Here’s an app note from ATMEL presenting a solution for microcontroller long delays.
The solution presented here shows how the AVR AT90 series microcontrollers (AT90S2313, AT90S4414 and AT90S8515) generate and handle long delays. On-chip timers are used without any software intervention, thus allowing the core to be in a low-power mode during the delay. Since the timers are clocked by the system clock, there is no need of any additional components.
Due to the very long timing capability, this implementation combines high system performance with long delay generation. For example, an AVR Microcontroller running at
20 Mips can generate delays as long as half an hour.