Using GPIO11 clobbers I2C?

Leif Bloomquist · 17 Apr 2020, 21:00

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!

Leif Bloomquist · 17 Apr 2020, 21:53

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.

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.

György Farkas · 19 Apr 2020, 08:16

@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

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)

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

Leif Bloomquist · 21 Apr 2020, 16:04

Thanks for the detailed explanation!

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