Frame types – Maxim Integrated High-Speed Microcontroller Users Guide: Network Microcontroller Supplement User 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.

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