AN1105 datasheet(11/100 Pages) STMICROELECTRONICS

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ST7 pCAN PERIPHERAL DRIVER

1.4.2 Message Acknowledgement

Any cell having received a message correctly, regardless of the result of the acceptance test,

must acknowledge it by sending a dominant bit during the ACK slot of the current frame, thus

overwriting the recessive bit sent by the current transmitter.

1.5 ERROR MANAGEMENT

The Error Management System of the CAN protocol is proof of the protocol’s robustness. This

system provides a way to automatically distinguish permanent failures from sporadic errors. It

can lead to self-disconnection of a failing node from the bus. This feature is named Error Con-

finement. An error is defined as a violation of one of the protocol’s rules. It can happen after

one of following events:

–

Bit error: A transmitter detects a difference between the actual bus state and what it

sends.

–

Acknowledgement error: A frame is not acknowledged (ACK slot recessive).

–

Stuffing error: More than five identical bits were received consecutively.

–

CRC error: The CRC received does not match the calculated CRC.

–

Format error: One of the fixed-format fields (CRC delimiter, ACK delimiter, End Of Frame)

does not have the expected format.

An Error frame is immediately sent by all the cells that have detected an error on the bus. Note

that the Error frame does not yield to the Stuffing rule. Upon reception of this frame, all cells in

the network detect a Stuffing error and react by sending an Error frame. The dominant state of

the bus caused by the occurrence of an error can last at most 12 bits (case when a node de-

tects the error only at the end of the error flag).

The Error Confinement system is based on two counters:

– A TransmissionError counter (TEC)

– A ReceptionError Counter (REC)

The rules handling the counters are basically the following:

– When an emitting node detects an error, it increments its TEC and sends an error frame.

– When a receiving node detects an error it increments its REC and sends an error frame.

– When a transmission is successful, the TEC counter is decremented.

– When a reception is successful, the REC counter is decremented.

Note: The complete set of rules is quite complicated taking into account the possibility of receiving over-

load frames, etc. Consequently, it’s beyond the scope of this document. This document describes

the basic reactions of the cell. For more information, refer to the original Robert Bosch GmBH doc-

ument or to the ISO 11898 norm.

The behaviour of the cell concerning the state of the counters is shown in the following dia-

gram.

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