MEGA128CAN Datasheet, PDF (230/413 Pages) ATMEL Corporation – Microcontroller WITH 128K BYTES OF ISP FLASH AND CAN CONTROLLER

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.
Can Standard Frame
Figure 111. CAN Standard Frames
Data Frame
Bus Idle _SOF
11-bit identifier
ID10..0
RTR IDE r0
4-bit DLC
DLC4..0
0 - 8 bytes
15-bit CRC
CRC
del.
ACK
ACK
del.
7 bits
Intermission Bus Idle
3 bits (Indefinite)
Interframe
Arbitration
Control
Data
CRC
ACK
End of
Space
Field
Field
Field
Field
Field
Frame
Remote Frame
Bus Idle
SOF
11-bit identifier
ID10..0
RTR IDE r0
4-bit DLC
DLC4..0
15-bit CRC
CRC
del.
ACK
ACK
del.
7 bits
Intermission Bus Idle
3 bits (Indefinite)
Interframe
Space
Interframe
Space
Arbitration
Field
Control
Field
CRC
ACK
End of
Field
Field
Frame
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 "IDen-
tifier 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 domi-
nant 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.
CAN Extended Frame
Figure 112. CAN Extended Frames
Data Frame
Bus Idle
SOF
11-bit base identifier
IDT28..18
SRR IDE
18-bit identifier extension
ID17..0
RTR r1
r0
4-bit DLC
DLC4..0
0 - 8 bytes
15-bit CRC
CRC
del.
ACK
ACK
del.
7 bits
Intermission Bus Idle
3 bits (Indefinite)
Interframe
Space
Arbitration
Field
Control
Field
Data
CRC
ACK
End of
Field
Field
Field
Frame
Remote Frame
Bus Idle
SOF
11-bit base identifier
IDT28..18
SRR IDE
18-bit identifier extension
ID17..0
RTR r1
r0
4-bit DLC
DLC4..0
15-bit CRC
CRC
del.
ACK
ACK
del.
7 bits
Intermission Bus Idle
3 bits (Indefinite)
Interframe
Space
Interframe
Space
Arbitration
Field
Control
Field
CRC
ACK
End of
Field
Field
Frame
Interframe
Space
230 AT90CAN128
4250E–CAN–12/04