Difference between combinational and sequential circuits

In this post, we will find the difference between combinational and sequential circuits, and as well as we will define different types of counters (Ripple counter or Asynchronous counter, Synchronous counter, down counter, and up counter).

Difference between combinational and sequential circuits

Digital logic circuits are classified into two groups, and these are Combinational logic circuits and sequential logic circuits.

Combinational logic circuits are circuits in which the output at any time depends upon the combination of the input signals present at that instant only, and does not depend upon any past conditions. The basic building block for combinational logic circuits is the logic gate.

Sequential logic circuits are circuits whose outputs at any instant of time depend not only on the present inputs but also on the past outputs. The basic building block for sequential logic circuits is the flip-flop (FF).

The combinational logic circuits are a part of digital systems and they have many applications such as decoder, encoder, adder, subtractor, multiplexer, demultiplexer, etc. Sequential logic circuits are used as counters, sequence generators, and finite state machines.

Counter Types & definition

Here, we will discuss and define different types of counters (Ripple counter or Asynchronous counter, Synchronous counter, down counter, and up counter).

Ripple counter | Asynchronous counter

Asynchronous counters are commonly referred to as ripple counters due to the way of flip-flop response one after another in a kind of rippling effect.

In an asynchronous counter, the external clock pulse clocks the first flip-flop. Therefore the effect of the input clock pulse is first “felt” by FF0. This effect cannot get to FF1 immediately because of the propagation delay through FF0. Then there is the propagation delay through FF1 before FF2 can be triggered.

Thus, the effect of an input clock pulse “ripples” through the counter, taking some time, due to propagation delays, to reach the last flip-flop. As the triggers move through the flip-flops like a ripple, it is called a ripple counter.

Synchronous counter

In a synchronous counter, the clock input terminals of all flip-flops are commonly connected. Therefore, the same clock pulse simultaneously triggers all flip-flops of the counters.

Down counter

Synchronous and asynchronous binary counters can be able to count either in increasing or decreasing order. In a count-down counter, the counter value sequentially decreased. For example, in a 3-bit down counter the counting sequence is 7,6,5,4,3,2,1, and 0.

Up counter

Synchronous and asynchronous binary counters can be able to count either in increasing or decreasing order. In a count-up counter, the counter value sequentially increased. For example, in a 3-bit up-counter the counting sequence is 0,1,2,3,4,5,6, and 7.

Author of this post

This post is co-authored by Professor Saraswati Saha, who is an assistant professor at RCCIIT, a renowned degree engineering college in India. Professor Saha teaches subjects related to digital electronics & microprocessors.