**3. Combinational logic circuits**

There are two types of circuits exists in the digital system, Combinational Logic Circuits and Sequential Circuits. A combinational logic circuit is a circuit where the output depends on the combination of present input state. The set of operations which these combinational circuits performs logically by a set of Boolean functions. A sequential logic circuit is a circuit where the output depends on the combination of present input state and past input or previous input values. The previous output values are stored in the memory elements.

A combinational circuit consist of variables for input and output, and basic logic gates to perform the boolean function. The output signals are generated according to the inputs as well as the logic circuits employed. Here both the input and outputs are binary values either 1 or 0. **Figure 1** shows the simple block diagram of combinational logic circuits with *n* input variables and *m* output variables. If there are *n* number of inputs to the circuit then *2<sup>n</sup>* possible combinations of input states but each combination can produce only one output state [2]. For instance if the combinational logic circuit has 2 inputs A and B then there can be 4 possible input states.
