CAN bit timing: where the numbers come from

October 14, 2026

One CAN bit is divided into time quanta: the sync segment (always one quantum), then BS1 and BS2. The sample is taken at the boundary between BS1 and BS2. The bit rate is:

bitrate = Fapb / (prescaler * (1 + BS1 + BS2))

Example

Fapb	48 MHz
Prescaler	6
BS1	13
BS2	2
Quanta per bit	16
Bit rate	500 kbit/s
Sample point	(1 + 13) / 16 = 87.5%

CANopen recommends a sample point around 87.5%. On long lines the propagation margin matters more, so it is sometimes moved earlier.

If the bus stays silent

Check the terminators first: with the bus powered down you should measure about 60 ohms between CANH and CANL. Then make sure there is a second node on the bus. Without an acknowledge the transmitter keeps retrying and ends up in an error state.