A combinational circuit is the digital logic circuit in which the output depends on the combination of inputs at that point of time with total disregard to the past state of the inputs. The digital logic gate is the
building block of combinational circuits. The function implemented by combinational circuit is depend upon the Boolean expressions. On the other hand, sequential logic circuits, consists of both logic gates and memory elements such as flip-flops. Figure below shows the combinational circuit having n inputs and and m outputs. The n number of inputs shows that there are 2^n possible combinations of bits at the input. Therefore, the output is expressed in terms m Boolean expressions.
The implementation steps in the design of a combinational logic circuit are,
1. Identify the input and output variables.
3. determine the relation between the input and output variables.
4. Construct truth table according to input–output specifications.
5. Find out the Boolean expressions for outputs in terms of inputs.
6. Minimize the Boolean expressions.
7. Realize the minimized Boolean expressions.
The various techniques to minimize the Boolean expressions are , Boolean algebra theorems, Karnaugh maps and Quinne–McCluskey method.
The below mentioned suggestions are used to reduce the hardware implementation.
1. The combinational circuit design should have minimum gates.
2. The combinational circuit design should have minimum interconnections.