509
Chapter 14
FCAN Interface Function
Preliminary User’s Manual U15839EE1V0UM00
14.4 Operating
Considerations
14.4.1 Rules to be observed for correct baud rate settings
Observing the following rules for the baud rate setting assures correct operation of a CAN module
and compliance to the CAN protocol specification.
(1)
Rule for sampling point (SPT) setting:
The sample point position needs to be programmed between 3 TQ and 17 TQ, which corresponds
to the SPTR4 to SPTR0 bits of the CxSYNC register (x = 1 to 4 for the derivatives µPD703129(A)
and µPD703129(A1), x = 1 to 2 for the derivative µPD703128(A)):
(2)
Rule for data bit time (DBT) setting:
The number of TQ per CAN frame bit is restricted to a range from 8 TQ to 25 TQ, which corre-
sponds to the DBTR4 to DBTR0 bits of the CxSYNC register:
(3)
Rule for synchronization jump width (SJW) setting:
The number of TQ allowed for soft-synchronization must not exceed the number of TQ for
PHASE_SEG2. The length of PHASE_SEG2 is given by the difference of data bit time (DBTR)
and the sampling point position (SPTR). Converted to register values the following condition
applies:
Remark:
The time quantum (TQ) is determined by the base clock f
BTL
for the CAN protocol layer,
which is defined in the CxBRP register:
Caution:
The rules above represent CAN protocol limits. Violating these rules may cause erro-
neous operation.
3 TQ
SPT
p
1
+
(
)
TQ
=
17 TQ
≤
≤
2
p
16
≤
≤
p
decimal value of bits SPTR4 to SPTR0
=
TQ
DBTR
q
1
+
(
)
TQ
=
25 TQ
≤
≤
7
q
24
≤
≤
q
decimal value of bits DBTR4 to DBTR0
=
SJW
s
1
+
(
)
TQ
DBT
SPT
–
≤
=
s
q
p
–
1
–
≤
s
decimal value of bits SJW1, SJW0
=
TQ
1
f
BTL
----------
=
Содержание mPD703128
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