Rainbow Electronics AT91CAP9S250A - Manual (Page 692)

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6264A–CAP–21-May-07
AT91CAP9S500A/AT91CAP9S250A
frozen. To go back to the standard mode, the ABM bit must be cleared in the CAN_MR
register.
39.6.4.2 Error Detection
There are five different error types that are not mutually exclusive. Each error concerns only
specific fields of the CAN data frame (refer to the Bosch CAN specification for their
correspondence):
• CRC error (CERR bit in the CAN_SR register): With the CRC, the transmitter calculates a
checksum for the CRC bit sequence from the Start of Frame bit until the end of the Data
Field. This CRC sequence is transmitted in the CRC field of the Data or Remote Frame.
• Bit-stuffing error (SERR bit in the CAN_SR register): If a node detects a sixth consecutive
equal bit level during the bit-stuffing area of a frame, it generates an Error Frame starting
with the next bit-time.
• Bit error (BERR bit in CAN_SR register): A bit error occurs if a transmitter sends a
dominant bit but detects a recessive bit on the bus line, or if it sends a recessive bit but
detects a dominant bit on the bus line. An error frame is generated and starts with the next
bit time.
• Form Error (FERR bit in the CAN_SR register): If a transmitter detects a dominant bit in
one of the fix-formatted segments CRC Delimiter, ACK Delimiter or End of Frame, a form
error has occurred and an error frame is generated.
• Acknowledgment error (AERR bit in the CAN_SR register): The transmitter checks the
Acknowledge Slot, which is transmitted by the transmitting node as a recessive bit,
contains a dominant bit. If this is the case, at least one other node has received the frame
correctly. If not, an Acknowledge Error has occurred and the transmitter will start in the next
bit-time an Error Frame transmission.
Fault Confinement
To distinguish between temporary and permanent failures, every CAN controller has two error
counters: REC (Receive Error Counter) and TEC (Transmit Error Counter). The counters are
incremented upon detected errors and respectively are decremented upon correct transmis-
sions or receptions. Depending on the counter values, the state of the node changes: the
initial state of the CAN controller is Error Active, meaning that the controller can send Error
Active flags. The controller changes to the Error Passive state if there is an accumulation of
errors. If the CAN controller fails or if there is an extreme accumulation of errors, there is a
state transition to Bus Off.