Maxim Integrated High-Speed Microcontroller Users Guide: DS80C390 Supplement - Manual (Page 134)

High-Speed Microcontroller User’s Guide: DS80C390 Supplement
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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 2.0 B), and four different frame types for CAN Bus communications.
The standard format seen below makes use of an 11-bit identifier.
Figure 19-1. CAN 2.0A 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
The extended format seen below makes use of a 29-bit identifier.
Figure 19-2. CAN 2.0B 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
End of
Frame
18-bit Identifier
S
R
R
r
1
Bus
Idle
The four different 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 include the Start of Frame, Arbitration Field, Control Field, Data Field, CRC Field,
Acknowledge Field and an End of Frame. A description of these fields follows.
Start of Frame (SOF) (Standard and Extended Format)
The Start of Frame is a dominant bit that signals the start of a Data or Remote Frame. The dominant
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 additional 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 bit is a dominant bit in Data Frames and a recessive bit in Remote Frames.
• Substitution Remote Request (SRR) bit: (Extended Format)