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^m discrete output states are possible, i.e. n is equal to 2^m. These are known as binary counters.

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 2⁴ 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.