Difference Between Combinational and Sequential Circuits

By Mona Kumari|Updated : April 20th, 2022

Difference Between Combinational and Sequential Circuits: Both circuits are the major components in digital electronics. Both combinational and sequential circuits have their own functionalities different from each other, we will be discussing the difference between combinational and sequential circuits after discussing the roles and responsibilities of each one individually.

The combinational circuits have outputs that are dependent only on the present inputs while on the other hand the sequential circuits have a series of inputs and outputs and the output of the sequential circuit depends both on the present inputs and the previous outputs. Being familiar with the basic difference between combinational and sequential circuits, let's dive into each one individually.

Table of Content

What is a Combinational circuit?

A combinational circuit is one in which we combine the various different gates to form a circuit. The output of the combinational circuit is determined by the present inputs, regardless of the previous inputs. We have a variety of combinational circuits available to us for performing a variety of tasks such as Adder, Subtractor, Multiplexer, De-multiplexer, Encoder, Decoder, etc.

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Characteristics of a Combinational Circuit

Having known the definition of combinational circuits and their functionalities, let us now discuss the characteristics of the combinational circuit. Following are the characteristics:

  • The Combinational circuits do not consist of any memory element.
  • At any point in time, the output only depends on the present inputs
  • There can be an “n” number of inputs and “m” number of outputs possible for a combinational circuit where n and m are integers.

What is a Sequential Circuit?

In simple language a combinational circuit with a memory element is a sequential circuit, this is the most basic difference between combinational and sequential circuits. It is a special circuit that has a sequence of inputs and outputs. Unlike combinational circuits, the output depends on the present inputs and previous outputs.

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Types of Sequential Circuits

Sequential circuits can be further divided into two types based on clock signal usage. These circuits are widely used in frequency division, pulse counting and finding the mod of the circuit, etc. Following are the two types:

  • Asynchronous sequential circuits: They do not use the clock signals instead they make use of the pulses. The un-clocked flip-flops are the memory elements of asynchronous circuits. Though many differences between combinational and sequential circuits the asynchronous sequential circuits are similar to combinational circuits along with feedback.
  • Synchronous Sequential circuits: They make use of clock signals and synchronize the memory element’s states with the clock signals. The output is stored in the memory elements called latches or flip-flops (Bi-stable multivibrator)

The Difference Between Combinational and Sequential Circuits

We have discussed the two circuits separately along with their characteristics and functionalities, now let us discuss the difference between the two:

Metrics

Combinational Circuit

Sequential Circuit

Usage of memory or Storage area

It does not require any memory element to store the output.

It requires a memory element to store the previous state output.

Feedback

It requires no feedback for generating the next output.

It requires feedback as it relies on the previous output/feedback and the current input.

Speed

It performs faster in comparison with sequential circuits.

Its performance is slow because it uses memory elements.

Elementary blocks

It uses logic gates as an elementary block.

It uses flip-flops as elementary blocks.

Complexity

It is easy to use and handle.

It is complex to use and handle in comparison to combinational circuits.

Example

Encoder, Decoder, Multiplexer, etc.

Flip-flops and Counters.

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FAQs

  • The combinational and sequential circuit plays a major role in digital electronics. A combinational circuit do not make use of clock signals while a sequential circuit uses clock signals.

  • A sequential circuit can be defined as a combinational circuit along with the memory element. Sequential circuits are of two types: asynchronous sequential circuits and Synchronous sequential circuits.

  • A combinational circuit is faster than a sequential circuit. Sequential circuits work slow due to the presence of memory elements and also due to the clock synchronization between outputs.

  • The elementary blocks for a sequential circuit are the flip-flops. Flip-flops are used as a memory element. They can store 1-bit information i.e. 1 or 0.

  • A combinational circuit is easy to handle and use while a sequential circuit is complex to handle. The complexity in sequential circuit is added due to the usage of memory elements and clock signals.

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