Beaglebone black GPIO registers not changing voltage on value change

Question

I'm trying to turn on and off the GPIO headers on the beaglebone, but i'm unable to get the physical pins to switch from high to low and vice versa. Ive written some code within my application to do this but even when I change the values in the command line I have the same issues.

Firstly, all of the pins I want to use have been correctly exported. For this example lets focus on GPIO 117. I'm able to change into /sys/class/gpio/gpio117 and when I run cat value, its in line with what I expected from my program. When I run echo 0 > value it changes to a zero and when I run echo 1 > value its a 1. Everything as expected. When I go to measure the voltage on that pin, it is always high, independent of the value.

Am I missing something here?

Answer 1

GPIO 117 is gpio3_21 on the "MCSAP0_AHCLKX" pin. So it is probably used by some audio device, probably HDMI. You can disable that by adding disable_uboot_overlay_audio=1 to your /boot/uEnv.txt file, see https://elinux.org/Beagleboard:BeagleBoneBlack_Debian#U-Boot_Disable_on-board_devices