This repo contains the source code for an AVR-based boost convertor controller. It uses fast PWM to control the switch and uses the ADC to read the output voltage (via a voltage divider). It then adjusts the PWM duty cycle to maintain a contstant voltage.
I've posted the schematic and step-by step instructions as an instructable:
http://www.instructables.com/id/Make-a-microcontroller-based-boost-converter/
With the right components, it can output several watts at 18V from a 5V supply. The voltage is adjustable by adjusting the feedback potentiometer.
The code is in main.c. I use the avr-gcc and avr-libc toolchain with AVR Eclipse. Lady Ada has an excellent tutorial on setting up the toolchain here: http://www.ladyada.net/learn/avr/.