The PC-Engines APU2 is a very nice board when you need a lot more horsepower than a Raspberry Pi but still want low power, small form factor, and no moving parts.
Among the many features of the The APU2 are three LEDs and one mode button/switch.
I needed a device driver so that my code could control the LEDs and read whether the button is pushed.
I wrote one. It is up and available on the InterWorking Labs website: at http://iwl.com/tech_examples/apu2led/.
This driver is for the APU2, not the predecessor APU version 1.