Table of Contents

    Asynchronous (Ripple) Counters
    Asynchronous Decade Counters
    Asynchronous Up-Down Counters
    Synchronous Counters
    Synchronous Decade Counters
    Synchronous Up-Down Counters


Circuits for counting events are frequently used in computers and other digital systems.  Since a counter circuit must remember its past states, it has to possess memory.  The chapter about flip-flops introduced how flip-flops are connected to make a counter.  The number of flip-flops used and how they are connected determine the number of states and the sequence of the states that the counter goes through in each complete cycle.

Counters can be classified into two broad categories according to the way they are clocked:

  1. Asynchronous (Ripple) Counters - the first flip-flop is clocked by the external clock pulse, and then each successive flip-flop is clocked by the Q or Q' output of the previous flip-flop.

  2. Synchronous Counters - all memory elements are simultaneously triggered by the same clock.

In this tutorial, pure binary, decade and up-down counters within the two categories will be introduced.