Its low-frequency RFID receiver has the disadvantages of short identification distance, low accuracy, high power consumption and poor anti-interference. In order to be suitable for various occasions requiring long identification distance, high read and write data rate and strong anti-interference, we designed a high-performance ultra-high frequency radio frequency identification receiving system. The system can directly convert and demodulate the frequency range of 800MHzZH to 1.5GHz, completely covering the UHF frequency band used by RFID readers, and uses 4 low-power, high-precision, and strong anti-interference Lingte chips in the radio frequency receiving circuit, achieving the purpose of simplifying the system circuit and improving the reliability of the receiving system.

2 Overall design of receiving system

The main function of the RFID receiving system is to receive, decode and process the code information of the electronic tag. Therefore, the receiving system is mainly composed of four parts: signal receiving circuit, amplifier circuit, low-pass filter circuit and information processing unit MSC1211. The signal receiving circuit, amplifier circuit and low-pass filter circuit constitute the baseband circuit of the RF receiving system. The overall structure of the system is shown in Figure 1.

Figure 1 Overall structure of the receiving system

As shown in Figure 1, the antenna shared by the transmitter and receiver is responsible for detecting the RF carrier and transmitting the signal to the RF input of the demodulator through a bandpass filter. The demodulator demodulates the signal and sends it to the operational amplifier for amplification and output, which is then used to drive the single-ended input of the low-pass filter. The low-pass filter performs baseband filtering on the signal and then sends it to the A/D pin of the MSC1211 microcontroller for analog-to-digital conversion.

2.1 MSC1211 microcontroller

The MSC1211 single-chip microcomputer is the latest high-performance chip integrating digital/analog mixed signals launched by Texas Instruments of the United States. It has a high computing speed and a clock frequency of 33MHZ, which reduces system noise and power consumption and improves the ability to process received signal RF data. The MSC1211 integrates a 24-bit resolution analog-to-digital converter, which makes the A/D conversion accuracy reach 24 bits and improves the accuracy of the converted data. The MSC1211 chip is the control core of the entire system. Its main function is to analyze and process the received signals from multiple tags and provide additional filtering processing.

2.2 Demodulator and Operational Amplifier

The RF demodulator uses the LT5516 chip from Linear Technology, which directly converts the demodulator's frequency range from 800MHzZH to 1.5GHz. The superior linearity of the LT5516 provides high sensitivity to low-level signals and will not be affected even by large interference signals. The operational amplifier uses the LT6231 chip from Linear Technology, which acts as a differential to single-ended amplifier to drive the single-ended input of the low-pass filter. The differential I or Q output of the LT5516 is converted to a single-ended output of the LT6231 differential amplifier. The circuit is shown in Figure 2.

Figure 2 Baseband interface circuit

As shown in Figure 2, adding a 270pF external capacitor across the 60Ω resistor limits the demodulator output to 10MHz to prevent any high-frequency interference from being transmitted to the LT6231 amplifier. Since the amplitude-shift-keyed RFID signal does not require DC coupling, AC coupling is used for the baseband amplifier. The high-pass pole provided by the AC coupling capacitor and the amplifier input resistor is set to 8kHz, and the input resistor of the differential amplifier is set to 140Ω to minimize input-related noise.

2.3 Low-pass filter circuit

The analog baseband filtering process is completed by the LT1568 chip of Lingte Company. It is a low-noise, precise RC filter unit component, and it also provides a simple solution for low-pass and band-pass filters with a cutoff frequency of 100kHz~10MHz. For the 250kHz~4MHz signal spectrum commonly used in UHF RFID systems, these cutoff frequencies can fully meet the needs of RFID communication. In the design of this system, two LT1568 chips are used to connect into a dual-channel, fourth-order filtering circuit. The single-ended input to differential output conversion gain of the LT1568 filter is 6dB, and the stopband attenuation is 34dB. The I and Q filter matching is guaranteed by the inherent matching of the A and B sides of the LT1568, realizing an elliptical low-pass filter function. The circuit is shown in Figure 3.

Figure 3 Dual-channel, fourth-order filter circuit

2.4 LCD display and USB interface circuit

The LCD display uses the character LCD display module DMC24138 chip, which integrates the dot matrix LCD display FRD7168, dot matrix driver HD44100 and controller HD44780 on the same double-sided printed circuit board. HD44780 is a dedicated control chip for LCD displays produced by Hitachi. The internal I/O buffer of the chip receives the control commands sent by the MSC1211 microcontroller using the read, write, RS and other pins, and realizes the transmission of instructions, data and status information between the microcontroller and HD44780. We chose the UART to USB chip CP2101 chip that does not require external components. The chip has the characteristics of low power consumption and high speed, complies with the USB2.0 specification, and meets the needs of high-speed communication between the MSC1211 microcontroller and the PC host.

3. Software Design

The main task of the RFID receiving system is to read the information from the tag when it enters the effective range of the reader. The read information is displayed on the LCD and can also be transmitted to the PC host through the USB interface for further processing. The main flow of the program is shown in Figure 4.

Figure 4 Main program flow chart

3.1 Initialization Procedure

Allocate and initialize the I/O ports and registers used by the program to avoid conflicts.

3.2 Sampling and processing procedures

First determine whether it has entered the sleep time range. If it has, start to detect whether there is a high level; if a high level arrives, prepare for sampling. The sampling point must be roughly located in the middle of each bit, so it is necessary to appropriately extend a period of time to detect the valid ID bits of the electronic tag. In order to improve the reliability and security of RIFD, the same tag must be sampled 5 times in a row. If the valid ID bits obtained by each sampling are the same, the sampling is successful.

3.3 LCD display program

Allocate and initialize the I/O ports to be used by the LCD screen, and display the corresponding electronic label information based on the processing results.

3.4 USB and host communication program

When the PC sends a control command to the reader to read the electronic tag ID, the reader starts sampling and sends the read data to the buffer, and then uploads the data to the PC through the USB interface circuit for further processing.

3.5 Data Verification

When using RF technology to transmit data, it is easy to be interfered by the outside world, causing the transmitted data to change and cause errors. Checksum is a method used to identify and correct data with certain measures. The checksum methods commonly used in circuit design include cyclic redundancy check (CRC), Hamming code, parity check code, etc. Our design uses the simplest error detection code-parity check code. Parity check is a simple and widely used checksum method. This method combines a parity check bit into each byte and transmits it, that is, nine bits are sent per byte. Before data transmission, it must be determined whether to use even checksum or odd checksum to ensure that both the transmitter and the receiver use the same method for checksum. This design uses even checksum.

4 Conclusion

The innovation of this system is that it designs a UHF radio frequency receiving system that directly converts RF radio frequency identification signals. It does not require an intermediate frequency down-conversion circuit, simplifies the receiving system circuit, and achieves the purpose of improving the reliability of the RF receiver. In addition, the receiving system can adapt and optimize the system's baseband circuit according to different application scenarios, meeting the requirements of existing and emerging RFID technology standards.

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