GPIO output value not changing

Question

I am trying to setup up a output GPIO pin on my Nitrogen6X board, but I can't change the file value. I navigated to /sys/class/gpio/ and I exported my pin (GPIO18) with echo 18 > export. I was then able to change direction with the command echo out > direction and it seems like I should be able to change the value file the same way, with echo 1 > value, but this doesn't seem to be working. I am logged in as root and the permissions on both the direction file and the value file are the same: -rw-r--r-- 1 root root.

Does anyone have an idea why this would not be writing to this file?

Thanks so much for all of your help!

Answer 1

The commands that you have listed should work, if the gpio number is correct. While I have not worked with Nitrogen6X in particular, I have found out that Linux GPIO pin numbers often do not match the labels on the board. I advice trying to find out the proper mapping experimentally by watching all possible GPIOs:

cd /sys/class/gpio
for x in `seq 1 128`; do echo $x > export; done

ground the pin in question via 10k resistor, run:

grep . gpio*/value > /tmp/values0

connect the pin in question to Vcc via 10k resistor, run:

grep . gpio*/value > /tmp/values1

diff the files, and pay attention which pin has changed.