The GPIO pins available on the 40-pin header can be easily controlled manually via sysfs.

Please note however, that the sysfs GPIO interface is deprecated and there may be more suitable alternatives for your application (like a real kernel driver).

First, identify which pins you would like to manipulate by reviewing the 40 Pin 4Kopen Connector page, and take a note of the number in the "sysfs gpio #" column.

Then, using a terminal, try the following:

$ cd /sys/class/gpio
$ echo "${GPIO_NUMBER}" > ./export
$ ls -l "./gpio${GPIO_NUMBER}/"
total 0
-rw-r--r--    1 root     root          4096 Nov 30 02:15 active_low
lrwxrwxrwx    1 root     root             0 Nov 30 02:15 device -> ../../../961f080.pin-controller-sbc
-rw-r--r--    1 root     root          4096 Nov 30 02:15 direction
-rw-r--r--    1 root     root          4096 Nov 30 02:15 edge
drwxr-xr-x    2 root     root             0 Nov 30 02:15 power
lrwxrwxrwx    1 root     root             0 Nov 30 02:15 subsystem -> ../../../../../class/gpio
-rw-r--r--    1 root     root          4096 Nov 30 02:15 uevent
-rw-r--r--    1 root     root          4096 Nov 30 02:15 value

Writing the number into the export file (e.g: with echo) will request that the kernel makes this pin available to you, in GPIO mode - it might fail if the pin is already in use by a driver, in which case you may need to investigate your device tree, or use a different pin.

A few important files are made available via the sysfs GPIO interface;

• active_low - whether the signal is inverted
• direction - to configure the pin as input or output
• value - the pin's current input or output value

More information about this interface can be found in the kernel documentation: https://www.kernel.org/doc/Documentation/gpio/sysfs.txt

Once you have exported your pin, you can manipulate it as follows:

$ cd "/sys/class/gpio/gpio${GPIO_NUMBER}/"

$ echo in > direction
$ cat value
1

$ echo out > direction
$ echo 1 > value
# check the pin's voltage
$ echo 0 > value
# check the pin's voltage

After you have made use of the pin, you may want to return it to the kernel. To do so, simply write the pin's number into the unexport file:

$ cd /sys/class/gpio
$ echo "${GPIO_NUMBER}" > ./unexport