Rainbow Electronics AT91CAP9S250A - Manual (Page 688)

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6264A–CAP–21-May-07
AT91CAP9S500A/AT91CAP9S250A
39.6.4 CAN 2.0 Standard Features
39.6.4.1 CAN Bit Timing Configuration
All controllers on a CAN bus must have the same bit rate and bit length. At different clock fre-
quencies 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 39-4. Partition of the CAN Bit Time
TIME QUANTUM:
The TIME QUANTUM (TQ) is a fixed unit of time derived from the MCK period. The total num-
ber 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:
SYNC_SEG PROP_SEG PHASE_SEG1 PHASE_SEG2
NOMINAL BIT TIME
Sample Point