STR720 datasheet(249/401 Pages) STMICROELECTRONICS

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STR720 - CONTROLLER AREA NETWORK (CAN)

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18.7.10 Configuring the 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 will amend a faulty configuration of the CAN bit

timing to such a degree that only occasionally an error frame is generated. However, in the

case of arbitration, 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 interaction of the CAN nodes on the CAN bus.

18.7.10.1 Bit Time and Bit Rate

CAN supports bit rates in the range of lower 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 (i.e. the reciprocal of the bit rate) can be configured individually for

each CAN node, creating a common bit rate even though the oscillator periods of the CAN

nodes (fosc) may be different.

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 inside a specific oscillator tolerance range (df), the CAN nodes are able to

compensate for the different bit rates by re-synchronizing to the bit stream.

According to the CAN specification, the bit time is divided into four segments (see Figure 35).

The Synchronization Segment, the Propagation Time Segment, the Phase Buffer Segment 1

and the Phase Buffer Segment 2. Each segment consists of a specific, programmable number

of time quanta (see Table 56). The length of the time quantum (tq), which is the basic time unit

of the bit time, is defined by the CAN controller’s system clock fAPB and the BRP bit of the Bit

Timing Register (CAN_BTR): tq = BRP / fAPB.

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 an edge, that occurs outside of

Sync_Seg, and the Sync_Seg is called the phase error of that edge. The Propagation Time

Segment, Prop_Seg, is intended to compensate for 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 re-synchronization may

move the Sample Point inside the limits defined by the Phase Buffer Segments to compensate

for edge phase errors.

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