[megous]

The project you're thinking about uses internal PLL in the SoC. This has nothing to do with GPIO reading/writing.

http://www.icrobotics.co.uk/wiki/index.php/Turning_the_Raspb...

[snovv_crash]

Ah, my bad. I was actually thinking afterwards about how they could be doing the frequency shifts and what kind of sampling speed they would need for accurate, small frequency shift adjustments.