Prescaler


A prescaler is an electronic counting circuit used to reduce a high frequency electrical signal to a lower frequency by integer division. The prescaler takes the basic timer clock frequency and divides it by some value before feeding it to the timer, according to how the prescaler register are configured. The prescaler values that may be configured might be limited to a few fixed values, or they may be any integer value from 1 to 2^P, where P is the number of prescaler bits.
The purpose of the prescaler is to allow the timer to be clocked at the rate a user desires. For shorter timers, there will often be a tradeoff between resolution and range. For example, one cannot achieve 1 µs resolution and a 1 sec maximum period using a 16-bit timer. In this example using 1 µs resolution would limit the period to about 65ms maximum. However the prescaler allows tweaking the ratio between resolution and maximum period to achieve a desired effect.

Example of use

Prescalers are typically used at very high frequency to extend the upper frequency range of frequency counters, phase locked loop synthesizers, and other counting circuits. When used in conjunction with a PLL, a prescaler introduces a normally undesired change in the relationship between the frequency step size and phase detector comparison frequency. For this reason, it is common to either restrict the integer to a low value, or use a dual-modulus prescaler in this application. A dual-modulus prescaler is one that has the ability to selectively divide the input frequency by one of two integers, such as 32 and 33. Common fixed-integer microwave prescalers are available in modulus 2, 4, 8, 5 and 10, and can operate at frequencies in excess of 10 GHz.

Nomenclature

A prescaler is essentially a counter-divider, and thus the names may be used somewhat interchangeably.