38.6.2.6 Bus Monitoring Mode

The CAN is set in Bus Monitoring Mode by programming CCCR.MON bit (CCCR <5>) 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 CANx_TX and CANx_RX to the CAN in Bus Monitoring Mode.