*61*

# 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 â€˜nâ€™ input variables come from an external source whereas the â€˜mâ€™ 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:

- The problem is stated.
- The total number of available input variables and required output variables is determined.
- The input and output variables are allocated with letter symbols.
- The exact truth table that defines the required relationships between inputs and outputs is derived.
- The simplified Boolean function is obtained from each output.
- 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.