LM3S2620 Datasheet, PDF (521/700 Pages) Texas Instruments – Stellaris® LM3S2620 Microcontroller

Stellaris^® LM3S2620 Microcontroller
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
Nominal CAN Bit Time
a
TSEG1
b
TSEG2
Sync
Prop
Phase1 ^c
Phase2
1 Time
Quantum
( t qq )
a. TSEG1 = Prop + Phase1
b. TSEG2 = Phase2
c. Phase1 = Phase2 or Phase1 + 1 = Phase2
Sample
Point
Table 14-3. CAN Protocol Ranges^a
Parameter
Range
Remark
BRP
[1 .. 64]
Defines the length of the time quantum t_q. The CANBRPE register can
be used to extend the range to 1024.
Sync
1 t_q
Fixed length, synchronization of bus input to system clock
Prop
[1 .. 8] t_q
Compensates for the physical delay times
Phase1
[1 .. 8] t_q
May be lengthened temporarily by synchronization
Phase2
[1 .. 8] t_q
May be shortened temporarily by synchronization
SJW
[1 .. 4] t_q
May not be longer than either Phase Buffer Segment
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 14-4 shows the relationship between
the CANBIT register values and the parameters.
Table 14-4. CANBIT Register Values
CANBIT Register Field
TSEG2
TSEG1
SJW
BRP
Setting
Phase2 - 1
Prop + Phase1 - 1
SJW - 1
BRP
Therefore, the length of the bit time is (programmed values):
[TSEG1 + TSEG2 + 3] × t_q
or (functional values):
June 18, 2012
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