How to use GPIO signals

References

• http://www.avrfreaks.net/wiki/index.php/Documentation:Linux/GPIO

GPIO Usage from a Linux Application

Configure the kernel for GPIO support in sysfs

Symbol: GPIO_SYSFS [=y]
  Prompt: /sys/class/gpio/... (sysfs interface)
    Defined at drivers/gpio/Kconfig:51
    Depends on: GPIOLIB && SYSFS && EXPERIMENTAL
     Location:
      -> Kernel configuration
        -> Device Drivers
         -> GPIO Support (GPIOLIB [=y])
 

Enable GPIO from user space

GPIO=22

cd /sys/class/gpio
ls
echo $GPIO > export
ls

Notice on the first ls that gpio22 doesn't exist, but does after you export GPIO 22 to user space.

cd /sys/class/gpio/gpio$GPIO
ls

There are files to set the direction and retrieve the current value.

echo "in" > direction
cat value

You can configure the GPIO for output and set the value as well.

echo "out" > direction
echo 1 > value

Viewing GPIO Configuration

You can use debugfs to videw the current GPIO configuration.

Configure the kernel to enable debugfs:

Symbol: DEBUG_FS [=y]
   Prompt: Debug Filesystem
     Defined at lib/Kconfig.debug:77
     Depends on: SYSFS     
     Location:
       -> Kernel configuration
         -> Kernel hacking         

Boot the target hardware and mount debugfs:

mount -t debugfs none /sys/kernel/debug

Dump the GPIO configuration.

cat /sys/kernel/debug/gpio