Several common factors affecting the normal operation of CAN
1. Normal operating voltage range of CAN network
Operating voltage: CAN network usually operates at 5V level, and the specific normal operating voltage range is 4.5V to 5.5V. The CAN transceiver is responsible for converting the physical level signal to the CAN bus standard level and is critical to the correct transmission of the signal.
Power supply voltage: The power supply voltage of the transceiver is usually 3.3V or 5V, depending on the design specifications of the device. It should be noted that the stability of the power supply voltage is very important for the reliability of the CAN signal, and voltage fluctuations may cause communication errors.
2. Cable resistance and capacitance parameters and environmental requirements of CAN network
Cable impedance: The characteristic impedance of the CAN bus is usually 120 ohms, which matches the terminal resistance to prevent signal reflection. In the application of electric vehicles, the CAN bus needs to use automotive-standard twisted pair cables to maintain consistent impedance and reduce electromagnetic interference.
Capacitive load: The maximum capacitive load of the bus should be less than 200 pF. Excessive capacitive load will increase the rise and fall time of the signal, thereby affecting the integrity of the signal.
Inductive effect: In electric vehicles, the wiring of cables may introduce inductive effect, which is usually reduced by reasonable wiring methods and adding filtering elements.
Cable length: The maximum effective length of the CAN network is 40 meters (at 1 Mbps). If the cable length needs to be extended, the communication rate needs to be reduced to ensure signal integrity. For example, at 125 kbps, the maximum cable length can reach 500 meters.
Working environment: CAN bus cables must be able to work reliably over the wide temperature range of the car (usually -40°C to +85°C, and up to +125°C under extreme conditions). The selection of cables and the design of sheaths should take into account requirements such as high temperature resistance, oil resistance, and UV resistance.
3. Interference and electromagnetic radiation of transceiver communication nodes
Electromagnetic interference (EMI) management: The electromagnetic environment in electric vehicles is complex due to the presence of motors and high-power electronic devices. CAN transceivers and buses are susceptible to EMI, which may cause communication errors or even system failures.
Anti-interference design: Shielded cable: Use shielded twisted pair (STP) or shielded twisted pair (F/UTP) to reduce the impact of electromagnetic radiation and external interference. Terminal resistance: Correctly configured terminal resistance (usually 120 ohms) plays an important role in suppressing signal reflection and interference.
Filter: Add common mode chokes and decoupling capacitors to the node design to filter high-frequency interference.
Electromagnetic radiation: The electromagnetic radiation of the CAN network must comply with automotive industry standards, such as CISPR 25 (radiation and interference immunity requirements for on-board electronic equipment). The specific radiation requirement is usually a level not exceeding 30 dBμV/m (in the 30 MHz to 1 GHz frequency band), and the specific value varies depending on the vehicle application environment.
4. Common fault analysis and troubleshooting methods
Fault type:
Bus open or short circuit: This may be caused by physical damage or loose connectors, which will cause the node to be unable to communicate.
Terminal resistance mismatch: Missing or incorrect value of terminal resistance will cause signal reflection, resulting in communication errors.
Abnormal power supply voltage: unstable or too low power supply voltage of CAN transceiver will cause signal distortion and communication interruption.
EMI interference: interference caused by strong external electromagnetic fields or internal high-frequency devices will cause packet loss or CRC errors.
Troubleshooting method:
Physical inspection: Check whether the cable connection, terminal resistance and node power supply voltage meet the standards. Oscilloscope test: Use an oscilloscope to detect the waveforms of CAN_H and CAN_L to observe whether there are abnormal signal reflections, too high or too low levels.
Power supply test: Detect the power supply voltage of CAN transceiver to ensure that it is within the normal operating range (for example, 4.5V to 5.5V).
EMI analysis: Use a spectrum analyzer to detect electromagnetic interference sources in the environment and take shielding or isolation measures.
Specific reference values
Normal operating voltage range: 4.5V to 5.5V.
Cable characteristic impedance: 120 ohms.
Maximum capacitive load: < 200 pF.
Maximum cable length: 40 meters at 1 Mbps; 500 meters at 125 kbps.
Electromagnetic radiation requirements: Under CISPR 25 standard, the level does not exceed 30 dBμV/m.
Through the above analysis, we can see that the stability of the CAN network depends on many factors, including voltage, cable parameters and electromagnetic compatibility (EMC). Reasonable design and troubleshooting methods are essential to ensure the reliable operation of the electric vehicle CAN network.
