Design of tire pressure monitoring system based on embedded microprocessor

1 Introduction

As the automobile consumer market heats up, consumers are paying more attention to the safety performance of automobiles. When driving at high speed, tire failure is the most worrying and difficult to prevent for all drivers, and it is also an important cause of sudden traffic accidents. According to statistics, 70% of traffic accidents on highways in China are caused by tire blowouts, while in the United States, this proportion is as high as 80%. Based on this, the National Highway Traffic Safety Administration (NHTSA) of the U.S. Department of Transportation (DOT) has issued regulations. The regulations stipulate that between November 2003 and October 31, 2006, new light vehicles manufactured in the United States will gradually introduce tire pressure monitoring systems (tyre pressure monitoring systems) [2]. In view of such a large market potential, many companies at home and abroad have begun to develop them. This paper proposes a tire pressure monitoring system based on Motorola's MPXY8040 temperature and pressure sensor, MC68HC908RF2 embedded integrated microprocessor and RF transmitter, MC33594 receiver and MC68H908KX8 microprocessor. The MPXY8040 and MC68HC908RF2 and their peripheral circuits constitute the measurement and transmission subsystem, and the MC33594 and MC6-8H908KX8 and their peripheral circuits constitute the receiving, processing and display main system. 2 TPMS system composition [2] The tire pressure monitoring system is divided into a direct system and an indirect system. The indirect system determines the tire pressure change through the vehicle speed sensor and uses the ABS wheel speed sensor to indirectly measure the tire pressure. It has low cost but poor accuracy and cannot work at any time. The direct pressure measurement method uses the pressure sensor and wireless transmitter inside the wheel to transmit signals to the RF receiver to directly measure the temperature and pressure. It has reliable accuracy and high cost. The direct measurement system requires a remote sensing module to be installed on each tire to collect data, and a receiving module to be installed in the car to receive and process the data and provide an appropriate display for the driver. 3 Hardware design and implementation The system circuit schematic is shown in Figures 1 and 2.

Figure 1 Schematic diagram of tire subsystem circuit

Figure 2 Schematic diagram of the instrument panel main system circuit

3.1 Tire subsystem (sub-machine) hardware design

(1) Air pressure and temperature sensor and signal conditioning circuit. Motorola's MPXY80-40A [4] sensor is used to measure tire pressure and temperature. The MPXY8040A pressure and temperature sensor chip is a highly integrated chip designed by Motorola for measuring automobile tire pressure and temperature. The chip embeds a capacitive absolute pressure sensor and a temperature sensor, including a pressure transducer, a positive temperature coefficient diffused resistor temperature sensor and all necessary circuits to generate a calibrated 8-bit temperature and pressure digital output. It can measure tire pressure from 0kPa to 637.5kPa and tire temperature in the range of 40℃ to 125℃, suitable for gas media and general tire environment. Each chip has an ID to identify which tire it is. The

typical electrical parameters of the MPXY8040A pressure sensor are: 3V operating voltage, low power consumption, standby current of 600nA, pressure sampling current of 15mA, and temperature sampling current of 500μA. It has four working modes: standby/reset, pressure detection, temperature detection, and data output; the four working modes can be flexibly used to meet the system power saving requirements. The built-in low-frequency oscillator can be used to wake up the MCU.

(2) Microcontroller and transmitter circuit. It uses the Motorola MC68HC908RF2[5] chip, which is a high-performance, low-power, 32-pin, LQFP surface mount device with a power supply voltage of 1.8V-3.6V, suitable for an ambient temperature of -40 degrees to +125 degrees, a standby current of 0.1nA, a transmission current of 11.6mA for FSK, and a transmission current of 8mA for ASK. The chip is packaged with two modules; the first is a 2kb flash HC08 MCU, and the second is a radio frequency transmitter module that can transmit digital data signals in free space up to several hundred meters.

The MC68HC908RF2 chip has an HC08 core, 2K flash user memory, 256 bytes of dedicated flash data memory, 128 bytes of RAM, 17-bit addressing, 16-bit index register and stack pointer, 8 general I/O ports, 6 inputs with keyboard interrupt function, and 2 LED outputs; in addition, it also integrates a radio frequency transmission module, a low-power PLL phase-locked loop radio frequency transmitter, which can complete the modulation and transmission of digital signals, support 315/434 and 868/915MHz FSK and ASK modulation methods, and has the function of controlling the transmission and output power. The output power is adjustable (up to 5dBm), the maximum data transmission rate is 11kbps, the integrated PLL phase-locked loop and VCO voltage-frequency oscillator, and the PCB printed antenna is used. The transmitter does not need to be adjusted, with few external components and stable and reliable operation.

(3) Power supply: It is powered by a 3V button-type lithium battery.

3.2 Hardware design of control and display part (host)

(1) Host receiving circuit: The host receiving and decoding circuit adopts Motorola's MC33594[3] chip. MC33594 is compatible with the transmitting circuit of Motorola's MC68HC908RF2 chip. MC33594 chip can receive and demodulate Manchester coded data and output data to the embedded microprocessor MC68H908KX8 through the universal SPI bus. Modulation mode: ASK and FSK; operating frequency: 315/433MHz or 866/915MHz; low current consumption, 4.3mA in run mode and 10mA in stop mode; internal and external selection; 1ms wake-up time; data transmission speed up to 11kbps; data manager (DM) with byte or tone detection; clock recovery with Manchester coded signal; can be fully configured by SPI interface; fewer external components.

(2) Host microcontroller: The MC68H908KX8[6] chip of Motorola Company is used; compared with the MC68HC908RF2 chip, this chip only lacks the UHF transmitter chip. Other performance indicators are the same.

4 Communication protocol and data format[1][5]

The system data is communicated using Manchester coding and FSK modulation. The data transmission rate is 9600bps. Manchester coding uses high frequency to low frequency to represent 0, and low frequency to high frequency to represent 1. FSK modulation uses two different frequencies to represent high and low signals of the digital. In this system, the lower frequency is used to represent 1 and the higher frequency is used to represent 0. The data manager of the receiver MC33594 chip has been set to such modulation and demodulation mode. When the tire module is set to send data, it first wakes up the receiver, then sends the data frame, and finally goes to sleep. A complete data frame consists of the following parts: (1) The leading same bit (preamble), a total of 16 bits, usually set to hexadecimal fb86, the first 4 bits (1111) are used to wake up the receiver and establish the internal circuit; the next 8 bits (b8) are used to configure the configuration value in register cr2, the purpose is to start the data manager, and the remaining 4 bits (0110) are used to activate mc33594 to make it work, indicating the beginning of valid data; (2) The valid data following the leading bit (preamble) is the device ID number, a total of 32 bits, used to distinguish different transmitters, (3) pressure and temperature signals, each occupying 8 bits; (4) status signal, occupying 8 bits; (5) 8 bits of checksum and 2 bits of stop bit. The checksum bit can help reduce interference caused by other factors and ensure the integrity and correctness of the data. The two stop bits notify the receiver that the data transmission is completed; the entire complete data frame contains a total of 82 data bits.

5 System software design

5.1 Design of signal detection processing and transmission program

The module is awakened every 3 seconds, measures temperature and pressure parameters, and stores the maximum and minimum values ​​in RAM. If there is no change, the counter count value is increased and the module returns to sleep mode. After 10 counts, about 30 seconds later, the data is transmitted to the receiver module. If there is a large change, the module enters the fast transmission mode and transmits 255 times continuously. The specific flow chart is shown in Figure 3.

Figure 3 Workflow diagram of tire microprocessor

5.2 Tire signal data transmission process

First, measure the battery voltage. If the voltage is normal, enter the working mode, measure data, send data, and enter the sleep mode after data transmission is completed. The specific flow chart is shown in Figure 4.

Figure 4 Tire pressure and temperature data measurement and transmission flow chart

5.3 Instrument panel receiving system program flow

After power-on reset, initialize mc68h908-kx8, configure mc33594, and self-check the host system. If the self-check passes, the LED flashes, otherwise the relevant LED is lit. Then wait for the SPI data frame. After receiving it, perform data verification. If it is correct, determine whether it is the tire sensor ID. If it is, display it, otherwise ignore it. The specific flow chart is shown in Figure 5.

Figure 5 Flowchart of the receiving and processing system of the car dashboard host

6 Conclusion

The tire pressure monitoring system based on Motorola's embedded microprocessor has high integration and good stability. It can simultaneously monitor three key parameters: tire pressure, temperature and monitoring module power supply voltage. When there are abnormal conditions such as tire leakage and temperature rise, the system will automatically alarm, thereby ensuring driving safety, extending the service life of the tire and reducing fuel consumption. The hardware structure uses low-power devices, and the software design uses appropriate sampling cycles and working modes to meet the requirements of low power consumption and safety. In addition, the host display module can also adapt to the needs of different users by expanding LCD display, voice alarm and providing an interface with the vehicle control system. With the improvement of people's requirements for active safety of automobiles and the positive role of TPMs in preventing major traffic safety accidents, the market demand for high-performance TPMs will further increase. Therefore, the automobile tire pressure monitoring system has a broad development space and a very good market prospect.