70353C datasheet(59/76 Pages) MICROCHIP

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DS70353C-page 21-59

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

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.

CAN Nominal Bit Time

Time Quanta

Time Segments

Propagation Segment

Phase Segment 1

Phase Segment 2

Sync

Seg

1 TQ

1 TQ - 8 TQ

Sample Point

BAUD RATE

PRESCALER

FCAN

Prop Seg

Phase Seg1

Phase Seg2

Sync Seg

FTQ

Time Quanta

CiCFG1<BRP>

CiCFG2<PRSEG>

CiCFG2<SEG1PH>

CiCFG2<SEG2PH>

CiCFG1<SJW>

1 TQ - 8 TQ

1:1

1:2

1:64

1 TQ - 8 TQ

1 TQ

8 TQ - 25 TQ

1 TQ - 8 TQ

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