CP3SP33SMSSLASHNOPB datasheet(169/407 Pages) TI1

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TxPIN

RxPIN

MODULE A

TxPIN

RECESSIVE

DOMINANT

MODULE A SUSPENDS TRANSMISSION

BUS LINE

RxPIN

MODULE B

DS019

CP3SP33

www.ti.com

SNOSCW5 – MAY 2013

22.2 Basic CAN Concepts

This section provides a generic overview of the basic concepts of the Controller Area Network (CAN).

The CAN protocol is a message-based protocol that allows a total of 2032 (211 –16) different messages in

the standard format and 512 million (229 – 16) different messages in the extended frame format.

Every CAN Frame is broadcast on the common bus. Each module receives every frame and filters out the

frames which are not required for the module's task. For example, if a dashboard sends a request to

switch on headlights, the CAN module responsible for brake lights must not process this message.

A CAN master module has the ability to set a specific bit called the “remote data request bit” (RTR) in a

frame. Such a message is also called a “Remote Frame”. It causes another module, either another master

or a slave which accepts this remote frame, to transmit a data frame after the remote frame has been

completed.

Additional modules can be added to an existing network without a configuration change. These modules

can either perform completely new functions requiring new data, or process existing data to perform a new

functionality.

As the CAN network is message oriented, a message can be used as a variable which is automatically

updated by the controlling processor. If any module cannot process information, it can send an overload

frame.

The CAN protocol allows several transmitting modules to start a transmission at the same time as soon as

they detect the bus is idle. During the start of transmission, every node monitors the bus line to detect

whether its message is over-written by a message with a higher priority. As soon as a transmitting module

detects another module with a higher priority accessing the bus, it stops transmitting its own frame and

switches to receive mode, as shown in Figure 22-2.

Figure 22-2. CAN Message Arbitration

If a data or remote frame loses arbitration on the bus due to a higher-prioritized data or remote frame, or if

it is destroyed by an error frame, the transmitting module will automatically retransmit it until the

transmission is successful or software has canceled the transmit request.

If a transmitted message loses arbitration, the CAN module will restart transmission at the next possible

time with the message which has the highest internal transmit priority.

Dual CAN Interfaces

169

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