Abstract: Taking the MC68376 integrated CAN controller as an example, this paper elaborates on the design points of implementing CAN bus communication using the SAE J1939 communication protocol in the electric control system of pure electric vehicles (EVs). The battery charging and discharging characteristic curves of the power battery monitoring system based on CAN communication are given. Experiments show that the CAN bus communication has high speed, accuracy and high reliability. Keywords: Electric control system, CAN bus communication, MC68376 With the increasing number of electronic control devices in automobiles, the wiring of the vehicle body is becoming more and more complex, which reduces the reliability of operation and increases the difficulty of fault repair. In order to improve the utilization rate of signals, a large amount of data information needs to be shared in different electronic control units, and a large number of control signals in the vehicle integrated control system can also be exchanged in real time. However, the traditional automotive electronic system uses serial communication methods, such as using standards such as SAE1587, which has a slow communication speed and a small amount of data transmitted, which is far from meeting the needs of high-speed communication. In recent years, the CAN bus has become the mainstream bus of automotive electronic systems, and the vehicle application layer communication standard SAE J1939[1-4] based on the CAN bus communication protocol has emerged. The communication network of the pure electric vehicle (EV) electronic control system developed using the CAN bus has the characteristics of high communication speed, accuracy, and high reliability. It is easy to connect and manage the whole vehicle control network, and provides a basic platform for sharing sensor signals, calculation information and operating status of various control units, as well as on-board or off-board fault diagnosis. At the same time, it also makes it possible to develop an online calibration and real-time monitoring system for controllers based on this communication network. This paper adopts the SAE J1939 communication protocol based on CAN2.0B, and takes MC68376 as an example to design and develop a CAN bus communication system for EV electronic control systems. 1 Design of CAN Communication for EV Electronic Control System 1.1 CAN Bus Communication Principle of EV Control System In the EV control system, the controllers include: brake controller (ABS/ASR), powertrain controller PTCM (Powertrain Control Module), battery pack controller BPCM (Battery Pack Control Module), drive motor controller DMCM (Driver Motor Control Module), power steering controller, and instrument pack controller IPCM (Instrument Pack Control Module), etc. Data is exchanged between the controllers through the CAN communication network to achieve data sharing and improve the control performance of each controller. Figure 1 shows the CAN communication principle diagram between the various controllers of the EV. Figure 1. CAN Communication Network Topology Diagram of Pure Electric Vehicle Control System 1.2 CAN Communication Design of EV Electronic Control System According to the CAN communication principle, the hardware mainly consists of a CAN controller and a CAN driver. The powertrain control module PTCM and battery management control module BPCM use a CAN controller integrated on a 32-bit high-performance microprocessor MC68376; the instrument controller IPCM module uses a CAN controller integrated on a FUJ 32-bit high-performance microprocessor; the motor control DMCM module, power steering control module, and brake control module use an SJA1000 controller. All CAN drivers use PCA82C250. Figure 2 is the CAN communication network node connection diagram of the EV. Each bus end is connected to a load resistor (denoted by RL) to suppress reflections. The load resistor is connected between CAN-H and CAN-L. For ECUs without integrated terminating resistors (commonly used), this resistor is 60Ω; for ECUs with integrated terminating resistors, this resistor is 120Ω. The terminal load resistor is best placed at the end of the bus, and the load resistor RL inside the ECU is eliminated, because if one of the ECUs is disconnected from the bus, the bus will lose its terminal. Figure 2 shows the connection diagram of the CAN communication network node of a pure electric vehicle. The design of CAN communication of the EV control system is introduced below using the 32-bit high-intelligence microprocessor MC68376 as an example. 1.3 Design of CAN communication of EV control system based on MC68376 [6～7] 1.3.1 Basic characteristics of TouCAN embedded in MC68376 The TouCAN module is a CAN controller embedded in MC68376 that implements the CAN communication protocol. Its maximum transmission speed is up to 1Mbit/s, and it can simultaneously support the standard (11-bit) and extended (29-bit) ID message modes in the CAN protocol. The TouCAN module contains 16 message buffers with sending and receiving functions. In addition, it also has a message filtering function, which is used to compare the received message ID code with the pre-set receive buffer ID code to determine whether the received message is valid. Figure 3 is a block diagram of TouCAN, where CANTX and CANRX are the sending and receiving pins, respectively. Figure 3 TOUCAN Structure Block Diagram 1.3.2 MC68376 CAN Communication Hardware Interface Design Figure 4 is the schematic diagram of the CAN node hardware interface circuit. CAN+5V is a dedicated power supply for the CAN bus interface circuit, isolating the CAN bus power supply from the CPU power supply, ensuring that voltage fluctuations in the CAN system do not affect the normal operating voltage of the CPU. 6N137 is an optocoupler chip that provides electrical isolation between electrical signals. PCA82C250 provides differential transmission capability to the bus and differential reception capability to the CAN controller, fully compatible with the ISO11898 standard. In motion environments, PCA82C250 exhibits resistance to transients, radio frequency, and electromagnetic interference. Its internal current-limiting circuit protects the transmission output stage in case of a short circuit. Figure 4 CAN Node Hardware Interface Circuit Schematic Diagram 1.3.3 MC68376 CAN Communication Software Design Each controller sends data (vehicle speed, battery voltage, current, and temperature, etc.) to the bus according to a specified format and cycle, while also receiving information from other controllers. Other controllers on the bus retrieve the required messages as needed. For receiving data, this system uses an interrupt-driven approach. Once an interrupt occurs, the received data is automatically loaded into the corresponding message register. A masking filter can also be used, selectively comparing the identifier of the received message with a pre-set identifier during the initialization of the receive buffer. Only messages with matching identifiers can enter the receive buffer; those that do not meet the requirements are masked out, thus reducing the CPU's processing load. Furthermore, different data are placed in different message registers, making it easy to determine which received message caused the interrupt in the receive interrupt service routine. Figure 5 shows the flowchart of the CAN communication program based on MC68376. Figure 5 Program Flowchart 2 Application of CAN Communication in EV Electric Control System Development The CAN communication system of the EV electric control system establishes a communication network between controllers, enabling information exchange between controllers and with the instrument panel. Through the developed online calibration and monitoring systems, the parameters of each controller can be monitored in real time on a PC. Figures 6 and 7 show the charge and discharge characteristic curves obtained by the real-time monitoring system for nickel-metal hydride batteries designed using CAN communication. The CAN communication data transmission rate is 500 kbit/s, and this system reflects the charging and discharging characteristics of the nickel-metal hydride battery in real time. As a reliable automotive computer network bus, the CAN bus has begun to be used in advanced vehicles, enabling various automotive computer control units to share all information and resources through the CAN bus. This simplifies wiring, reduces the number of sensors, avoids redundant control functions, improves system reliability and maintainability, reduces costs, and better matches and coordinates various control systems. This elevates the vehicle's power, operational stability, and safety to new heights. With the development of automotive electronics technology, the CAN bus communication protocol, with its high flexibility, simple scalability, excellent anti-interference capabilities, and error handling abilities, will undoubtedly find wider application in automotive electronic control systems. 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