The CAN bus edge time will affect the sampling accuracy, and sampling errors will cause continuous error frames to appear, affecting CAN bus communication. So what is the CAN bus edge time standard? How is the edge time measured?
First, the significance of CAN test edge time
At present, there is no clear standard in the domestic automotive electronics industry, which results in uneven quality of auto parts. The replacement of spare parts into the car will cause abnormal communication of the CAN bus, which brings safety hazards to the car driving. The following are the specifications for the CAN bus edge of the GMW3122 signal edge standard.
According to different requirements, the baud rate is divided into high-speed CAN and medium-speed CAN. The signal edge time is measured, and the edge time is the total time from the recessive level to the dominant level time and the dominant level to the recessive level change. The recessive level (logic value 0) to the dominant level (logic value 1) time is the rising edge, and the dominant level to the recessive level is the falling edge.
The edge time is divided into rising edge time and falling edge time. The falling edge time is according to the voltage (20%~80% voltage range, some measured the edge time according to the 10%~90% voltage interval, and the edge time is measured in the temperature range of 20%~80%). The time range is given in the table. If the specified time is exceeded, the waveform bit width will increase, the sampling point value will be inaccurate, the baud rate will be abnormal, and a large number of error frames will appear. Retransmitting the data frame will also cause CAN bus communication.
Due to the length of the fieldbus, the mounting capacitance on the bus increases, resulting in an increase in line impedance. Resistance and capacitance matching need to be considered in the edge time test. The analog test line is short, and it is necessary to artificially add a capacitor to simulate the actual situation at the scene. Typical values ​​in the above table are common values ​​based on field capacitance and resistance.
Second, CAN edge time test steps
Oscilloscope test CAN waveform
Use the oscilloscope to acquire the CAN bus waveform, set the amplitude cursor to 20%~80%, and record the rising edge time and falling edge time.
? Record multiple data, confirm that the rising edge and falling edge time are within the standard range.
CAN test problem
Using only an oscilloscope to measure CAN edge time requires multiple operations for human operation. The vehicle CAN bus has multiple components. It takes a lot of time and manpower to test the CAN edge time. This is only one of the vehicle CAN conformance tests. To complete all the test requirements, one person needs to test for three days. As efficiency requirements become higher and higher, OEMs are more likely to spend their time developing new technologies for automotive applications.
Third, CANDT
Based on the complicated testing methods of CAN bus in the automotive industry, the CANDT conformance test system independently developed by Zhiyuan Electronics can construct a CAN bus security guarantee system and automate the CAN bus physical layer, link layer and application layer automation test. The four steps to ensure the safety of the whole vehicle are as follows:
Step 1: CAN Consistency Automation Test
By connecting the car or node components to the CAN conformance test system, the CAN bus physical layer, link layer and other automated tests can be automated, covering the standard requirements of the mainstream host factory, ensuring the safety and stability of the vehicle CAN bus.
Step 2: Detailed test report export
After the single-node test is completed, the detailed CAN conformance test report can be automated everywhere, including the test standard reference, the judgment basis, the judgment result and other data, and accurately determine the working condition of the node, which can be seen as a basis for the host factory.
Step 3: CAN bus fault location and troubleshooting
After the CANDT conformance test, the new energy automobile main engine factory and the component enterprise can use CANScope to locate and check the faults in the test report, and accelerate the stable development speed of the CAN bus.
Test results: to ensure the safety of the vehicle CAN network
After passing the conformance test, the CAN nodes of the new energy vehicles are highly consistent in quality, avoiding the security risks caused by the unevenness of the CAN nodes, thus ensuring the security and uniformity of the vehicle CAN network.
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