Editing Counter (digital) (section)

===Periodic interval timer===

Binary counters are commonly used as periodic interval timers (PITs), which output periodic pulses at an integer fraction of the clock frequency. PITs are used to generate [[system clock]] interrupts in computers, as clock dividers in [[phase lock loop]]s and [[frequency synthesizer]]s, and in many other applications.

In the circuit below, a binary down-counter is used to implement a PIT. The interval between output pulses, measured in clock cycles, is stored in the Interval register, resulting in output frequency <math>f_{out} = f_{clock} / (interval + 1)</math>. When the count reaches zero, the NOR gate issues a pulse on the timer output. The output pulse is also used internally to reload the interval into the counter, thus restarting the timer. Each output pulse has a duration of one clock cycle.

[[File:Programmable interval timer.jpg|350px|border|Programmable interval timer]]

For example, to obtain a 1 MHz output frequency from a 6 MHz clock, the interval would be set to 5 as shown in the following timing diagram:

[[File:Programmable interval timer timing diagram.jpg|350px|border|Timing diagram of a programmable interval timer]]