Datasheet

20.3.2.2. CAN extended frame
Figure 20-2. CAN extended frames
11-bit base identifier
IDT28..18
Interframe
Space
CRC
del.
ACK
del.
15-bit CRC
0 - 8 bytes
SOF
SOF
SRR
IDE ACK
7 bits
Intermission
3 bits
Bus Idle Bus Idle
(Indefinite)
Arbitration
Field
Arbitration
Field
Data
Field
Data frame
Control
Field
Control
Field
End of
Frame
CRC
Field
ACK
Field
Interframe
Space
11-bit base identifier
IDT28..18
18-bit identifier extension
ID17..0
18-bit identifier extension
ID17..0
Interframe
Space
4-bit DLC
DLC4..0
CRC
del.
ACK
del.
15-bit CRC
SOF
SOF
SRR
IDE r0
4-bit DLC
DLC4..0
RTR
RTR
r0r1
r1 ACK
7 bits
Intermission
3 bits
Bus Idle Bus Idle
(Indefinite)
Remote frame
End of
Frame
CRC
Field
ACK
Field
Interframe
Space
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 (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.
20.3.2.3. 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
20.3.3. 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.
20.3.3.1. 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.
Atmel ATmega16M1/32M1/64M1 [DATASHEET]
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