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TCAN1042-Q1: Fall into bus-off state when CAN H/L is open

Kai Nomiyama
Expert 5615 points
Part Number: TCAN1042-Q1
Other Parts Discussed in Thread: AM4376

Hi Team,

I have a question about TCAN1042 bus-off state.

with below condition, some AM4376s detect bus-off but almost all AM4376s don't detect bus-off.
AM4376 ⇔ TCAN1042 (CAN H/L open)

From the datasheet, when VCANH/L are open, OUTPUT RXD is H.
in that case, why does the system fall into bus-off? Is it normal behavior?

If it is normal behavior, could you let me nkow the reason why some AM4376s detect bus-off but almost all AM4376s don't detect bus-off?

Regards,
Kai

  • 0
    TI__Guru 53575 points

    Hi Kai,

    From my understanding, AM4376 switching into a bus-off state means that the controller has recognized as series of errors and has therefore stopped trying to communicate with the CAN bus. This may be occurring because the controller is expecting some behavior from the bus which has been disconnected.

    While the bus is open, if TCAN1042 TXD is driven by the controller, it will still attempt to drive the bus. This will in turn loop-back on the RXD pin (as the transceiver has not been disconnected from itself) and the controller will see RXD toggle in response to its own transmission. If this disconnected node expects an acknowledgement or other information from elsewhere on the bus and gets NACKed, errors may accumulate until it switches to a bus-off state.

    If you still believe there to be an issue with TCAN1042, please share scope shots of the CAN bus, TXD, and RXD so we can diagnose if the device is functioning correctly in the open-bus configuration. Otherwise I would recommend inquiring what errors are being thrown by AM4376 to cause a bus-off state to find the root cause.

    Let me know if you have any more questions.

    Regards,
    Eric

  • 0 in reply to Eric Schott1
    TI__Expert 5615 points

    Hi Eric,

    You are right, bus-off state means that the controller has recognized as series of errors and has therefore stopped trying to communicate with the CAN bus as you mentioned.
    From your comment, I understand when AM4376 drives TXD, TCAN1042 loops back on the RXD pin and the AM4376 see RXD toggle in response to its own transmission.

    Here is additional question.
    Some of AM4376 switch to bus-off state but almost all AM4376 don't switch to bus-off.
    Do you know the reason why we see this two different cases?

    There is one different to Rid.
    - No bus-off: Rid=49kohm
    - Bus-off: Rid=47kohm

    If you need more information, please let me know.

    Regards,
    Kai

  • 0 in reply to Kai Nomiyama
    TI__Guru 53575 points

    Hi Kai,

    Could you share the termination scheme at each node and weather they are detecting bus-off? It would also be helpful to see the differences between nodes which do and do not detect bus-off on the scope. Please share the state of the CAN lines as well as TXD and RXD if possible. 

    I am not too familiar with AM4376 so please excuse any vague questions. Are any other errors reported by DCAN in the faulty cases? What interrupts are present before the issue occurs?

    Regards,
    Eric