Difference Between Sequential Logic Circuit Vs Combinational Logic Circuit | Examples

 A sequential logic circuit is the one kind of logic circuit which's output function depends not only present state but also on the sequence of past inputs and the input history as well. The sequential logic circuit requires a memory unit to store the previous input history as feedback.

Examples of sequential circuits are flip-flop, register, counter, clocks, etc.

Fig.1 Sequential circuit

The digital sequential logic circuit is divided into sub-categories. They are synchronous and asynchronous sequential logic circuit. The main difference between Synchronous and Asynchronous sequential logic circuit is:

The synchronous circuit is governed by the clock signal, where the Asynchronous circuit is not governed by the clock signal.

A combinational logic circuit is another type of logic circuit whose output function only depends on the present state value. Sometimes it is referred to as time-independent logic. The output is a pure function of the present input only. And an important feature is that memory unit is not required in the combinational logic circuit. 

In contrast, the sequential logic circuit requires a memory unit to store the previous input history.

Examples of combinational circuits are a half adder, full adder, magnitude comparator, multiplexer, demultiplexer, etc.

Fig 2. Full adder Combination circuit

Here A, B and Cin are input variable and Sum and Carry are output. The output Sum and carry do not depend on the previous output of this circuit. 

Note: Someone may assume that Cin comes from the previous output of that circuit. Right. But The full adder circuit is a Combinational Circuit. Because though Cin comes from previous output, that circuit hasn't any memory component to store and it's current outputs depend on only instant input A, B, Cin.

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Summary:

1. Sequential logic circuit has the feedback of previous output with the memory unit.

2. Combinational logic circuit has not any memory elements.

3. Sequential logic circuit two types; Synchronous and Asynchronous

4. Clock governed circuit is Synchronous, otherwise Asynchronous

5. Full adder is a combinational circuit.