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TMS570LS1227: DCAN unexpected transmission of dominant bit during MCU power-cycle

Luis Lanca
Prodigy 250 points
Part Number: TMS570LS1227

Dear all,

During a power-cycle of the PCBA that embeds (and powers) the MCU and CAN transceiver we observed a disturbance in the DCAN TX pin (unexpected dominant bit in CAN1TX signal, CAN_1_TX pin) and this behaviour was fixed with an external pull up resistor. (Additional info: The signal reset pull state was pull up. The behaviour was observed after MCU power on (OFF->ON) and after MCU power off (ON->OFF). The behaviour is reproducible using https://www.ti.com/tool/LAUNCHXL2-TMS57012 and measured with a logic analyser.)

The applied solution is documented for TIOC and RIOC registers used to configure CAN_TX and CAN_RX as GIO pins—SPNU515C – March 2018, for TX, page 1596, Table 27-30, bit '0' description note—for easy access quoted here "Note: When CAN_TX pin is connected to a CAN transceiver, an external pull up resistor has to be used to ensure that the CAN bus will not be disturbed (for example, while the DCAN module is reset)."

In this e2e post is also claimed the note should only be applicable to GIO mode.

In our case the CAN_TX and RX are not configured as GIO but anyway the external pull up fixed the issue.

Could you please confirm that the note concerning the external pull up resistor is only applicable to IOC and RIOC registers used to configure CAN_TX and CAN_RX as GIO pins?

Thanks in advance for your time and support.

Best regards,

Luis

  • +1
    TI__Guru**** 193136 points

    Hi Luis,

    TI CAN transceivers have the input and output pins passively pulled high internally, so that in the absence of any input, the device automatically defaults to a recessive bus state on all input and output pins. So no external pull-up is required.

    If the transceiver doesn't have internal pull-up, it is better to add an external pull to CAN TX and RX signals.