In computing and in embedded systems, a programmable interval timer (PIT) is a counter which triggers an interrupt when it reaches the programmed count.
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PITs counters may be one-shot or periodic. One-shot timers interrupt only once, and then stop counting. Periodic timers interrupt every time they reach a specific value. This interrupt is received at regular intervals from the programmable interval timer. This interrupt is used to invoke kernel activities that must be performed on a regular basis. Counters are usually programmed with fixed increment intervals which determine how long the counter counts before it triggers an interrupt. The interval increments therefore determine the resolution for which the counter may be programmed to generate its one-shot or periodic interrupt.
The Intel 8253 PIT is the original timing device used on IBM PC compatibles. It uses a 1.193182 MHz crystal oscillator (one third of the color burst frequency used by NTSC[1]) and contains three timers. Timer 0 is used by Microsoft Windows (uniprocessor) and Linux as a system timer, timer 1 was historically used for dynamic random access memory refreshes and timer 2 for the PC speaker.[2]