A prescaler is an electronic
counting circuit used to reduce a high
frequency
Frequency is the number of occurrences of a repeating event per unit of time. Frequency is an important parameter used in science and engineering to specify the rate of oscillatory and vibratory phenomena, such as mechanical vibrations, audio ...
electrical signal to a lower frequency by
integer division
Division is one of the four basic operations of arithmetic. The other operations are addition, subtraction, and multiplication. What is being divided is called the ''dividend'', which is divided by the ''divisor'', and the result is called the ...
. The prescaler takes the basic timer clock frequency (which may be the CPU clock frequency or may be some higher or lower frequency) and divides it by some value before feeding it to the timer, according to how the prescaler register(s) are configured. The prescaler values, referred to as prescales, that may be configured might be limited to a few fixed values (powers of 2), 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 (8 and 16-bit) timers, there will often be a tradeoff between resolution (high resolution requires a high clock rate) and range (high clock rates cause the timer to overflow more quickly). For example, one cannot (without some tricks) 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 counter
A frequency counter is an electronics, electronic measuring instrument, instrument, or Electronic component, component of one, that is used for measuring frequency. Frequency counters usually measure the number of cycles of oscillation or pulses p ...
s,
phase locked loop
A phase-locked loop or phase lock loop (PLL) is a control system that generates an output signal whose phase is fixed relative to the phase of an input signal. Keeping the input and output phase in lockstep also implies keeping the input and ou ...
(PLL) 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
A phase detector or phase comparator is a frequency mixer, analog multiplier or Digital logic, logic circuit that generates a signal which represents the difference in phase between two signal inputs.
The phase detector is an essential elemen ...
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 (normally consecutive)
integer
An integer is the number zero (0), a positive natural number (1, 2, 3, ...), or the negation of a positive natural number (−1, −2, −3, ...). The negations or additive inverses of the positive natural numbers are referred to as negative in ...
s, such as 32 and 33. Common fixed-integer
microwave
Microwave is a form of electromagnetic radiation with wavelengths shorter than other radio waves but longer than infrared waves. Its wavelength ranges from about one meter to one millimeter, corresponding to frequency, frequencies between 300&n ...
prescalers are available in modulus 2, 4, 8, 5 and 10, and can operate at frequencies in excess of 10
GHz
The hertz (symbol: Hz) is the unit of frequency in the International System of Units (SI), often described as being equivalent to one event (or Cycle per second, cycle) per second. The hertz is an SI derived unit whose formal expression in ter ...
.
[For example, in 2011, the Hittite Microwave Corporation advertised prescalers from 100MHz to 10GHz: ]
Nomenclature
A prescaler is essentially a
counter-divider, and thus the names may be used somewhat interchangeably.
See also
*
Frequency divider
Frequency is the number of occurrences of a repeating event per unit of time. Frequency is an important parameter used in science and engineering to specify the rate of oscillatory and vibratory phenomena, such as mechanical vibrations, audio ...
References
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Electronic circuits