A given bit rate may be met by different bit-time configurations, but for the proper function of the
CAN network, the physical delay times and the oscillator's tolerance range have to be considered.
Figure 14-4. 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 Phase1 + 1 = Phase2
TSEG1
a
TSEG2
b
Phase1
c
Table 14-1. CAN Protocol Ranges
a
RemarkRangeParameter
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 clock1 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 14-2 shows the relationship between
the CANBIT register values and the parameters.
Table 14-2. CANBIT Register Values
SettingCANBIT Register Field
Phase2 - 1TSEG2
Prop + Phase1 - 1TSEG1
SJW - 1SJW
BRPBRP
Therefore, the length of the bit time is (programmed values):
[TSEG1 + TSEG2 + 3] × t
q
or (functional values):
June 22, 2010378
Texas Instruments-Production Data
Controller Area Network (CAN) Module
