The bit synchronization of the CANBUS bus of the car is only generated when the node detects the transition from "recessive bit" to "dominant bit". When the edge of the jump is not within the sync segment of the bit period, a phase error will occur. . This phase error is the distance between the edge of the transition and the end of the sync segment.

If the edge of the transition occurs before the sync segment, the phase error is positive, if the edge of the jump is a negative phase error after the sample point before the sync segment.

The phase error is due to the oscillating drift of the node, the propagation delay between the network nodes, and noise interference.

On the car bus, the synchronization mechanism is generally divided into two types:

Hard synchronization

Hard synchronization is only done with a falling edge (frame start) when the CAN bus is idle, regardless of the phase error, the bit time of all nodes restarts. Forcing a jump along the bit time that causes the hard sync to occur along the bit time to feed the fish back.

Resync

In the subsequent bits of the message frame, whenever there is a jump from "recessive bit" to "dominant bit", and the jump falls outside the sync segment, it will cause a resynchronization, and the resynchronization mechanism can The position of the sampling point is adjusted according to the edge of the jump or the bit time is shortened to ensure correct sampling.

In addition to the transparency of the data of the actual arbitration field, control field, data field and CRC sequence, the bit filling mechanism of the CAN bus protocol also increases the chance of jumping from "recessive bit" to "dominant bit", that is, increasing Resynchronize data to improve synchronization quality.