Maxim Integrated MAXQ7666 - Manual (Page 194)

MAXQ7665/MAXQ7666 User’s Guide
4-64
4.15 Bit Timing
Bit timing in the CAN2.0B specification is based on a unit called the nominal bit time. The nominal bit time is further subdivided into
four specific time periods.
1) The SYNC_SEG time segment is where an edge is expected when synchronizing to the CAN bus.
2) The PROP_SEG time segment is provided to compensate for the physical times associated with the CAN bus network.
3) The PHASE_SEG1 and PHASE_SEG2 time segments compensate for edge phase errors.
4) The PHASE_SEG1 and PHASE_SEG2 time segments can be lengthened or shorted through the use of the SJW1 and SJW0 bits
in the CAN 0 bus timing register zero.
The CAN bus bit data is evaluated at a specific sample point. A time quantum (t QU ) is a unit of time derived from the division of the
microcontroller system clock by both the baud-rate prescaler (programmed by the BPR7:BPR0 bits of the CAN 0 operation control reg-
ister and CAN 0 bus timing register) and the system clock divider (programmed by the CD1:CD0 and PMME bits of the CKCN regis-
ter). Combining the PROP_SEG and PHASE_SEG1 time segments into one time period termed t
TSEG1 , and equating the SYNC_SEG
time segment to t SYNC_SEG and PHASE_SEG2 to t TSEG2 , provides the basis for the time segments outlined in Figure 4-14 and in the
CAN bus timing peripheral register descriptions.
Figure 4-14. Bit Timing
1 BIT TIME
1 t QU
TIME QUANTA
1 t QU
TIME QUANTA
TRANSMIT
SYNC_SEG PROP_SEG PHASE_SEG1 PHASE_SEG2
NOMINAL BIT TIME
t SYNC-SEG
t TSEG1
t TSEG2
SAMPLE
POINT
SAMPLE
POINT
2 t QU - 16 t QU 2 t QU - 8 t QU
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