70353C Datasheet, PDF (59/76 Pages) Microchip Technology – Section 21. Enhanced Controller Area Network

Section 21. Enhanced Controller Area Network (ECAN™)
21.9
BIT TIMING
The nominal bit rate is the number of bits per second transmitted on the CAN bus.
Nominal Bit Time = 1 ÷ Nominal Bit Rate
There are four time segments in a bit time to compensate for any phase shifts due to oscillator
drifts or propagation delays. These time segments do not overlap each other and are represented
in terms of TQ. One TQ is a fixed unit of time derived from the oscillator clock. The total number
of time quanta in a nominal bit time must be programmed between 8 TQ and 25 TQ.
Figure 21-17 illustrates how the time quantum frequency (FTQ) is obtained from the system clock
and also how the different time segments are programmed.
Figure 21-17: ECAN™ Bit Timing
FCAN
1:1
1:2
BAUD RATE
PRESCALER
1:64
CiCFG1<BRP>
Prop Seg
1 TQ - 8 TQ
2
FTQ
Time Quanta
CiCFG2<PRSEG>
1 TQ - 8 TQ
Phase Seg1
CiCFG2<SEG1PH>
1 TQ - 8 TQ
Phase Seg2
CiCFG2<SEG2PH>
Sync Seg
1 TQ
CiCFG1<SJW>
21
Time Quanta
Time Segments
Sync
Seg
Propagation Segment
1 TQ
1 TQ - 8 TQ
Phase Segment 1
1 TQ - 8 TQ
Phase Segment 2
1 TQ - 8 TQ
Sample Point
CAN Nominal Bit Time
8 TQ - 25 TQ
21.9.1 Bit Segments
Each bit transmission time consists of four time segments:
• Synchronization Segment – This time segment synchronizes the different nodes
connected on the CAN bus. A bit edge is expected to be within this segment. Based on
CAN protocol, the Synchronization Segment is assumed to be 1 TQ.
• Propagation Segment – This time segment compensates for any time delay that may
occur due to the bus line or due to the various transceivers connected on that bus.
• Phase Segment 1 – This time segment compensates for errors that may occur due to
phase shift in the edges. The time segment may be lengthened during resynchronization to
compensate for the phase shift.
• Phase Segment 2 – This time segment compensates for errors that may occur due to
phase shift in the edges. The time segment may be shortened during resynchronization to
compensate for the phase shift. The Phase Segment 2 time can be configured to be either
programmable or specified by the Phase Segment 1 time.
© 2008-2011 Microchip Technology Inc.
DS70353C-page 21-59