Design Case of RFID System Used in Vehicle

This project focuses on the key issues of data collection, transmission and application in the vehicle Internet of Things, and designs a new generation vehicle radio frequency identification system based on short-range wireless radio frequency communication technology. The system consists of a short-range wireless communication on-board unit (OBU) and a base station system (Base StaTIon System, BSS) to form a point-to-multipoint wireless identification system (Wireless idenTIficaTIon system, WIS), which can be used within the coverage of the base station Vehicle identification and intelligent guidance.

1 System hardware design

The system hardware is mainly composed of the control part, the radio frequency part and the external expansion application part. Take the low-power MCU as the control unit, integrated single-chip narrow-band UHF transceiver, built-in optimized design antenna. Using advanced photovoltaic battery power supply, practical high integration short-range wireless identification radio frequency terminal (OBU). The terminal is small in size, low in power consumption, wide in range, and establishes an open protocol and standard interface, which is convenient for interfacing with existing systems or other systems.

The system working diagram is shown in Figure 1.

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1.1 Control circuit design

The control unit uses the MSP430 series produced by TI, which is a relatively mature industry with low power consumption. This series is a 16-bit ultra-low power mixed signal processor (Mired Signal Proessor) that TI began to market in 1996. Application requirements Integrate many analog circuits, digital circuits and microprocessors on a single chip to provide a "single-chip solution. The working principle of OBU and BSS is the same in the WIS system, so focus on the design of the OBU part and the schematic diagram of its control part. as shown in picture 2.

The input voltage of MSP430F2274 is 1.8 ~ 3.6V. When operating under a 1 MHz clock, the chip consumes about 200 to 400 μA, and the minimum power consumption in clock-off mode is only 0.1 μA. Due to the different functional modules that are turned on during system operation, standby, operation, and sleep 3 are used. A variety of working modes effectively reduce system power consumption.

The system uses two clock systems; the basic clock system and the digitally-oscillator clock system (Digitally Controlled Oscillator, DCO), using an external crystal oscillator (32 768 Hz). After power-on reset, the MCU (Microprogrammed Control Unit) is started by DCOCLK to ensure that the program starts from the correct position and the crystal oscillator has sufficient start-up and stabilization time. Then the software can set the control bits of the appropriate register to determine the final system clock frequency. If the crystal oscillator fails when used as the MCU clock MCLK, the DCO will automatically start to ensure the normal operation of the system; if the program runs away, it can be reset by the watchdog. This design uses on-chip peripheral module watchdog (WDT), analog comparator A, timer A (Timer_A), timer B (Timer_B), serial port USART, hardware multiplier, 10-bit / 12-bit ADC, SPI bus, etc. .

1.2 RF circuit

The radio frequency part adopts TI's CC1020 as the radio frequency control unit. The chip is the industry's first true single-chip narrow-band UHF transceiver. It has FSK / GFSK / OOK three modulation methods, and the minimum channel interval is 50 kHz, which can meet multiple channels. Strict requirements for narrowband applications (402-470 MHz and 804-94O MHz bands), multiple operating frequency bands can be freely switched, and the operating voltage is 2.3-3.6 V, which is very suitable for integration and expansion to mobile devices as wireless data transmission or electronic tags. The chip complies with EN300 220.ARIB STD-T67 and FCC CFR47 part15 specifications. The carrier frequency of 430 MHz is selected as the working frequency band. This frequency band is the ISM frequency band, which complies with the standards of the National Radio Regulatory Commission, and there is no need to apply for a frequency point. The FSK modulation method has high anti-interference ability and low error rate. The forward error correction channel coding technology is used to improve the ability of data to resist burst interference and random interference. The channel error rate is 10-2 When the actual bit error rate is 10-5 ~ 10-6, the data transmission distance can be reached when the line-of-sight conditions are open, the baud rate is 2A Kbs, and the large suction cup antenna (length 2m, gain 7.8 dB from the ground height 2m) 800 m. The standard configuration of the RF chip can provide 8 channels to meet multiple communication combinations. Due to the use of narrow-band communication technology, communication stability and anti-interference are enhanced. The schematic diagram of the RF part is shown in Figure 3.

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1.3 System power supply

The power supply part of the system is a combination of photovoltaic battery as the daily work power supply and lithium sub-battery as the backup battery. Under good light conditions, the energy storage battery is charged by solar energy, and a certain amount of light time is guaranteed every day to basically meet the daily work needs of OBU, which greatly extends the service life of the backup battery and the working life of OBU. It is suitable for vehicles that often run outdoors, and can collect sufficient sunlight for photovoltaic cells to work.

1.4 System development environment

The system development environment is as follows: 1) IAR Embedded Workbench fMSP430 compiler; 2) PADS PCB Design Solutions 2007 Bisi circuit board design tool.

2 System programming

The program adopts modular design and is written in C language. It is mainly composed of 4 parts: main program module, communication program module, peripheral circuit processing module, interrupt and storage module. The main program mainly completes the initialization of the control unit, the configuration of various parameters, and the configuration and initialization of each peripheral module; the communication program module mainly handles the configuration of the RF chip and the 433 MHz transceiver processing; the peripheral circuit processing module mainly indicates the external LED Voltage detection, voice prompts by keys and other processing; interrupt and storage modules mainly deal with system interruption and record storage. The main program flow is shown in Figure 4.

Main program flow

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