“The edge time of the CAN bus will affect the correctness of the sampling, and the sampling error will cause continuous error frames to appear, affecting the CAN bus communication. So what is the CAN bus edge time standard? How is edge time measured?
“
The edge time of the CAN bus will affect the correctness of the sampling, and the sampling error will cause continuous error frames to appear, affecting the CAN bus communication. So what is the CAN bus edge time standard? How is edge time measured?
1. The meaning 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 and components. The assembly of spare parts on the car will cause abnormal CAN bus communication and bring safety hazards to car driving. The following is the specification requirements of the GMW3122 signal edge standard for the CAN bus edge.
According to different needs and different baud rates in the table, it is divided into high-speed CAN and medium-speed CAN. What is tested is the signal edge time, and the edge time refers to the total time from the recessive level to the dominant level and the change from the dominant level to the recessive level. The time from the recessive level (logic value 0) to the dominant level (logic value 1) is the rising edge, and the time from the dominant level to the recessive level is the falling edge.
Edge time is divided into rising edge time and falling edge time. The falling edge time is based on voltage (20%~80% voltage range, some edge time is measured according to 10%~90% voltage range, in this paper, edge time is measured in 20%~80% voltage range). The time range is given in the table. If it exceeds the specified time, 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. Repeated data frames will also cause CAN bus communication to be paralyzed.
Because the field bus is too long, the capacitance mounted on the bus increases, resulting in an increase in line impedance. Resistor and capacitor matching needs to be considered during edge time testing. The analog test line is short, and it is necessary to artificially add capacitors to simulate the actual situation on site. The typical values in the above table are common values based on field capacitance and resistance.
2. CAN edge time test steps
Oscilloscope to test CAN waveform
l Use the oscilloscope to collect the CAN bus waveform, set the amplitude cursor to 20%~80%, and record the rising edge time and falling edge time.
l Record the data multiple times, and confirm that the rising and falling times are within the standard range each time.
CAN test problem
Only using an oscilloscope to measure the CAN edge time requires manual operation to record multiple times. The vehicle CAN bus has multiple components, and it takes a lot of time and manpower to test the CAN edge time, and this is only one of the vehicle CAN conformance tests. To complete all the test requirements, it takes three days for one person to test. With higher and higher efficiency requirements, OEMs are more likely to spend their time developing new technologies for automotive applications.
3. CANDT
Based on the complex testing methods for CAN bus in the automotive industry, the CANDT conformance test system independently developed by ZHIYUAN Electronics can build a CAN bus security guarantee system and automatically complete the CAN bus physical layer, link layer and application layer automation test. The four steps to ensure vehicle safety are as follows:
Step 1: CAN Conformance Automated Test
By connecting the vehicle or node components to the CAN conformance test system, the CAN bus physical layer, link layer and other automated tests can be automatically completed, covering the standard requirements of mainstream OEMs, and ensuring the safety and stability of the vehicle CAN bus.
Step 2: Export the detailed test report
After the single node test is completed, the detailed CAN conformance test report can be automated everywhere, which includes test standard reference, judgment basis, judgment result and other data, and can accurately judge the working status of the node.
Step 3: CAN bus fault location and troubleshooting
New energy vehicle OEMs and parts companies can use CANScope to locate and troubleshoot problems that fail the test in the test report after the CANDT conformance test, speeding up the stable development of the CAN bus.
Test results: ensure the safety of the vehicle CAN network
After passing the conformance test, the quality of the CAN nodes of new energy vehicles is highly consistent, which avoids the potential safety hazards caused by the uneven quality of CAN nodes, thereby ensuring the safety and unity of the vehicle CAN network.
The Links: CLAA170EA09 PD064VL1
