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Chapter 13
FCAN Interface Function
Preliminary User’s Manual U14913EE1V0UM00
(9)
CAN 1 to 3 synchronization control registers (C1SYNC to C3SYNC)
A bit in a CAN frame is built by a programmable number of time quanta (TQ), as shown in the Fig-
ure 13-41 below.
Figure 13-41: CAN Bus Bit Timing
For the CAN modules in the FCAN system the bit length of segments SYNC_SEG, PROP_SEG,
PHASE_SEG1 and PHASE_SEG2 must not be defined explicitly. All necessary CAN bit timings are
programmed by defining
•
the total number of time quanta TQ per CAN bit (i.e. DBT).
•
the location of the sample point (i.e. SPT) as a number of TQ.
The CAN protocol segmentation is done by the CAN module automatically.
Due to re-synchronisation mechanisms the CAN module may lengthen PHASE_SEG1 or shorten
PHASE_SEG2 by one or more TQ. The total number of TQ for which the CAN module is permitted
to lengthen or shorten the phase segments is called synchronisation jump width (SJW). The SJW
value must be less or equal the difference of DBT and SPT, which corresponds to PHASE_SEG2,
and can be specified in the range of 1 TQ to 4 TQ.
For additional information on the CAN bus bit timing please refer to ISO 11898.
The relation between CAN memory clock and CAN bus baud rate is:
Valid values for DBT and BRP are:
Note: BRP is the resulting bit rate prescaler value specified in the CxBRP register, where the variable
k corresponds to the contents of bits BRP5 to BRP0 when TLM bit = 0, and bits BRP7 to BRP0
bits when TLM bit = 1, respectively (x = 1 to 3).
TLM bit
DBT [TQ]
BRP
Note
0
8, 9, 10, ... ,25
2, 4, 6, ... ,128
2
×
(k + 1)
1
8, 9, 10, ... ,25
1, 2, 3, ... ,256
k + 1
SYNC_SEG
PROP_SEG PHASE_SEG1 PHASE_SEG2
1
time quanta
1..8
time quanta
1..8
time quanta
1..8
time quanta
DBT
SPT
Sampling Point
f
C ANBUS
1
DBT
TQ
×
---------------------------
f
BTL
DBT
------------
f
MEM
DBT
BRP
×
--------------------------------
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=
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