Controller Area Network (CAN) - The HCS12/9S12: An Introduction to Software and Hardware Interfacing - page 636

stuffing to avoid a long sequence of identical bit values. With these measures implemented, the
residual probability for undetected corrupted messages in a CAN system is as low as
Message error rate 3 4.7 3 10 2 11
CAN nodes are able to distinguish short disturbances from permanent failures. Defective
nodes are switched off from the CAN bus.

13.3 CAN Messages

The CAN protocol defines four different types of messages.
• Data frame . A data frame carries data from a transmitter to the receivers.
• Remote frame . A remote frame is transmitted by a node to request the
transmission of the data frame with the same identifier.
• Error frame . An error frame is transmitted by a node on detecting a bus error.
• Overload frame . An overload frame is used to provide for an extra delay between
the preceding and the succeeding data or remote frames.
Data frames and remote frames are separated from preceding frames by an interframe space.
Applications do not need to send or handle error and overload frames.

13.3.1 Data Frame
As shown in Figure 13.2, a data frame consists of seven different bit fields: start of frame,
arbitration, control, data, CRC, ACK, and end of frame.

Interframe
space or
overload
fr a me

Interframe
space

Data frame

Data
field

Start-of-
frame

Arbitration
field

Control
field

CRC
field

ACK
field

End-of-
frame

Figure 13.2 ■ CAN data frame

S TART - OF -F RAME F IELD
This field is a single dominant bit that marks the beginning of a data frame. A node is
allowed to start transmission only when the bus is idle. All nodes have to synchronize to the
leading edge of the field of the node that starts transmission first.

A RBITRATION F IELD
The format of the arbitration field is different for standard format and extended format
frames, as illustrated in Figure 13.3. The identifier's length is 11 bits for the standard format
and 29 bits for the extended format.
The identifier of the standard format corresponds to the base ID in the extended format.
These bits are transmitted most significant bit first. The most significant 7 bits cannot all be
recessive.