A combinational circuit can be defined as a circuit whose output is dependent only on the inputs at the same instant of time where as a sequential circuit can be defined as a circuit whose output depends not only on the present inputs but also on the past history of inputs.
For a sequential circuit, the values of the variable are usually specified at certain discrete time instants rather than over the whole continuous time. Half Adder, Full Adder, Half Subtractor, Full Subtractor are examples of combinational circuits whereas Flip-Flops, Counters form the sequential circuit.
A sequential circuit consists of combinational circuit and memory elements are connected to it to form a feedback path as shown in the block diagram below:
|1.||It contains no memory elements||It contains memory elements|
|2.||The present value of it’s outputs are determined solely by the present values of it’s inputs||The present value of it’s outputs are determined by the present value of it’s inputs and it’s past state|
|3.||It’s behavior is described by the set of output functions||It’s behavior is described by the set of next-state(memory) functions and the set of output functions|
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