Here is another how-to from Jeff, this time he describes using a 555 timer as an external clock for the Arduino. It’s a really interesting technique on how to get a fairly accurate external interrupt at lower frequencies.
The key here is an ‘external clock’. Rather than have the Arduino keep track of when to perform the next task, you have an outside signal that says “Now!” and fires an interrupt which the Arduino responds to. If your source fires every 100 milliseconds, then 10 times a second, the Arduino will receive an interrupt which will stop any processing currently happening and immediately execute the interrupt handler. For every tick of the clock, the interrupt handler will execute.
In the Maker Shed: Make: Arduino