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{{Short description|Counter that generates an output signal when it reaches a programmed count}}
In [[CPU|computing]] and in [[embedded system]]s, a '''programmable interval timer''' ('''PIT''') is a [[Counter (digital)|counter]] that generates an output signal when it reaches a programmed count. The output signal may trigger an [[interrupt]].

==Common 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.

==IBM PC compatible==
The [[Intel 8253]] PIT was the original timing device used on [[IBM PC compatible]]s. 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]],<ref>{{cite web|url=http://www.osdever.net/bkerndev/Docs/pit.htm |title=Bran's Kernel Development Tutorial: The Programmable Interval Timer |publisher=Osdever.net |date= |accessdate=2013-10-30}}</ref> 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]].<ref>{{cite web|url=http://wiki.osdev.org/Programmable_Interval_Timer |title=Programmable Interval Timer - OSDev Wiki |publisher=Wiki.osdev.org |date=2012-06-26 |accessdate=2013-10-30}}</ref>

The [[LAPIC]] in newer Intel systems offers a higher-resolution (one microsecond) timer.<ref name="timer">Uwe Walter, Vincent Oberle [http://telematics.tm.kit.edu/publications/Files/61/walter_ibm_linux_challenge.pdf μ-second precision timer support for the Linux kernel]</ref> This is used in preference to the PIT timer in [[Linux kernel]]s starting with 2.6.18.<ref>[http://kb.vmware.com/selfservice/microsites/search.do?language=en_US&cmd=displayKC&externalId=1005802 Determining and changing the rate of timer interrupts a guest operating system requests (1005802)]</ref>

==See also==
* [[High Precision Event Timer]]
* [[Monostable multivibrator]]
* [[NE555]]

==References==
{{Reflist}}

== External links ==
* [http://www.luxford.com/high-performance-windows-timers High Performance Windows Timers]
* [http://www.phatcode.net/articles.php?id=246 Timing on the PC family under DOS]

[[Category:IBM PC compatibles]]
[[Category:Digital electronics]]

Programmable interval timer - Revision history

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