Counters

A sequential circuit that goes through a prescribed sequence of states upon the application of input pulses is called a counter. The input pulses, called count pulses, may be clock pulses. In a counter, the sequence of states may follow a binary count or any other sequence of states. Counters are found in almost all equipment containing digital logic. They are used for counting the number of occurrences of an even and are useful for generating timing sequences to control operations in a digital system. A counter is a sequential circuit with 0 inputs and n outputs. Thus, the value after the clock transition depends only on old values of the outputs. For a counter, the values of the outputs are interpreted as a sequence of binary digits (see the section on binary arithmetic). We shall call the outputs o0, o1, ..., on-1. The value of the outputs for the counter after a clock transition is a binary number which is one plus the binary number of the outputs before the clock transition. We can explain this behavior more formally with a state table. As an example, let us take a counter with n = 4. The left side of the state table contains 4 columns, labeled o0, o1, o2, and o3.