Figure 19-4. CAN Bit Time
Sync
Prop
Phase2
Sample
Point
1 Time
Quantum
( t
q
)
q
Nominal CAN Bit Time
a.
TSEG1
= Prop + Phase1
b.
TSEG2
= Phase2
c. Phase1 = Phase2
or
1 = Phase2
TSEG1
a
TSEG2
b
Phase1
c
Table 19-3. CAN Protocol Ranges
a
Remark
Range
Parameter
Defines the length of the time quantum t
q
. The
CANBRPE
register can
be used to extend the range to 1024.
[1 .. 64]
BRP
Fixed length, synchronization of bus input to system clock
1 t
q
Sync
Compensates for the physical delay times
[1 .. 8] t
q
Prop
May be lengthened temporarily by synchronization
[1 .. 8] t
q
Phase1
May be shortened temporarily by synchronization
[1 .. 8] t
q
Phase2
May not be longer than either Phase Buffer Segment
[1 .. 4] t
q
SJW
a. This table describes the minimum programmable ranges required by the CAN protocol.
The bit timing configuration is programmed in two register bytes in the
CANBIT
register. In the
CANBIT
register, the four components
TSEG2
,
TSEG1
,
SJW
, and
BRP
have to be programmed to a
numerical value that is one less than its functional value; so instead of values in the range of [1..n],
values in the range of [0..n-1] are programmed. That way, for example, SJW (functional range of
[1..4]) is represented by only two bits in the
SJW
bit field. Table 19-4 shows the relationship between
the
CANBIT
register values and the parameters.
Table 19-4. CANBIT Register Values
Setting
CANBIT Register Field
Phase2 - 1
TSEG2
Prop + Phase1 - 1
TSEG1
SJW - 1
SJW
BRP
BRP
Therefore, the length of the bit time is (programmed values):
[
TSEG1
+
TSEG2
+ 3] × t
q
or (functional values):
[Sync + Prop + Phase2] × t
q
The data in the
CANBIT
register is the configuration input of the CAN protocol controller. The baud
rate prescaler (configured by the
BRP
field) defines the length of the time quantum, the basic time
1371
June 18, 2014
Texas Instruments-Production Data
Tiva
™
TM4C1294NCPDT Microcontroller