SAM7X128_14 Datasheet, PDF (488/662 Pages) ATMEL Corporation – ARM-based Flash MCU

36.6.4 CAN 2.0 Standard Features
36.6.4.1 CAN Bit Timing Configuration
All controllers on a CAN bus must have the same bit rate and bit length. At different clock frequencies of the individual
controllers, the bit rate has to be adjusted by the time segments.
The CAN protocol specification partitions the nominal bit time into four different segments:
Figure 36-4. Partition of the CAN Bit Time
NOMINAL BIT TIME
SYNC_SEG
PROP_SEG
PHASE_SEG1 PHASE_SEG2
Sample Point
 TIME QUANTUM
The TIME QUANTUM (TQ) is a fixed unit of time derived from the MCK period. The total number of TIME QUANTA in a
bit time is programmable from 8 to 25.
SYNC SEG: SYNChronization Segment.
This part of the bit time is used to synchronize the various nodes on the bus. An edge is expected to lie within this
segment. It is 1 TQ long.
 PROP SEG: PROPagation Segment.
This part of the bit time is used to compensate for the physical delay times within the network. It is twice the sum of the
signal’s propagation time on the bus line, the input comparator delay, and the output driver delay. It is programmable to
be 1,2,..., 8 TQ long.
This parameter is defined in the PROPAG field of the ”CAN Baudrate Register”.
 PHASE SEG1, PHASE SEG2: PHASE Segment 1 and 2.
The Phase-Buffer-Segments are used to compensate for edge phase errors. These segments can be lengthened
(PHASE SEG1) or shortened (PHASE SEG2) by resynchronization.
Phase Segment 1 is programmable to be 1,2,..., 8 TQ long.
Phase Segment 2 length has to be at least as long as the Information Processing Time (IPT) and may not be more than
the length of Phase Segment 1.
These parameters are defined in the PHASE1 and PHASE2 fields of the ”CAN Baudrate Register”.
 INFORMATION PROCESSING TIME:
The Information Processing Time (IPT) is the time required for the logic to determine the bit level of a sampled bit. The
IPT begins at the sample point, is measured in TQ and is fixed at 2 TQ for the Atmel CAN. Since Phase Segment 2
also begins at the sample point and is the last segment in the bit time, PHASE SEG2 shall not be less than the IPT.
 SAMPLE POINT:
The SAMPLE POINT is the point in time at which the bus level is read and interpreted as the value of that respective bit.
Its location is at the end of PHASE_SEG1.
 SJW: ReSynchronization Jump Width.
The ReSynchronization Jump Width defines the limit to the amount of lengthening or shortening of the Phase Segments.
SJW is programmable to be the minimum of PHASE SEG1 and 4 TQ.
If the SMP field in the CAN_BR register is set, then the incoming bit stream is sampled three times with a period of half a
CAN clock period, centered on sample point.
SAM7X Series [DATASHEET]
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