Simply put, each message is like a speeding car on the CAN bus. If the messages are sent too quickly and frequently, congestion will occur, leading to transmission blockage, delay, or even loss of information. Therefore, the number of messages on a CAN bus and the frequency at which they are transmitted should be fully considered, which is measured by the load rate.
Load rate: The ratio of the communication parameters used by the CAN bus to the bus capacity within a given time. Specifically, it can be understood as follows: assuming the CAN bus has the ability to transmit 1000 bit data in 1 second, if the design is to transmit 500 bit data in 1 second, then the load rate is 50%. If the design is to transmit 1000 bit data in 1 second, then the load rate is 100%.
How to calculate the load rate for different CAN frame types? In short, the logic of load rate calculation is:
Firstly, it is necessary to clarify the target CAN bus baud rate;
Then clarify what types of CAN frames are present on the bus, how many are there, and what is the transmission cycle;
Consider the frame interval again, and finally calculate the load rate based on the previous information.
The specific calculation process will not be carried out here. The load rates of different CAN frames are as follows:
Assuming the current load rate is 50%, the baud rate of CAN is 500Kbps, the baud rate arbitration segment of CAN FD is 500Kbps, the data segment is 2Mbps, and the DLC is 8. Based on the above data, what changes will occur in the load rate if a new message is added?
Assuming adding a CAN standard frame with a transmission cycle of 10ms, the load rate will increase by 2.22%
Assuming adding a CAN extension frame with a transmission cycle of 10ms, the load rate will increase by 2.62%
Assuming an additional CANFD frame with a transmission cycle of 10ms, the load rate will increase by 1.035%
Once the above foundation is established, we can clearly know how much the load rate will increase with each additional message. On the other hand, we can set the CAN frame type reasonably, striving to transmit more data while minimizing the load rate of the CAN bus.
