70353C datasheet(63/76 Pages) MICROCHIP

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DS70353C-page 21-63

Section 21. Enhanced Controller Area Network (ECAN™)

21.10

ECAN ERROR MANAGEMENT

21.10.1 CAN Bus Errors

The CAN specification defines five different ways of detecting errors:

• Bit Error

• Acknowledge Error

• Form Error

•Stuffing Error

• CRC Error

The bit error and the acknowledge error occur at the bit level; the other three errors occur at the

message level.

21.10.1.1 BIT ERROR

A node that is sending a bit on the bus also monitors the bus. A bit error is detected when the bit

value that is monitored is different from the bit value that is sent. An exception is when a

recessive bit is sent during the stuffed bit stream of the Arbitration field or during the ACK slot. In

this case, no bit error occurs when a dominant bit is monitored. A transmitter sending a passive

error frame and detecting a dominant bit does not interpret this as a bit error.

21.10.1.2 ACKNOWLEDGE ERROR

In the Acknowledge field of a message, the transmitter checks if the Acknowledge Slot (which it

has sent out as a recessive bit) contains a dominant bit. If not, this implies that no other node has

received the frame correctly. An acknowledge error has occurred, and as a result, the message

must be repeated. No error frame is generated in this case.

21.10.1.3 FORM ERROR

A form error is detected when a fixed-form bit field (EOF, Inter-frame Space, Acknowledge

Delimiter or CRC Delimiter) contains one or more illegal bits. For a receiver, a dominant bit during

the last bit of EOF is not treated as a form error.

21.10.1.4 STUFFING ERROR

A stuffing error is detected at the bit time of the sixth consecutive equal bit level in a message

field that should be coded by the method of bit stuffing.

21.10.1.5 CRC ERROR

The node transmitting a message computes and transmits the CRC corresponding to the

transmitted message. Every receiver on the bus performs the same CRC calculation as the

transmitter. A CRC error is detected if the calculated result is not the same as the CRC value

obtained from the received message.

21.10.2 Fault Confinement

Every CAN controller on a bus tries to detect the errors outlined above within each message. If

an error is found, the discovering node transmits an error frame, thus destroying the bus traffic.

The other nodes detect the error caused by the error frame (if they have not already detected the

original error) and take appropriate action (that is, discard the current message).

The ECAN module maintains two error counters:

• Transmit Error Counter (CiEC<15:8>)

• Receive Error Counter (CiEC<7:0>)

There are several rules governing how these counters are incremented and/or decremented.

That is, a transmitter detecting a fault increments its transmit error counter faster than the

listening nodes will increment their receive error counter. This is because there is a good chance

that it is the transmitter that is at fault.

Note:

The error counters are modified according to the CAN Specification 2.0B.

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