Strange GPIO voltage issue

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  • #1979

    I’m having a bizarre issue with the PTT GPIO output that I’m wondering if anyone has come across before. Logic high on the PTT GPIO is way lower voltage than it should be. Now seeing it across two different known-good Pi3s.

    Setup:
    – Raspberry Pi 3
    – 2x Kenwood TK-805d with I/O mods
    – Opto-isolated COS (3.3V active-high)
    – Easy Digi opto-isolated PTT

    The issue is that for some reason no matter which GPIO I select as PTT, the logic high signal is only a few hundred mV. Additionally other GPIOs will also go high at a few hundred mV when they should stay at 0V. The level is not consistent between trials either. Scope captures below.

    It should be noted that the COS input GPIO is working fine. I hear audio passed via the sound card as expected when a signal is received and all the levels look ok.

    I was thinking this could be a fried GPIO controller in some way, but I’m seeing identical behavior after switching in another known-good Raspberry Pi 3. Input pin works fine, audio is passed, but PTT GPIO voltage is way too low.

    Really scratching my head on this one. Unless I managed to destroy the GPIO outputs in exactly the same way on two different boards this points to something software-related… but I’m not sure how it could.

    COS signal working at a full scale 3.3V, as a sanity-check:

    PTT output on pin 12 (with output set to pin 12), note different voltage levels at different times but both way off. Keep in mind this is with nothing external connected to the pin other than the oscilloscope probe.

    On an unrelated pin (15) that should be off or floating, we somehow get a higher output signal than on pin 12:

    Any input would be appreciated!

    #1980
    Aaron, N3MBH
    Forum Administrator

    A few thoughts. One are you trying to check this with circuitry attached? If so I would remove your interface and use a basic LED and resistor between your gpio and ground pin. Also, verify that the gpio pins are properly registered and showing up under /sys/gpio. I would also recommend toggling the high/low values outside of ORP from just the command line to make sure the low level functionality is working. To me it sound like you might have a short in your circuit that is. I would check that first.

    73,
    Aaron – N3MBH / WRFV871

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    #1982
    Aaron, N3MBH
    Forum Administrator

    Check out this link for instructions on controlling the GPIO pins from command line:
    https://sites.google.com/site/semilleroadt/raspberry-pi-tutorials/gpio

    73,
    Aaron – N3MBH / WRFV871

    OpenRepeater is offered free of charge. Find out how you can support us.

    #1989

    All those measurements were taken with the circuitry not connected – just the oscilloscope probe straight to the GPIO pins. Though the behavior is the same when it is connected. Not enough voltage is produced to activate the optocoupler, which is how I noticed the problem originally.

    I’ll give manually setting the pins a try. Could you point me to where in the code the PTT pin gets set in case I need to tinker with it?

    Thanks!

    #1990
    Aaron, N3MBH
    Forum Administrator

    SSH into the board and shut down the SVXLink Core (services svxlink stop). If you don’t know how to SSH into the board, see this: https://openrepeater.com/knowledgebase/topic/connecting-via-ssh

    Once you do that use the tutorial above (just the top portion of the page) to register pins and manually toggle the values. You can cat the values on both in and out pins to see what their current state is and you can echo out a new value to output pins.

    Also, make sure that you are specifying that GPIO pin numbers and not the actual header pin numbers. If the RPI seems to function already, try another GPIO pin. Maybe try the inputs you know are good.

    73,
    Aaron – N3MBH / WRFV871

    OpenRepeater is offered free of charge. Find out how you can support us.

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