Datasheet

39.6.2.6 Bus Monitoring Mode
The CAN is set in Bus Monitoring Mode by programming CCCR.MON to ‘1’. In Bus Monitoring Mode (see
ISO 11898-1, 10.12 Bus monitoring), the CAN is able to receive valid data frames and valid remote
frames, but cannot start a transmission. In this mode, it sends only recessive bits on the CAN bus. If the
CAN is required to send a dominant bit (ACK bit, overload flag, active error flag), the bit is rerouted
internally so that the CAN monitors this dominant bit, although the CAN bus may remain in recessive
state. In Bus Monitoring Mode register TXBRP is held in reset state.
The Bus Monitoring Mode can be used to analyze the traffic on a CAN bus without affecting it by the
transmission of dominant bits. The figure below shows the connection of signals CAN_TX and CAN_RX
to the CAN in Bus Monitoring Mode.
Figure 39-3. Pin Control in Bus Monitoring Mode
CAN_TX CAN_RX
=1
TX
HANDLER
RX
HANDLER
CAN
Bus Monitoring Mode
39.6.2.7 Disabled Automatic Retransmission
According to the CAN Specification (see ISO 11898-1, 6.3.3 Recovery Management), the CAN provides
means for automatic retransmission of frames that have lost arbitration or that have been disturbed by
errors during transmission. By default automatic retransmission is enabled. To support time-triggered
communication as described in ISO 11898-1, chapter 9.2, the automatic retransmission may be disabled
via CCCR.DAR.
Frame Transmission in DAR Mode
In DAR mode all transmissions are automatically cancelled after they started on the CAN bus. A Tx
Buffer’s Tx Request Pending bit TXBRP.TRPx is reset after successful transmission, when a transmission
has not yet been started at the point of cancellation, has been aborted due to lost arbitration, or when an
error occurred during frame transmission.
• Successful transmission:
– Corresponding Tx Buffer Transmission Occurred bit TXBTO.TOx set
– Corresponding Tx Buffer Cancellation Finished bit TXBCF.CFx not set
• Successful transmission in spite of cancellation:
– Corresponding Tx Buffer Transmission Occurred bit TXBTO.TOx set
– Corresponding Tx Buffer Cancellation Finished bit TXBCF.CFx set
• Arbitration lost or frame transmission disturbed:
– Corresponding Tx Buffer Transmission Occurred bit TXBTO.TOx not set
– Corresponding Tx Buffer Cancellation Finished bit TXBCF.CFx set
SAM D5x/E5x Family Data Sheet
CAN - Control Area Network
© 2019 Microchip Technology Inc.
Datasheet
DS60001507E-page 1211