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SN65HVD1040: SN65HVD1040: CAN Transceiver : Come out of low power mode

Prasant J
Intellectual 510 points
Part Number: SN65HVD1040

Hi ,

I hope this is the right forum for this query.

In SN65HVD1040, there is a STB pin for putting CAN Transceiver in low power mode. I have not clearly understood the Low Power Mode.

As per section : 9.3.1.2: Low Power Mode: A dominant bit > 5us is passed to the receiver output. Does this mean that in low power mode, CAN messages will be received by the micro-controller? 

Any inputs on the above will be of help.

Thanks & regards, Pj

  • 0
    TI__Intellectual 2090 points

    That's a good question.  

    The low power mode of the SN65HVD1040, along with most CAN transceivers, is not meant to process CAN messages to the CAN controller.  The reason for the 5us dominant bit pass-through is to give the CAN controller an indication that the bus is awake, and the CAN node is ready to transition to Normal Mode by setting the STB pin low.  It is a wake indicator the CAN Controller. 

    The low power mode is generally used to conserve power when there is no expected traffic on the CAN bus.

    Max Megee

    TRX Applications

  • 0 in reply to Max Megee
    Intellectual 510 points

    : Thanks for the response

    I have understood the following:
    * In low power mode CAN messages will NOT be received by the micro-controller.
    * In low power mode, a dominant activity > 5 us is passed on to the micro-controller
    * Micro-controller CAN peripheral should have CAN activity interrupt to realise that CAN bus has activity
    * Until micro-controller switches CAN transceiver to high speed mode, it will not receive CAN messages.

    Are the above statements correct?


    I intend to run the CAN bus at 500 kbps. With 500 Kbps a CAN message SOF (start of Frame) dominant bit is 2 usec long. Can this baud rate generate activity on the transceiver receiver output?

    Thanks & regards, Pj

  • 0 in reply to Prasant J
    TI__Intellectual 2090 points

    You are correct.

    In low power mode, only bit-widths greater than 5us will pass through the receiver.  So until the micro-controller switches to high speed mode, you will be unable to pass CAN traffic at 500kbps.  You will be able to run just fine at this speed in Normal Mode.

    Thanks,
    Max

  • 0 in reply to Max Megee
    Intellectual 510 points

     : Thanks again.

    Do you think If the transceiver in low power mode receives 3 continuous dominant bits at 500 kbps [1 bit = 2us & 3 bits = 6 us] then it will pass the 6 us long dominant bit to the receiver output? Then this receiver output can be used by my micro-controller to wake up on CAN activity/wakeup interrupt.

    Will the above work?

    Thanks & regards, Pj

  • 0 in reply to Prasant J
    TI__Intellectual 2090 points

    Yes, in low-power mode a 6us long dominant pulse should work to use as a wake-interrupt on the RXD pin.  The RXD pin will reflect a 6us long Low logic signal to the micro-controller.

    Thanks,

    Max

  • 0 in reply to Max Megee
    TI__Intellectual 2090 points
    I do want to clarify that the 6us long dominant pulse will be constrained by the 5us filter time of the low-power filter. Therefore, given a 6us long dominant pulse, the response on the RXD pin will be a logic Low of 1us or less, given the filter constraints. But a 6us dominant pulse should be sufficient to serve as a wake interrupt to pass through the low-power receiver.

    Thanks,
    Max
  • 0 in reply to Max Megee
    TI__Intellectual 2090 points
    Correction to the above statement:
    "logic Low of 1us or less" should be
    "logic Low as short as 1us".

    Thanks,
    Max