Atmel SAM9X25, Manual

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915
11054A–ATARM–27-Jul-11
SAM9X25
915
11054A–ATARM–27-Jul-11
SAM9X25
Figure 41-6. CAN Resynchronization
Autobaud Mode
The autobaud feature is enabled by setting the ABM field in the CAN_MR register. In this mode,
the CAN controller is only listening to the line without acknowledging the received messages. It
can not send any message. The errors flags are updated. The bit timing can be adjusted until no
error occurs (good configuration found). In this mode, the error counters are frozen. To go back
to the standard mode, the ABM bit must be cleared in the CAN_MR register.
41.7.4.2 Error Detection
There are five different error types that are not mutually exclusive. Each error concerns only spe-
cific 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.
SYNC_
SEG
PROP_SEG PHASE_SEG1 PHASE_SEG2
SYNC_
SEG
PROP_SEG PHASE_SEG1 PHASE_SEG2
Phase error
Phase error (max Tsjw)
SYNC_
SEG
SYNC_
SEG
SYNC_
SEG
PROP_SEG PHASE_SEG1
PHASE_SEG2
SYNC_
SEG
PHASE_SEG2
SYNC_
SEG
PROP_SEG PHASE_SEG1
PHASE_
SEG2
SYNC_
SEG
PHASE_SEG2
Phase error
Nominal
Sample point
Sample point
after resynchronization
Nominal
Sample point
Sample point
after resynchronization
THE PHASE ERROR IS POSITIVE
(the transmitter is slower than the receiver)
Received
data bit
Received
data bit
Nominal bit time
(before resynchronization)
Bit time with
resynchronization
Bit time with
resynchronization
Phase error (max Tsjw)
Nominal bit time
(before resynchronization)
THE PHASE ERROR IS NEGATIVE
(the transmitter is faster than the receiver)