Design of automobile instrument based on MB90F428

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Design of automobile instrument based on MB90F428 [Copy link]

Introduction
The automotive instrument is the interactive interface between people and cars. It provides drivers with the required information such as vehicle operating parameters, faults, mileage, etc. It is an indispensable part of every car. It has gone through the development process of mechanical, electrical, analog circuit and electronic. With the networking of automotive electronics, CAN bus technology has been more and more widely used in the automotive field. Therefore, CAN bus and embedded have become the inevitable trend of the future development of automotive instruments.

The basic structure and function of automobile instruments

There are four commonly used indicator instruments on cars, namely the speedometer, engine water temperature meter, engine tachometer, fuel meter, etc. They respectively display the car's driving speed, single mileage and total mileage, engine coolant temperature, engine speed when the car is driving, and the amount of oil in the car's fuel tank. There are often more than a dozen indicator and warning signal lights installed on the car dashboard, such as left and right turn signals, brake signals, high beam signals, ABS, battery charging, battery life alarm, oil pressure alarm, oil alarm, water temperature alarm, etc. These indicator lights are different in different dashboards and are usually displayed with LEDs.

Advantages of CAN bus and its application in automobile field

Controller Area Network (CAN) is a serial data communication protocol developed by Bosch in Germany in the early 1980s to solve the data exchange between numerous control and test instruments in modern automobiles. It is a multi-master bus whose communication medium can be twisted pair, coaxial cable or optical fiber. In the field of automation electronics, such as automotive engine control components, sensors, anti-skid systems, etc., the bus bit rate can reach up to 1Mbit/s. CAN networks are being continuously applied in various aspects of automotive electronics. The CAN bus has the following main features: (1) Multiple master stations access the bus based on priority; (2) Non-destructive bus arbitration based on priority competition; (3) Multi-address frame transmission with the help of receive filtering; (4) Remote data request; (5) Configuration flexibility; (6) Full system data compatibility; (7) Error detection and error signaling; (8) Automatic retransmission of frames that lose arbitration or are damaged due to errors during transmission.

Car dashboard hardware and software design

Combined with the technical and performance indicators of automotive instruments, as well as the requirements of simplifying hardware circuits, Fujitsu's MB90F428 chip is selected as the microcontroller for the design of automotive instruments. MB90F428 chip is a 16-bit single-chip microcomputer with an internal CAN bus interface and FLASH ROM. It is mainly used in automobiles and industries. Its CAN bus complies with V2.0 Part A and Part B, and can support more flexible information buffer processing. It supports high-level languages, expandable address modes, enhanced multiplication and division instructions, enhanced bit operation instructions, etc. The microcontroller has a 32-bit accumulator (long word processing); peripheral resources include: 8-channel 8/10Bit A/D converter, UART, extended I/O serial interface, 8/16Bit timer, I/O timer (input capture, output comparison), 8-way external interrupt, CANBUS interface, 4-way stepper motor driver module, LCD module (can drive 24 4 pen segment LCD module), etc. The 4-channel 16-bit input capture channel can capture the pulse signal input by the vehicle speed sensor and the engine speed sensor. The A/D converter can be used to convert the voltage signal input by the water temperature and oil level sensor. The I/O port is used to input the signal of many signal indicator lights. The CAN interface mainly sends and receives CAN signals with the CAN bus transceiver PCA82C250 chip. The stepper motor driver module and LCD display module are used to drive the 4 stepper motor indicators and mileage time display on the instrument panel. This design method of on-chip self-contained driver module improves the reliability of the system and reduces the cost.
This design is mainly divided into two modules: the detection circuit control module and the instrument driver module, as shown in Figure 1. The detection circuit module is mainly composed of input signal acquisition, signal processing, and signal conversion circuits. First, the vehicle status is detected by the corresponding sensor, converted into voltage and pulse signals, filtered and amplified, and then input into the MB90F428 chip for built-in A/D conversion and digital processing to obtain the required digital quantity signal, and send the processed digital quantity to the CAN bus in real time.

The instrument drive module is mainly composed of signal receiving, data storage, and device drive circuits. When the driver board receives the CAN signal, it will process the data through the MB90428 chip to drive the stepper motor, LCD, LED, etc. When the ignition switch is turned on, after the instrument detects this signal, it will first detect itself (the diagnostic indicator light is always on during this process), and determine whether the current instrument needs to be corrected based on the historical working conditions recorded in the FLASH RAM, and initialize the instrument through several procedures. After the self-test program is passed, the instrument begins to enter the working state from the non-working state, and will process the vehicle speed, speed, water temperature, oil level and other signals accordingly, and display the current working conditions through the pointer and indicator light.

Dashboard Hardware Design

Power Circuit

The car battery provides about 12V power, and the dashboard requires two power supplies: +5V and +12V. The 5V power supply is used to power MB90F428, CAN interface chip (PCA82C250) and EEPROM, and the 12V power supply is used to power LEDs, buzzers, etc. Considering the cost and availability, we chose the 7805 chip as the power conversion chip. In order to save the mileage data in time when the power is off, a 1000 F electrolytic capacitor is added to the input end of the power supply. When the power is disconnected, the large capacitor can maintain the power supply of the microcontroller for a long enough time so that the microcontroller can complete the service program of the external interrupt. As shown in Figure 2.

Conditioning circuit

Automobile speed sensors and engine speed sensors usually use Hall devices. When the wheel starts to rotate, the Hall effect sensor starts to generate a series of pulse signals. The number of pulses will increase as the vehicle speed increases, but the position duty cycle remains constant at any speed. In order to improve the waveform, a conditioning circuit is added outside the input capture timer pin to shape and amplify the pulse signal. Here we process it through RC filtering and triode amplification. As shown in Figure 3 and Figure 4.

We use the CAN transceiver PCA82C250 chip for data transmission and reception. It was originally designed for automotive high-speed communication (up to 1Mbps) applications. The device can provide differential transmission capabilities for the bus and differential reception capabilities for the CAN controller. It is fully compatible with the ISO/DIS11898 standard. The CANH and CANL double lines also prevent electrical transients that may occur in the automotive environment. Since automobiles often work in harsh environments, the transceiver is subjected to certain anti-interference treatment. Here, an optocoupler (6N137 chip) is selected, as shown in Figure 5.

Other circuits

In addition to the above circuits, this hardware design also includes EEPROM circuit, LED drive circuit, LCD display circuit, and stepper motor circuit. Since the MB90F428 chip is specially designed for automobiles and has most of the drivers built into the chip, it simplifies the design of the hardware drive circuit, saves costs, and improves system reliability.

Dashboard software design

Figure 6 shows the software flow chart of the dashboard main program. The program is controlled by the ignition signal. When the ignition switch is turned on, the dashboard enters the main program loop. The entire system software consists of the main program, data acquisition subroutine, AD conversion subroutine, data processing subroutine, CAN communication subroutine, LCD/LED display subroutine, stepper motor working subroutine, etc.

in conclusion

With the bus-based automotive electrical system, high integration, embedded and bus-based are the inevitable trends in the development of automotive instruments. This paper proposes a bus-based automotive instrument design, including the signal acquisition and processing part and the drive display part. The design of embedded automotive instruments with CAN communication is introduced in detail from the overall and software and hardware aspects. The solution has been debugged and has good performance in all aspects. The accuracy, response speed and anti-interference of the instrument have reached the domestic leading level.