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CAN Bit Timing
Each member of the CAN network has its own clock generator, typically derived from a crystal oscillator.
The Bit timing parameters can be configured individually for each CAN node, creating a common Bit rate
even though the CAN nodes' oscillator periods (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
resynchronizing to the bit stream.
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. 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.
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.
23.12.1 Bit Time and Bit Rate
According to the CAN specification, the Bit time is divided into four segments (see
10):
•
Synchronization Segment (Sync_Seg)
•
Propagation Time Segment (Prop_Seg)
•
Phase Buffer Segment 1 (Phase_Seg1)
•
Phase Buffer Segment 2 (Phase_Seg2)
Sync_
Seg
1 time quantum
Each segment consists of a specific number of time quanta. The length of one time quantum (t
the basic time unit of the bit time, is given by the CAN_CLK and the Baud Rate Prescalers (BRPE and
BRP). With these two Baud Rate Prescalers combined, divider values from 1 to 1024 can be programmed:
t
= Baud Rate Prescaler / CAN_CLK
q
Apart from the fixed length of the synchronization segment, these numbers are programmable.
describes the minimum programmable ranges required by the CAN protocol.
A given bit rate may be met by different bit time configurations.
1530
M3 Controller Area Network (CAN)
Figure 23-11. Bit Timing
Nominal CAN bit time
Prop_Seg
Phase_Seg1
(t
)
q
Copyright © 2012–2019, Texas Instruments Incorporated
Figure
23-11Figure 15-
Phase_Seg2
Sample point
SPRUH22I – April 2012 – Revised November 2019
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), which is
q
Table 23-1
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