16.2.2.2
CAN Extended Frame
Figure 16-2. CAN Extended Frames
Data Frame
Bus Idle
11-bit base identifier
IDT28..18
18-bit identifier extension
ID17..0
4-bit DLC
DLC4..0
CRC
del.
ACK
del.
Intermission Bus Idle
3 bits (Indefinite)
SRR IDE
RTR r1
r0
15-bit CRC
ACK
7 bits
SOF
0 - 8 bytes
Interframe
Space
Arbitration
Field
Control
Field
Data
Field
CRC
Field
ACK
Field
End of
Frame
Interframe
Space
Remote Frame
Bus Idle
11-bit base identifier
IDT28..18
18-bit identifier extension
ID17..0
4-bit DLC
DLC4..0
CRC
del.
ACK
del.
Intermission
3 bits
Bus Idle
(Indefinite)
SRR IDE
RTR r1
r0
15-bit CRC
ACK
7 bits
SOF
Interframe
Space
Arbitration
Field
Control
Field
CRC
Field
ACK
Field
End of
Frame
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 dis-
tinction 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.
16.2.2.3
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 mes-
sage 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
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 con-
structed from an integer multiple of the Time Quantum. The Time Quantum or TQ is the smallest
discrete timing resolution used by a CAN node.
170
ATmega16/32/64/M1/C1
7647F–AVR–04/09
ATMEL [ ATMEL ]
