MEGA128CAN Datasheet, PDF (233/413 Pages) ATMEL Corporation – Microcontroller WITH 128K BYTES OF ISP FLASH AND CAN CONTROLLER

Errors
Error at Message Level
Error at Bit Level
Error Signalling
AT90CAN128
The bus access conflict is resolved during the arbitration field mostly over the identifier
value. If a data frame and a remote frame with the same identifier are initiated at the
same time, the data frame prevails over the remote frame (c.f. RTR bit).
Figure 114. Bus Arbitration
Arbitration lost
node A
TXCAN
node B
TXCAN
Node A loses the bus
Node B wins the bus
CAN bus
SOF ID10 ID9 ID8 ID7 ID6 ID5 ID4 ID3 ID2 ID1 ID0 RTR IDE - - - - - - - - -
The CAN protocol signals any errors immediately as they occur. Three error detection
mechanisms are implemented at the message level and two at the bit level:
• Cyclic Redundancy Check (CRC)
The CRC safeguards the information in the frame by adding redundant check bits at
the transmission end. At the receiver these bits are re-computed and tested against
the received bits. If they do not agree there has been a CRC error.
• Frame Check
This mechanism verifies the structure of the transmitted frame by checking the bit
fields against the fixed format and the frame size. Errors detected by frame checks
are designated "format errors".
• ACK Errors
As already mentioned frames received are acknowledged by all receivers through
positive acknowledgement. If no acknowledgement is received by the transmitter of
the message an ACK error is indicated.
• Monitoring
The ability of the transmitter to detect errors is based on the monitoring of bus
signals. Each node which transmits also observes the bus level and thus detects
differences between the bit sent and the bit received. This permits reliable detection
of global errors and errors local to the transmitter.
• Bit Stuffing
The coding of the individual bits is tested at bit level. The bit representation used by
CAN is "Non Return to Zero (NRZ)" coding, which guarantees maximum efficiency
in bit coding. The synchronization edges are generated by means of bit stuffing.
If one or more errors are discovered by at least one node using the above mechanisms,
the current transmission is aborted by sending an "error flag". This prevents other nodes
accepting the message and thus ensures the consistency of data throughout the net-
work. After transmission of an erroneous message that has been aborted, the sender
automatically re-attempts transmission.
4250E–CAN–12/04
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