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. You may also be able to use debugfs to see if the GPIO pin is multiplex as a GPIO or is dedicated to some other function.

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

Dump the pin multiplexing configuration.

cat /sys/kernel/debug/omap_mux/board