what is the difference between sequential and combinational logic circuits?

what is the difference between sequential and combinational logic circuits? In order to answer this question lets just first get the idea of what are the combinational circuits.

Combinational circuit:

These types of the circuit are the memory-less circuit. In the combinational circuit the output at any time only depends on the combinations of the inputs. In this type of circuit, the output depends on the input. These Type of circuits are made up of the NAND, NOR and NOT Gates. In the combinational logic circuits these gates are combined to form a complicated Switching Systems.

The combinational circuits are the memoryless circuits which means that one of the input states changes the condition from 0-1 and 1-0, so it will change the resulting output of the combinational circuit.

Diagram:

The diagram of the combinational logic circuits is discussed as under.

These types of circuits have no feedback which means that any change is applied to the input directly affects the output of the logic circuit. The difference between the sequential and the combinational circuits are discussed as under.

• In this circuit there is no memory element is present.
• The main building block of these Circuits are the logic gates.
• The present value of the output is determined by the present values of the input.
• In the combinational circuits the combination of the operands is not essential in the same sequence.
• The behavior of these circuits is explained by the output function test.
• There is no feedback is required in the combinational circuits, that’s is the reason they are less complex than the sequential circuits.
• The hardware required for the realization of the combinational circuit is large.
• The data is not stored in the combinational circuits.
• It is easy to use and handle.
• The operation of the combinational circuits can be explained by the truth table.
• These circuits are very expensive because the large number of gates are required in the operation of the combinational circuits.
• These circuits are fast in speed.
• In the combinational circuits the combination of the operands is not essential in the same sequence.
• On the other hand, the combinational circuits have not the ability to store the present state and to retain the previous state.
• These circuits don’t depend on clock.
• Any change in the input will be reflected at the output terminal of the combinational circuits.
• The combinational circuits are not time dependent.
• The designing of these type of circuit is easier.
• The combinational circuits are more costly than the sequential circuits.
• The feedback is not required in this type of circuit.
• There are many types of the combinational circuits are existing in which the multiplexer and the de- multiplexer are significant. This is because they are used in the various applications.
• The example of these circuits is Half Adder, Full Adder, magnitude compactor and de-multiplexer.
• These circuits are mainly used for the Arithmetic(Addition, Subtraction, Multiplication and Division) and the Boolean Functions.

 

What are the sequential Circuits?

In the sequential circuit the present output depends upon the present inputs and also depends on the past output. Sequential circuit is the extension of the combinational circuit. In this type of circuit, we attach the memory element  with the combinational circuit. This means that when we applied inputs than the output is generated, the output  not only depends on the input but also for the previous output. The previous output is saved in the memory element.

Sequential logic circuits are usually named as the Bi Stable device or the two-state device. which can have the output or outputs set in one of two basic states, a logic level “1” or a logic level “0” and will remain “latched” (hence the name latch) indefinitely in this current state or condition until some other input trigger pulse or signal is applied which will cause the bistable to change its state once again.

 

Diagram:

The diagram of the sequential logic circuits is discussed as under.

• Memory element is present in these circuit.
• The main building block of these Circuits are the flip-flop.
• The present value of the output is determined by the present values of the input and it also depends on the present state of the circuit.
• In the sequential circuits the combination of the operands essential in the same sequence.
• The behavior is expressed by the set of next-state and the set of output functions.
• There is feedback is required in the sequential circuits, that’s is the reason they are more complex than the combinational circuits.
• The sequential circuits are time dependent.
• The hardware required for the realization of the combinational circuit is small.
• The Sequential circuit stores very small amount of data.
• The operation of the sequential circuits can be explained by the truth table and also by the timing diagram.
• The cost of the sequential circuit is very low.
• It is not easy to use and handle.
• In the sequential circuits the combination of the operands essential in the same sequence.
• The speed of the sequential circuit is very low. This is due to because the memory elements are present in the sequential circuits.
• These circuits have an ability to store the present state and retain the previous state.
• In the Sequential circuit the clock is utilized to perform the Triggering functions.
• The designing of these type of circuit is complicated.
• The feedback is not required in this type of circuit.
• The sequential circuits are inexpensive than the combinational circuits.
• These are the circuits that are mainly used for storing data.
• Flip-flop, Registers and the Counters are the example of the sequential circuits.

 

These are the main differences between the sequential circuits and the combinational circuits.

Conclusion:

The combinational and the sequential circuits are designed to meet the various types of applications. But in general, most of the circuits used both of the above logic circuits to meet the need .Output base of the function of input provide by the combinational logic circuits and the sequential circuits will provide the output based on the state of the memory element.

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