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Using GPIO11 clobbers I2C?



  • Hi all,

    I'm trying to use the PWM Expansion module along with a GPIO (11) as an output. Though as soon as I set the output mode on the GPIO, all the I2C devices vanish, disabling the PWM Expansion module.

    Nothing is connected, just the Omega2 on a Power Dock with the PWM Expansion on top. I even have a rubber spacer in between the two to avoid any shorts. I've used the PWM Expansion on its own in past projects, has worked fine.

    On bootup, everything looks fine:

    root@Omega-10D9:~# i2cdetect -y 0
         0  1  2  3  4  5  6  7  8  9  a  b  c  d  e  f
    00:          -- -- -- -- -- -- -- -- -- -- -- -- --
    10: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
    20: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
    30: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
    40: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
    50: -- -- -- -- -- -- -- -- -- -- 5a -- -- -- -- --
    60: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
    70: 70 -- -- -- -- -- -- --
    

    Great! 5A is the PWM Expansion (not sure what 70 is). But then I run this little python script:

    import onionGpio
    
    gpio11 = onionGpio.OnionGpio(11)
    gpio11.setOutputDirection(0)
    

    And wham, no more I2C devices:

    root@Omega-10D9:~# i2cdetect -y 0
         0  1  2  3  4  5  6  7  8  9  a  b  c  d  e  f
    00:          -- -- -- -- -- -- -- -- -- -- -- -- --
    10: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
    20: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
    30: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
    40: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
    50: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
    60: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
    70: -- -- -- -- -- -- -- --
    

    Any suggestions? I specifically chose GPIO11 because it's not associated with any other function.

    Thanks!



  • Dug a bit deeper and it seems GPIO11 is reserved for the reset button, etc. It's just not shown on the dock pinouts, it appears to be free i.e.

    alt text

    Reference:

    https://docs.onion.io/omega2-docs/using-gpios.html#pins-at-boot
    http://onioniot.github.io/wiki/Documentation/Hardware/Omega-Hardware.html

    So I think I'll be OK using a different GPIO, but would be nice to understand what's going on here a little more and have that shown in the dock pinouts.



  • @Leif-Bloomquist This issue is a hardware compatibility side effect.

    Omega2: During boot the boot loader sets GPIO11 to Output and High. This ~3.3V high level will be the "power suply" (instead of Omega's 2.5V)

    • for the RESET BUTTON (FW_RST) on Omega2 Expansion Dock, Power Dock, Power Dock 2 and Arduino Dock
    • for the I2C level shifter's lower voltage section on a PWM/Servo Expansion and/or an OLED Expansion

    The Expansion Header of the "original" Omega vs. Omega2
    expansion-headers.png

    I2C logic level shifters: VDD25 ie. 2.5V or 3.3V of the GPIO11 Output High is the power supply of the lower voltage section - these circuits don't work without it.
    ( GPIO20, GPIO21 are GPIO4 and GPIO5 on Ommega2 respectively)
    pwm-expansion-resized.png

    You can find lots of details about Omega2 hardware for example here:
    https://github.com/OnionIoT/Onion-Hardware



  • Thanks for the detailed explanation!

    Though it makes me wonder why GPIO11 is on the breakout header at all. It's doesn't seem usable by end users.



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