In
computing
Computing is any goal-oriented activity requiring, benefiting from, or creating computing machinery. It includes the study and experimentation of algorithmic processes, and development of both hardware and software. Computing has scientific, ...
and in
embedded system
An embedded system is a computer system—a combination of a computer processor, computer memory, and input/output peripheral devices—that has a dedicated function within a larger mechanical or electronic system. It is ''embedded'' ...
s, a programmable interval timer (PIT) is a
counter
Counter may refer to:
Mathematics and computing
* Counter machine, a subclass of register machines
* Counter (digital), an electronic device, mechanical device, or computer program for counting
* Loop counter, the variable that controls the iter ...
that generates an output signal when it reaches a programmed count. The output signal may trigger an
interrupt
In digital computers, an interrupt (sometimes referred to as a trap) is a request for the processor to ''interrupt'' currently executing code (when permitted), so that the event can be processed in a timely manner. If the request is accepted, ...
.
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
The Intel 8253 and 8254 are programmable interval timers (PITs), which perform timing and counting functions using three 16-bit counters.
The 825x family was primarily designed for the Intel 8080/ 8085-processors, but were later used in x86 ...
PIT was the original timing device used on
IBM PC compatible
IBM PC compatible computers are similar to the original IBM Personal Computer, IBM PC, IBM Personal Computer XT, XT, and IBM Personal Computer/AT, AT, all from computer giant IBM, that are able to use the same software and expansion cards. Such ...
s. It used a 1.193182
MHz
The hertz (symbol: Hz) is the unit of frequency in the International System of Units (SI), equivalent to one event (or cycle) per second. The hertz is an SI derived unit whose expression in terms of SI base units is s−1, meaning that one he ...
clock signal (one third of the
color burst
Colorburst is an analog video, composite video signal generated by a video-signal generator used to keep the chrominance subcarrier synchronized in a color television signal. By synchronizing an oscillator with the colorburst at the back porc ...
frequency used by
NTSC
The first American standard for analog television broadcast was developed by National Television System Committee (NTSC)National Television System Committee (1951–1953), Report and Reports of Panel No. 11, 11-A, 12–19, with Some supplement ...
, one twelfth of the system clock
crystal oscillator
A crystal oscillator is an electronic oscillator circuit that uses a piezoelectric crystal as a frequency-selective element. The oscillator frequency is often used to keep track of time, as in quartz wristwatches, to provide a stable clock ...
, therefore one quarter of the 4.77 MHz CPU clock) and contains three timers. Timer 0 is used by
Microsoft Windows (uniprocessor) and
Linux
Linux ( or ) is a family of open-source Unix-like operating systems based on the Linux kernel, an operating system kernel first released on September 17, 1991, by Linus Torvalds. Linux is typically packaged as a Linux distribution, w ...
as a system timer, timer 1 was historically used for
dynamic random access memory refreshes and timer 2 for the
PC speaker
A PC speaker is a loudspeaker built into some IBM PC compatible computers. The first IBM Personal Computer, model 5150, employed a standard 2.25 inch magnetic driven (dynamic) speaker. More recent computers use a tiny moving-iron or pie ...
.
The
LAPIC in newer Intel systems offers a higher-resolution (one microsecond) timer.
[Uwe Walter, Vincent Oberl]
μ-second precision timer support for the Linux kernel
/ref> This is used in preference to the PIT timer in Linux kernels starting with 2.6.18.Determining and changing the rate of timer interrupts a guest operating system requests (1005802)
/ref>
See also
* High Precision Event Timer
* Monostable multivibrator
A multivibrator is an electronic circuit used to implement a variety of simple two-state devices such as relaxation oscillators, timers, and flip-flops. The first multivibrator circuit, the astable multivibrator oscillator, was invented by Henri ...
* NE555
The 555 timer IC is an integrated circuit (chip) used in a variety of timer, delay, pulse generation, and oscillator applications. Derivatives provide two (556) or four (558) timing circuits in one package. The design was first marketed in 197 ...
References
{{Reflist
External links
* http://www.luxford.com/high-performance-windows-timers
* https://stackoverflow.com/questions/10567214/what-are-linux-local-timer-interrupts
Timing on the PC family under DOS
IBM PC compatibles
Digital electronics