AN2352

Configuration of the CAN bit timing

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1

Configuration of the CAN bit timing

Even if minor errors in the configuration of the CAN bit timing do not result in immediate

failure, the performance of a CAN network can be reduced significantly.

In many cases, the CAN bit synchronization compensates a faulty configuration of the CAN

bit timing to such a degree that an error frame is generated only occasionally. In the case of

arbitration however, when two or more CAN nodes simultaneously try to transmit a frame, a

misplaced sample point may cause one of the transmitters to become error passive.

The analysis of such sporadic errors requires a detailed knowledge of the CAN bit

synchronization inside a CAN node and of the CAN nodes’ interaction on the CAN bus.

1.1

Bit time and bit rate

CAN supports bit rates in the range of less than 1 Kbit/s up to 1000 Kbit/s. Each member of

the CAN network has its own clock generator, usually a quartz oscillator. The timing

parameter of the bit time (that is, the reciprocal of the bit rate) can be configured individually

for each CAN node, creating a common bit rate even though the CAN nodes’ oscillator

The frequencies of these oscillators are not absolutely stable. Small variations are caused

by changes in temperature or voltage and by deteriorating components. As long as the

variations remain within a specific oscillator tolerance range (df), the CAN nodes can

compensate the different bit rates by resynchronizing to the bit stream.

According to the CAN specifications, the bit time is divided into four segments (

Figure 1):

●

Synchronization segment

●

Propagation time segment

●

Phase buffer segment 1

●

Phase buffer segment 2

Each segment consists of a specific, programmable number of time quanta (see

Table 1).

register.

The synchronization segment (Sync_Seg) is that part of the bit time where edges of the

CAN bus level are expected to occur; the distance between the Sync_Seg and an edge that

occurs outside of Sync_Seg is called the phase error of that edge. The propagation time

segment (Prop_Seg) is intended to compensate the physical delay times within the CAN

network. The phase buffer segments (Phase_Seg1 and Phase_Seg2) surround the sample

point. The (Re)synchronization jump width (SJW) defines how far a resynchronization may

move the sample point within the limits defined by the phase buffer segments to

compensate edge phase errors.