MEGA128CAN Datasheet, PDF (231/413 Pages) ATMEL Corporation – Microcontroller WITH 128K BYTES OF ISP FLASH AND CAN CONTROLLER

AT90CAN128
Format Co-existence
CAN Bit Timing
Bit Construction
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 identifier) and an 18-bit extension (identi-
fier 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.
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
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 fall-
ing edge SOF and on each recessive to dominant edge.
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 113. CAN Bit Construction
CAN Frame
(producer)
Transmission Point
(producer)
Time Quantum
(producer)
Segments
(producer)
Segments
(consumer)
Nominal CAN Bit Time
SYNC_SEG
propagation
delay
PROP_SEG
SYNC_SEG
PROP_SEG
PHASE_SEG_1
PHASE_SEG_2
PHASE_SEG_1
PHASE_SEG_2
Sample Point
Synchronization Segment
Propagation Time 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.
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.
4250E–CAN–12/04
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