Datasheet

corresponding binary values and cannot be changed dynamically. The identifier with the lowest binary
number has the highest priority.
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" automatically become receivers of
the message with the highest priority and do not re-attempt transmission until the bus is available again.
20.3.2. 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, supports a length of 29 bits for the
identifier.
20.3.2.1. CAN standard frame
Figure 20-1. CAN standard frames
11-bit identifier
ID10..0
Interframe
Space
4-bit DLC
DLC4..0
CRC
del.
ACK
del.
15-bit CRC
0 - 8 bytes
SOF
SOF
RTR
IDE r0 ACK
7 bits
Intermission
3 bits
Bus Idle Bus Idle
(Indefinite)
Arbitration
Field
Data
Field
Data frame
Control
Field
End of
Frame
CRC
Field
ACK
Field
Interframe
Space
11-bit identifier
ID10..0
Interframe
pace
4-bit DLC
DLC4..0
CRC
del.
ACK
del.
15-bit CRC
SOF
SOF
RTR
IDE r0 ACK
7 bits
Intermission
3 bits
Bus Idle Bus Idle
(Indefinite)
Arbitration
Field
Remote frame
Control
Field
End of
Frame
CRC
Field
ACK
Field
Interframe
Space
A message in the CAN standard frame format begins with the "Start Of Frame (SOF)", this is followed 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 messages 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.
Atmel ATmega16M1/32M1/64M1 [DATASHEET]
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