Figure 396. Pin Control In Bus Monitoring Mode - ST STM32G0 1 Series Reference Manual

RM0444 FD controller area network (FDCAN)
Bus Monitoring mode
The FDCAN is set in Bus Monitoring mode by setting CCCR.MON bit. In Bus Monitoring
mode (for more details refer to ISO11898-1, 10.12 Bus monitoring), the FDCAN 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 FDCAN is required to send a
dominant bit (ACK bit, overload flag, active error flag), the bit is rerouted internally so that
the FDCAN can monitor it, even if the CAN bus remains in recessive state. In Bus
Monitoring Mode the TXBRP register 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. Figure 396 shows the connection of FDCAN_TX and
FDCAN_RX signals to the FDCAN in Bus Monitoring mode.

Figure 396. Pin control in Bus Monitoring mode

FDCAN_TX FDCAN_RX
=1
Tx Rx
FDCAN
MS41462V1
Disabled Automatic Retransmission (DAR) mode
According to the CAN Specification (see ISO11898-1, 6.3.3 Recovery Management), the
FDCAN 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.
Frame transmission in Disabled Automatic Retransmission (DAR) mode
In DAR mode all transmissions are automatically canceled after they have been started on
the CAN bus. A Tx Buffer Tx Request Pending bit TXBRP.TRPx is reset after successful
transmission, when a transmission has not yet been started at the point of cancellation, or
has been aborted due to lost arbitration, or when an error has occurred during frame
transmission.
RM0444 Rev 5 1207/1390
1261