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ST7 pCAN PERIPHERAL DRIVER

Message prioritization

– The identifier of a message also indicates the priority of its carrier. During the arbitration

phase, a bitwise comparison of the different identifiers is performed. The message with the

higher identifier being the more urgent, it wins the arbitration and is immediately transmit-

ted.

Error management system (See “Error Management” on page 11)

– Detection: every node continuously monitors the bus, even if it is not concerned by the cur-

rent transmission, to detect potential violations of the protocol.

– Signalization: any node detecting an error, signals it to its peers and aborts the current

transmission meanwhile. The sender automatically retries transmission within a certain

amount of time. Thus, the consistency of data throughout the network is preserved.

– Confinement: each node implements counters which are incremented (or decremented)

after each detection of an error (or successful operation). A defective station can then

switch itself off when its counters reach a certain value, and thus stop disturbing the bus

operations.

1.1.2 CAN in the OSI reference model

The protocol occupies parts of the two lowest levels (the physical and data link layers) of the

7-level ISO/OSI (Open System Interconnection) telecommunications reference model.

Table 1. ISO/OSI Standard Telecommunication Layers

In this reference model, the data link layer deals with physically passing data from one node to

another. It defines arbitration, message framing, error management and transmission timing

for example.

The physical layer deals with putting data on the physical network, and with taking it off. Con-

sequently, it defines signal levels and timings, as well as the transmission support (wire types,

connectors, etc.).

The CAN protocol splits the Data Link layer into Object and Transfer sublayers, corre-

sponding to the ISO/OSI Logical Link Control and Medium Access Control sublayers. The

Transfer layer is the interface between the user application and the CAN. The Object layer

OSI Layer

OSI Model

CAN Protocol

Application

No (User defined)

Presentation

No (User defined)

Session

No (User defined)

Transport

No (User defined)

Network

No (User defined)

Data Link

Yes

Physical

Partly

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