See the top level README for information on where to find documentation for the raspberry pi and the ARM processor inside. Also find information on how to load and run these programs. This example is for the older pi1 boards, see other directories for other flavors of raspberry pi. There is a free-running 64 bit timer, super easy to use, just read it. //0x01000000 17 seconds //0x00400000 4 seconds //#define TIMER_BIT 0x01000000 #define TIMER_BIT 0x00400000 I think what they are doing is using a 1GHz clock for the ARM core (and GPU?) That is divided by 4 to get the 250MHz system clock (and or there is a 250MHz reference clock that is multiplied by 4). Either way there is a 250MHz system clock, and my guess is this is divided by 256 to get 977KHz. 0x01000000 ticks would be 17.18 seconds and I am using a watch with a second hand, so very plausible. Can change the TIMER_BIT to a different one to change the blink rate of the LED for example try: #define TIMER_BIT 0x00100000