This is my simple C app that drives my Binary Clock Shield for Raspberry Pi.
To use this, you'll need to have a Raspbian install built and running on your Raspberry Pi (I would recommend using the latest "Raspbian Lite" as there's no need of X-Windows, but that's just me).
You need to have installed Mike McCauley's excellent BCM2835 library first (from http://www.airspayce.com/mikem/bcm2835/). That's the only dependency.
If you're using one of the oldest Raspberry Pis (probably with a 26-way GPIO connector) then you'll need to change a constant near the top of the code. This is because the GPIO pin arrangement was changed after the first version of the Pi was released. It's not a hard change - there's a comment next to it.
To compile, enter: gcc -o bcd-clock bcd-clock.c -l bcm2835
To run it: ./bcd-clock [brightness]
... where [brightness] is a number between 1 and 15. It defaults to 8.
If you want your clock to run on startup, then add this line near the end of your /etc/rc.local file:
/home/pi/bcd-clock 15 &
(The '&' forces it to the background.)
See my blog post on this project, here: http://danceswithferrets.org/geekblog/?p=1087 It includes more detailed notes about installation.