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ATmega16M1/32M1/64M1 [DATASHEET]
8209E–AVR–11/2012
A message in the CAN extended frame format is likely the same as a message in CAN standard frame format. The
difference is the length of the identifier used. The identifier is made up of the existing 11-bit identifier (base identi-
fier) and an 18-bit extension (identifier extension). The distinction between CAN standard frame format and CAN
extended frame format is made by using the IDE bit which is transmitted as dominant in case of a frame in CAN
standard frame format, and transmitted as recessive in the other case.
19.3.2.3
Format co-existence
As the two formats have to co-exist on one bus, it is laid down which message has higher priority on the bus in the
case of bus access collision with different formats and the same identifier / base identifier: The message in CAN
standard frame format always has priority over the message in extended format.
There are three different types of CAN modules available:
2.0A - Considers 29 bit ID as an error
2.0B Passive - Ignores 29 bit ID messages
2.0B Active - Handles both 11 and 29 bit ID Messages
19.3.3
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.
19.3.3.1
Bit construction
One CAN bit time is specified as four non-overlapping time segments. Each segment is constructed from an inte-
ger multiple of the Time Quantum. The Time Quantum or TQ is the smallest discrete timing resolution used by a
CAN node.
Figure 19-3. CAN bit construction.
19.3.3.2
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.
19.3.3.3
Propagation time segment
This segment is used to compensate for signal delays across the network.
Time quantum
(producer)
Nominal CAN Bit Time
Segments
(producer)
SYNC_SEG
PROP_SEG
PHASE_SEG_1
PHASE_SEG_2
propagation
delay
Segments
(consumer)
SYNC_SEG
PROP_SEG
PHASE_SEG_1
PHASE_SEG_2
Sample Point
Transmission point
(producer)
CAN frame
(producer)