Combinational and Sequential Circuits

Nov 29 2013 • Engineering Notes, Notes • 2248 Views • No Comments on Combinational and Sequential Circuits

Circuit Designing is a process of designing for use our device how to make it perfect in processing speed, working, portable, space consumer, feed back, reliability, good efficiency and space consumer.  Their are two types of circuit in circuit designing i.e; Combinational Circuits and Sequential Circuits. Combinational logic are used in computer circuits on input signals in order to perform Boolean algebra. Present post includes of explanation Digital Electronics Notes on Combinational and Sequential Circuits explaining classification of sequential and combinational circuits with examples.

Practical computer circuits normally contain a mixture of combinational and sequential circuits.

1: Combinational Circuit
2: Sequential Circuit

Combinational Circuit

When logic gates are connected together to produce a specified output for certain specified combination of the input variable, with no storage involved, the resulting circuit is called a combinational logic circuits and respective circuits are known as combinational circuits.

Its output is dependent on the present input. A combinational logic circuit consists variables, logic gates and output variable. the logic gates accepts the signals from the input variable and generates output signal as shown in Block Diagram of Combinational Logic Circuits, in following figure:

Combinational and Sequential Circuits

Block Diagram of Combinational Logic Circuit

Analysis Procedure of Combinational Circuits:

 The analysis of a combinational circuit is that we determine the function that the circuit implements. In this analysis, the logic diagram is given and culminates with a set of Boolean function, a truth table or possible explanation of the circuit operation.

Figure (a) Example of combinational logic circuit.

Combinational and Sequential Circuits

Figure (a)

Figure (b) Not a combinational logic circuit.

Combinational and Sequential Circuits

Figure (b)

Example of Combinational Circuits: Combinational Circuits are used in micro-controller as computer hardware and software designing and in microprocessor.

Classification of Combinational Logic Circuits

Combinational Logic Circuits are classified into three major parts as Arithmetic and Logic functions, Date transmission and code converters. Following chart elaborate their further classification.

Example of Combinational logic Circuit


Sequential  Circuit

When the output of a circuit depends on the present input and past output then these logic are termed as Sequential logics and respective circuits are known as Sequential circuits. This requires a memory element in order to store the past output. Following figure represent the block diagram of sequential logic circuits.

Combinational and Sequential Circuits

Block Diagram of Sequential Logic Circuits

The combinational logic circuits operates these inputs to produce various outputs. The o/p of the memory elements is given tot he inputs of combinational circuits.

Logic symbol of Flip flop as a memory element is represented as below: 

 Combinational and Sequential Circuits

Symbol of Flip Flop

Example of Sequential Circuits: Sequential  circuits work like a master and slab as the input of master, goes to slab input. So it is also recognised as Master Slab flip-flop. In this process the output of the first device goes to second devices input and so on.

Classification of Sequential Logic Circuits

Sequential Logic Circuits are classified into three major parts as Event driven, Clock drive and Pulse driven. Following chart elaborate their further classification.

Combinational and Sequential Circuits


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