ATmega16/32/64/M1/C1
Bus access conflicts are resolved by bit-wise arbitration on the identifiers involved by each node
observing the bus level bit for bit. This happens in accordance with the "wired and" mechanism,
by which the dominant state overwrites the recessive state. The competition for bus allocation is
lost by all nodes with recessive transmission and dominant observation. All the "losers" automat-
ically become receivers of the message with the highest priority and do not re-attempt
transmission until the bus is available again.
16.2.2
Message Formats
The CAN protocol supports two message frame formats, the only essential difference being in
the length of the identifier. The CAN standard frame, also known as CAN 2.0 A, supports a
length of 11 bits for the identifier, and the CAN extended frame, also known as CAN 2.0 B, sup-
ports a length of 29 bits for the identifier.
16.2.2.1
Can Standard Frame
Figure 16-1. CAN Standard Frames
Data Frame
Bus Idle
11-bit identifier
ID10..0
4-bit DLC
DLC4..0
CRC
del.
ACK
del.
Intermission
3 bits
Bus Idle
(Indefinite)
RTR IDE 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 identifier
ID10..0
4-bit DLC
DLC4..0
CRC
del.
ACK
del.
Intermission
3 bits
Bus Idle
(Indefinite)
RTR IDE 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 standard frame format begins with the "Start Of Frame (SOF)", this is fol-
lowed by the "Arbitration field" which consist of the identifier and the "Remote Transmission
Request (RTR)" bit used to distinguish between the data frame and the data request frame
called remote frame. The following "Control field" contains the "IDentifier Extension (IDE)" bit
and the "Data Length Code (DLC)" used to indicate the number of following data bytes in the
"Data field". In a remote frame, the DLC contains the number of requested data bytes. The "Data
field" that follows can hold up to 8 data bytes. The frame integrity is guaranteed by the following
"Cyclic Redundant Check (CRC)" sum. The "ACKnowledge (ACK) field" compromises the ACK
slot and the ACK delimiter. The bit in the ACK slot is sent as a recessive bit and is overwritten as
a dominant bit by the receivers which have at this time received the data correctly. Correct mes-
sages are acknowledged by the receivers regardless of the result of the acceptance test. The
end of the message is indicated by "End Of Frame (EOF)". The "Intermission Frame Space
(IFS)" is the minimum number of bits separating consecutive messages. If there is no following
bus access by any node, the bus remains idle.
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7647F–AVR–04/09
ATMEL [ ATMEL ]
