Calculating CAN Bus load

CAN Bus Load

How to calculate the time it takes to send a CAN message. This can be used to estimate the bus load on the CAN bus. This will always be an estimation since the number of stuff bits is impacted by the data being sent. Bus arbitration also makes the entire bus non-deterministic.

Fields in a CAN frame

Field Name	Size (in bits)
Start	1
Identifier	11 (standard), or 27 (extended)
RTR	1
Control	6
Data	64 (worst case standard), or 512 (worst case CAN-FD)
CRC	15, or (CAN-FD is 16 for 0-16 data bytes and 21 for 17-64 data bytes)
ACK	3
EOF	7
Intermission	1
Stuffing*	CEIL( (Identifier + Data + CRC) / 5) (worst case is all 0 or 1)

*: The 5 here represents the maximum number of stuffing bits

Therefore a single standard CAN frame has a worst case size of 128 bits

A single CAN-FD frame with an extended identifier would have a worst case size of 702 bits

The CAN-FD example assumes 64 bytes of data, and therefore a CRC size of 21 bits.

It is unrealistic that the CRC and/or Identifier would be all 0 or all 1, but it makes for a reasonable worst-case estimation.

Calculating the bit time

• The bit time is simply: 1 / bit rate, so for a 500 K CAN bus
• bit time = 1 / bit rate = 1 / (500 * 1000) s = 2 µs

Based on the worst case numbers above this would mean the time it takes is:

• 256 µs for a standard CAN frame at 500K
• 1404 µs for a CAN-FD frame with an extended identifier

Estimating the bus load

Assuming that every 100 ms one message will be sent every 100 ms the bus will be occupied for 256 µs. So the bus load from these cyclic messages is:

256 µs / 100 ms = (256 / (100 * 1000)) * 100% = 25600 / 100000% = 0.256%

Assuming the following transmission intervals on the bus as:

1 frame every 10 ms     =  100 frames every 1000 ms
1 frame every 100 ms    =   10 frames every 1000 ms
1 frame every 1000 ms   =    1 frame  every 1000 ms

Frame total:           111 frames every 1000 ms
Total time on bus is:  111 * 256 µs
Total time is:         1000 ms = 1000 * 1000 µs
Bus load is:           ((111 * 256) / (1000 * 1000)) * 100% = 2.84%

References

• https://support.vector.com/kb?id=kb_article_view&sysparm_article=KB0012332
• http://esd.cs.ucr.edu/webres/can20.pdf