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Sync
Prop
Phase2
Sample
Point
1 Time
Quantum
(t
q
)
a. TSEG1 = Prop + Phase1
B. TSEG2 = Phase2
C. Phase1 = Phase2 or 1=Phase 2
TSEG2
Nominal CAN Bit Time
Functional Description
798
SLAU723A – October 2017 – Revised October 2018
Copyright © 2017–2018, Texas Instruments Incorporated
Controller Area Network (CAN) Module
According to the CAN specification, the bit time is divided into four segments (see
): the
Synchronization Segment, the Propagation Time Segment, the Phase Buffer Segment 1, and the Phase
Buffer Segment 2. Each segment consists of a specific, programmable number of time quanta (see
). The length of the time quantum (t
q
), which is the basic time unit of the bit time, is defined by
the CAN controller's input clock (fsys) and the Baud Rate Prescaler (BRP):
t
q
= BRP / fsys
The fsys input clock is the system clock frequency as configured by the RSCLKCFG register (see
).
The Synchronization Segment Sync is that part of the bit time where edges of the CAN bus level are
expected to occur; the distance between an edge that occurs outside of Sync and the Sync is called the
phase error of that edge.
The Propagation Time Segment Prop is intended to compensate for the physical delay times within the
CAN network.
The Phase Buffer Segments Phase1 and Phase2 surround the Sample Point.
The (Re)Synchronization Jump Width (SJW) defines how far a resynchronization may move the Sample
Point inside the limits defined by the Phase Buffer Segments to compensate for edge phase errors.
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 must be considered.
Figure 11-4. CAN Bit Time
Table 11-2. CAN Protocol Ranges
Parameter
Range
Remark
BRP
[1 to 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 to 8] t
q
Compensates for the physical delay times
Phase1
[1 to 8] t
q
May be lengthened temporarily by synchronization
Phase2
[1 to 8] t
q
May be shortened temporarily by synchronization
SJW
[1 to 4] t
q
May not be longer than either Phase Buffer Segment
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 must be programmed to a numerical value
that is one less than its functional value; so instead of values in the range of [1 to n], values in the range
of [0 to n-1] are programmed. That way, for example, SJW (functional range of [1 to 4]) is represented by
only two bits in the SJW bit field.
shows the relationship between the CANBIT register values
and the parameters.