A counter is a sequential circuit that counts in a cyclic sequence. It is essentially a register that goes through a predetermined sequence of states upon the application of input pulses. There are two types of counters – Synchronous Counter & Asynchronous Counter.
In a synchronous counter, the input pulses are applied to all clock pulse inputs of all flip flops simultaneously (directly). Synchronous counter is also known as parallel sequential circuit. Examples of Synchronous Counters are as below:
In an asynchronous counter, the flip flop output transition serves as a source for triggering other flip flops. In other words, the clock pulse inputs of all flip flops, except the first, are triggered not by the incoming pulses, but rather by the transition that occurs in previous flip flop’s output.. Asynchronous counter is also known as serial sequential circuit. Example of Asynchronous Counters are as below:
Synchronous counters are faster than asynchronous counter because in synchronous counter all flip flops are clocked simultaneously.