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TCA6408A: input state when floating unuse pin

Jason Dai
Expert 3720 points
Part Number: TCA6408A

Dear E2E,

we would like to know the input voltage or state by reading TCA6408A through I2C when we floating unuse pin.

BR,

Jason

  • 0
    TI__Guru 72361 points

    Jason,

    The P-Port of this device does not reference ground or Vcc when it is set as an input so the pin really is floating. A floating pin's value could be anything. Typically the parasitic capacitance on the pin will discharge through the air or any parasitic path it can find. So my guess would be the value would read GND in most cases. If the environment or board sees a lot of electrical noise, the pin could see spikes above the ViH levels. Any crosstalk on the board would also cause the parasitic cap to charge and discharge. A floating pin's value is really unknown and can change depending on multiple factors.

    -Bobby

  • 0 in reply to BOBBY
    TI__Expert 3720 points

    Bobby,

    The issue customer suffer is that they swap TCA6408A on the same PCB board, but the result they read from floating pin is different.

    They think it is IC issue.

    what is your thought?

    BR,

    Jason

  • 0 in reply to Jason Dai
    TI__Guru 72361 points

    Jason,

    What do you mean the customer issue suffer?

    Do you have a schematic we can review?

    -Bobby

  • 0 in reply to BOBBY
    TI__Expert 3720 points

    Bobby,

    Customer left P03-P07 open or floating.

    They will read all of pin status back in production line.

    For floating pin, they could read high or low status in different board always. They expect to read "Low" status.

    The question is that the board they could read high status. after they change a new IC, the status could become low.

    our question is why change IC could change the status at the same test environments and PCB board? 

    BR,

    Jason

  • 0 in reply to Jason Dai
    TI__Guru 72361 points

    Hey Jason,

    The only thing two things I could think are is:

    1) Noise (electrical)  in the environment could have an affect.

    2) leakage current into the floating nodes may be different between ICs

    We suggest customers reference all input pins. If you have the pins floating you can reference them by setting the unused pins to outputs.

    -Bobby