# Combinational logic circuits

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Evolution of Digital IC Technologies. Finite State Machines Finite State Machines Topics. Programmable Logic Devices Architectures 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. Implementation steps The implementation steps in the design of a combinational logic circuit are, 1.

Identify the input and output variables. Find out the Boolean expressions for outputs in terms of inputs. Minimize the Boolean expressions. In digital circuit theory, combinational logic sometimes also referred to as time-independent logic [1] is a type of digital logic which is implemented by Boolean circuits , where the output is a pure function of the present input only.

This is in contrast to sequential logic , in which the output depends not only on the present input but also on the history of the input. In other words, sequential logic has memory while combinational logic does not.

Combinational logic is used in computer circuits to perform Boolean algebra on input signals and on stored data. Practical computer circuits normally contain a mixture of combinational and sequential logic. For example, the part of an arithmetic logic unit , or ALU, that does mathematical calculations is constructed using combinational logic. Other circuits used in computers, such as half adders , full adders , half subtractors , full subtractors , multiplexers , demultiplexers , encoders and decoders are also made by using combinational logic.

An alternate term is combinatorial logic , [2] though this usage may be considered controversial. Combinational logic is used to build circuits that produce specified outputs from certain inputs. The construction of combinational logic is generally done using one of two methods: Consider the following truth table:.