Maxim Integrated High-Speed Microcontroller Users Guide: Network Microcontroller Supplement - Manual (Page 148)

High-Speed Microcontroller User’s
Guide: Network Microcontroller
Supplement
148
Frame Types
The CAN 2.0B protocol specifies two different message formats, the standard 11-bit (CAN 2.0A) and the extended 29-bit (CAN 2.0B),
and it also specifies four different frame types for CAN bus communications.
The standard format makes use of an 11-bit identifier, as follows.
Figure 19-1. CAN 2.0A (Standard) Format
The following extended format makes use of a 29-bit identifier.
Figure 19-2. CAN 2.0B (Extended) Format
The four frame types for CAN bus communications are the data frame, the remote frame, the error frame, and the overload frame.
Data frame: The data frame is formulated to carry data from a transmitter to a receiver. The preceding two figures are examples of
data frames in the standard and extended formats. The data frame is composed of seven fields. These are the start of frame, arbitra-
tion field, control field, data field, CRC field, acknowledge field, and end of frame. A description of these fields follows.
Start of frame (SOF): (Standard and extended format) The start of frame (SOF) is a dominant bit, which signals the start of a data or
remote frame. The dominant bit forces a hard synchronization, initiating the CAN controller receive mode.
Arbitration field: (Standard and extended format) The arbitration field contains the identifier of the message and a dominant remote
request (RTR) bit. The identifier is composed of one field in the standard 11-bit format or two fields in the extended 29-bit format. Two addi-
tional bits, the substitution remote request (SRR) bit and the identifier extension (IDE) bit, separate the two fields in the extended format.
Remote request (RTR) bit: (Standard and extended format) The remote request (RTR) bit is a dominant bit in data frames and a reces-
sive bit in remote frames.
Substitution remote request (SRR) bit: (Extended format) The substitution remote request (SRR) bit is a recessive bit and is substi-
tuted for the RTR bit when using the extended format.
Identifier extension (IDE) bit: (Extended format) The identifier extension (IDE) bit is a dominant bit in the standard format and a reces-
sive bit in the extended format. The IDE bit is located in the arbitration field in the standard format and is located in the control field in
the extended format.
S
O
F
R
T
R
I
D
E
r
0
11-bit Identifier 0 to 8 Bytes DLC 15-bit CRC
Arbitration Field Control Field Data Field CRC Field
ACK
Field INTER
Bus
Idle
End of
Frame
S
O
F
R
T
R
I
D
E
r
0
11-bit Identifier 0 to 8 Bytes DLC 15-bit CRC
Arbitration Field
Control
Field Data Field CRC Field
ACK
Field INTER
End of
Frame
18-bit Identifier
S
R
R
r
1
Bus
Idle
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