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Original question:

TCAN1145-Q1: TCAN1145-Q1

TCAN1145-Q1: CANH and CANL short to GND detection

Manu Chang
Intellectual 1440 points
Part Number: TCAN1145-Q1

Hi Team,

My customers are facing a problem when using TCAN1145. Currently they are communicating with a positive common voltage on CAN bus. When they short the CANH and CANL to GND, TCAN1145 is not entering the fail safe mode, but it is still communicating with 0V common mode voltage on CAN bus.

However, it should be considered as a fault from customer's view since it is already shorted. And because there is no diagnostic function of TCAN1145, it cannot be read by SPI as well when there is no fail safe mode.

Why are the TCAN1145 not entering fail safe mode when CANH and CANL are short to GND? Could the big common mode choke be a reason?

If the TCAN1145 consider the above scenario is not a fault, how could it be detected via SPI?

Your support will help us a lot, thanks !

Manu

  • 0
    TI__Guru 50835 points

    Hi Manu,

    Fail-safe mode is used to protect the transceiver from conditions that could impact its own function such as overtemperature or undervoltage conditions. Because CAN bus conditions do not have an impact on this (apart from possibly overetemperature after some time), Failsafe mode is not associated with CAN bus faults. 

    If bus fault detection is a requirement for this application, the design should switch to using the TCAN1146 which supports this feature. This will allow CAN bus faults to be reporting via SPI.

    Let me know if you have any more questions. 

    Regards,
    Eric Schott 

  • 0 in reply to Eric Schott1
    TI__Intellectual 1440 points

    Hi Eric,

    Thanks for the reply. So could I understand this way, TCAN1145 may think that the CANH and CANL short is not a essential fault that can damage the IC so it will not turn to fail safe mode and continue communicating. If I would like to detect this kind of error, the only way is to use a transceiver with bus fault detection function? Are there some other way to detect it with using TCAN1145?

    Regards,

    Manu Chang

  • 0 in reply to Manu Chang
    TI__Intellectual 1440 points

    Hi Eric,

    I would like to add some information from customers.

    Customers added a relative big value common mode choke on CAN Bus, and the TCAN1145 continue communicating when CANH and CANL short to GND. But after removing the choke, it won't communicate when CANH and CANL short to GND.

    So Customers wondered, whether it is a fault for the TCAN1145?

    Are there any descriptions about all the fault mode of TCAN1145 and how it acts during all the faults?

  • 0 in reply to Manu Chang
    TI__Guru 50835 points

    Hi Manu,

    If the bus fault does not impact actual bus communication as would be recognizable by the CAN controller, then there would be no convenient way for the system to identify a bus fault without the use of a CAN transceiver with the bus fault detection feature.  

    The inductance of a CMC can have a decent impact on the recognition capability of bus faults as it changes the timing of the fault current that flows through the CAN driver. The transmission possibilities of a CAN network are largely dependant on the type of fault and the characteristics of the network. The CAN transceiver itself has minimal impact here because they all function on the same principle of dominant and recessive drive states. 

    The CAN physical layer is mostly capable of dealing with the following fault conditions while maintaining the ability to transmit data, albeit may need slower data rate as the common-mode will no longer be stable:
    CANH Open*
    CANL Open*
    CANH short to Vcc
    CANH short to Vsup
    CANL short to GND 

    *assuming there is enough capacitance in the system to keep to open line stable relative to the active communication line.

    The remaining faults generally result in complete communication failure because a differential cannot be driven between CANH and CANL
    CANH short to GND
    CANL short to Vcc
    CANL short to Vsup
    CANH short to CANL

    Regards, 
    Eric Schott