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DS90UB954-Q1: GPIO3/INTB shorting intput.

Sebastian Graf
Prodigy 30 points
Part Number: DS90UB954-Q1

Hello,

in this application GPIO3 is used as an input which is connected to a buffer output.

Now I do have some strange behavior. Once I set the input to "high", the supplying part (Buffer) will get shorted, since for some reason the GPIO3 is pulling down the signal.
Alternating the input high->low, low->high is not recognized by 954's status registers. The only other way to make the Pin "poll" it's actual state is by setting the line buffer in tri-state.
The exact same happens on DS90UB936.
Is there any chance, that the "INTB" can somehow change the state of GPIO3, while the pin is configured as input?


This behavior is as follows:

1. Buffer is in tri-state
->(Startup)


2. Buffer output is active low
-> signal is recognized by the deserializer


3. Buffer output is active high
-> signal is not recognized by the deserializer. We could also measure a voltage drop at the gpio. We are sure that the deseriaizer pulls the voltage down.


4. Buffer is tri-state
-> if the buffer is not going into tri-state for a short moment, step 5 would show the same result as step 3


5. Buffer is active high
-> we can measure the full supply voltage at the gpio. gpio is also recognized by deserializer


best regards

Sebastian

  • 0
    TI__Genius 11340 points
    Hi Sebastian,

    GPIO3 is actually open-drain, we recommend connecting a 4.7 kOhm pull-up resistor to VDDIO for proper operation.

    Best,
    Jiashow
  • 0 in reply to Jiashow Ho
    Prodigy 30 points

    Hi Jiashow,

    thanks for your answer.
    I know GPIO is open drain. That's why I ask.
    As soon as I put GPIO 3 to input, I expect the Pin to be high resistance.
    However it seems to open up on certain occasions and short the driving Buffer.
    I tested this with 4.7kOhm and without. No difference.

    As I see it, the 4.7k only is relevant, as soon as GPIO3 acts as an output, since it is open drain and can't output any voltage.

    Best regards

    Sebastian

  • 0 in reply to Sebastian Graf
    TI__Genius 11340 points
    Hi Sebastian,

    Regardless of whether you are configuring this as input or output, you need the 4.7kOhm pull-up to VDDIO.

    Can you measure the voltage level at that pin when the input is high and when the input is low?

    Best,
    Jiashow
  • 0 in reply to Jiashow Ho
    Prodigy 30 points

    Hi Jiashow,

    The 4.7kOhm is in place. I only removed it for the test.
    I can measure te voltages. Only when I had a High to low transition, the level was Stuck at ~1,27V some times and no "Lo" was detected.
    I resolved the problem myself in the meanwhile by increasing the pullup to 10kOhms. Now the Transitions work every time.
    Might have to do with the corresponding part, since it's an active I/O, which interfered with the pullup. With the reduces current through the pullup it works fine.
    I'm aware I might have some problems with very fast transitions on the GPIO, due to the limited current through the 10k, but it might work for my application.

    Thanks for your help anyways

    Regards

    Sebastian