## Quick Reference

A clocked digital electronic device whose output takes up one and one only of a number, *n*, of distinct states upon the application of each clock pulse (see clock). The output thus reflects the total number of clock pulses received by the counter up to its maximum capacity, *n*. All *n* states are displayed sequentially for *n* active transitions of the clock, the sequence then repeating. Since *n* clock pulses are required to drive the output between any two identical states, counters provide a “divide-by-*n* action” and are thus also known as **dividers**.

A counter whose output is capable of displaying *n* discrete states before producing an overflow condition can also be called a mod-n counter (or modulo-n counter), since it may be considered to be counting input pulses to a base of *n*. The value of *n* is often an integer power of 2. Counters are generally formed by a cascaded series of clocked flip-flops (see cascadable counter), each of which provides a divide-by-two action. For a counter consisting of *m* flip-flops, the maximum capacity of the counter will be 2* ^{m}* since 2

*discrete output states are possible, i.e.*

^{m}*n*is equal to 2

*. These are known as binary counters.*

^{m}Count lengths of other than integer multiples of two are possible. For example, a decade counter (or mod-10 counter) exhibits 10 separate and distinct states. To achieve this digitally requires a counter having at least four individual flip-flop elements, giving 2^{4} or 16 possible output states; six of these states are prevented from occurring by a suitable arrangement of logic gates around the individual flip-flops. In multimode counters the number, *n*, of distinct states can be selected by the user.

See also ripple counter, synchronous counter, shift counter, Johnson counter.

**From:**
counter
in
A Dictionary of Computing »

*Subjects:*
Computing.

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