NUC970 Technical Reference Manual
Publication Release Date: Dec. 15, 2015
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Revision V1.30
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According to the CAN specification, the bit time is divided into four segments (see the following figure).
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 Table 5.24-3). The length of the time quantum (tq), which is the basic time unit of the bit time, is
defined by the CAN controller’s APB clock fAPB and the BRP bit of the Bit Timing Register
(CAN_BTR): tq = BRP / fAPB.
The Synchronization Segment, Sync_Seg, 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_Seg, and the
Sync_Seg is called the phase error of that edge. The Propagation Time Segment, Prop_Seg, is
intended to compensate for the physical delay times within the CAN network. The Phase Buffer
Segments Phase_Seg1 and Phase_Seg2 surround the Sample Point. The (Re-)Synchronization Jump
Width (SJW) defines how far a re-synchronization may move the Sample Point inside the limits
defined by the Phase Buffer Segments to compensate for edge phase errors.
Sync_Seg
Prop_Seg
Phase_Seg1
Phase_Seg2
Normal CAN Bit Time
1 Time Quantum(
t
q
)
Sample Point
Figure 5.24-7 Bit Timing
Parameter
Range
Remark
BRP
[1 .. 32]
Defines the length of the time quantum tq
Sync_Seg
1 tq
Fixed length, synchronization of bus input to APB clock
Prop_Seg
[1.. 8] tq
Compensates for the physical delay times
Phase_Seg1
[1..8] tq
Which may be lengthened temporarily by synchronization
Phase_Seg2
[1.. 8] tq
Which may be shortened temporarily by synchronization
SJW
[1 .. 4] tq
Which may not be longer than either Phase Buffer Segment
This table describes the minimum programmable ranges required by the CAN protocol
Table 5.24-3 CAN Bit Time Parameters
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.
Propagation Time Segment
5.24.7.17
This part of the bit time is used to compensate physical delay times within the network. These delay
times consist of the signal propagation time on the bus and the internal delay time of the CAN nodes.
Any CAN node synchronized to the bit stream on the CAN bus will be out of phase with the transmitter
of that
bit stream, caused by the signal propagation time between the two nodes. The CAN protocol’s
non-destructive bitwise arbitration and the dominant acknowledge bit provided by receivers of CAN