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Combinational Circuits

A combinational circuit comprises of logic gates whose outputs at any time are determined directly from the present combination of inputs without any regard to previous inputs.

A combinational circuit performs a specific information-processing operation fully specified logically by a set of Boolean functions.

The basic components of a combinational circuit are: input variables, logic gates, and output variables.

The ‘n’ input variables come from an external source whereas the ‘m’ output variables go to an external destination. In many applications, the source or destination are storage registers.

Design procedure of a Combinational Circuit

The design procedure of a combinational circuit involves the following steps:

1. The problem is stated.
2. The total number of available input variables and required output variables is determined.
3. The input and output variables are allocated with letter symbols.
4. The exact truth table that defines the required relationships between inputs and outputs is derived.
5. The simplified Boolean function is obtained from each output.
6. The logic diagram is drawn.

The combinational circuit that performs the addition of two bits is called a half adder and the one that performs the addition of three bits (two significant bits and a previous carry) is a full adder.