Programmable interval timer
Counter that generates an output signal when it reaches a programmed count
In computing and in embedded systems, a programmable interval timer (PIT) is a counter that generates an output signal when it reaches a programmed count. The output signal may trigger an interrupt.
01Common features
PITs may be one-shot or periodic. One-shot timers will signal only once and then stop counting. Periodic timers signal every time they reach a specific value and then restart, thus producing a signal at periodic intervals. Periodic timers are typically used to invoke activities that must be performed at regular intervals.
Counters are usually programmed with fixed intervals that determine how long the counter will count before it will output a signal.
02IBM PC compatible
The Intel 8253 PIT was the original timing device used on IBM PC compatibles. It used a 1.193182 MHz clock signal (one third of the color burst frequency used by NTSC, one twelfth of the system clock crystal oscillator, therefore one quarter of the 4.77 MHz CPU clock) 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.
The LAPIC in newer Intel systems offers a higher-resolution (one microsecond) timer. This is used in preference to the PIT timer in Linux kernels starting with 2.6.18.
Sources and credits
This article is adapted from the Wikipedia article “Programmable interval timer”, written by its contributors and licensed under CC BY-SA 4.0. Fathomly has changed the layout, removed citation markers, navigation and maintenance notices, and adjusted punctuation. This adapted version is shared under the same license. For references, see the original article.
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