16.2.3 CAN Bit Timing
To ensure correct sampling up to the last bit, a CAN node needs to re-synchronize throughout the entire frame. This is done
at the beginning of each message with the falling edge SOF and on each recessive to dominant edge.
16.2.3.1 Bit Construction
One CAN bit time is specified as four non-overlapping time segments. Each segment is constructed from an integer multiple
of the time quantum. The time quantum or TQ is the smallest discrete timing resolution used by a CAN node.
Figure 16-3. CAN Bit Construction
CAN Frame
(producer)
Transmission Point
(producer)
Nominal CAN Bit Time
Time Quantum
(producer)
Segments
(producer)
PROP_SEG
PHASE_SEG_1
PHASE_SEG_2
SYNC_SEG
Propagation
delay
Segments
(consumer)
PROP_SEG
PHASE_SEG_1
PHASE_SEG_2
SYNC_SEG
Sample Point
16.2.3.2 Synchronization Segment
The first segment is used to synchronize the various bus nodes.
On transmission, at the start of this segment, the current bit level is output. If there is a bit state change between the previous
bit and the current bit, then the bus state change is expected to occur within this segment by the receiving nodes.
16.2.3.3 Propagation Time Segment
This segment is used to compensate for signal delays across the network.
This is necessary to compensate for signal propagation delays on the bus line and through the transceivers of the bus
nodes.
16.2.3.4 Phase Segment 1
Phase Segment 1 is used to compensate for edge phase errors.
This segment may be lengthened during re-synchronization.
16.2.3.5 Sample Point
The sample point is the point of time at which the bus level is read and interpreted as the value of the respective bit. Its
location is at the end of phase segment 1 (between the two phase segments).
16.2.3.6 Phase Segment 2
This segment is also used to compensate for edge phase errors.
This segment may be shortened during re-synchronization, but the 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.
16.2.3.7 Information Processing Time
It 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 2TQ for the Atmel CAN. Since phase segment 2 also
begins at the sample point and is the last segment in the bit time, PS2 minimum shall not be less than the IPT.
ATmega16/32/64/M1/C1 [DATASHEET]
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MICROCHIP [ MICROCHIP ]
