Sequential Circuits are those which have the notion of an internal state. This notion of Internal State is necessary because in sequential circuits, the output of the circuit is function of both the present input as well as the past inputs. The Internal State of a sequential circuit is nothing but the reflection of the past inputs to the circuit. Now the Internal State of a Sequential Circuit is represented by a number of State Variables. Each state variable can be in 1 of 2 possible states. This is because State Variables are physically implemented with the help of Flip-Flops, and each Flip-Flop can only represent 2 possible states. Therefore, if we have ‘N’ Flip-Flops, we can represent a maximum of 2N states.
Max. No. of States with 'N' Flip-Flops =
This means that a Sequential Circuit having ‘N’ Flip-Flops can be in at most
In both the above circuits
Synchronous Circuit | Asynchronous Circuit |
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All the State Variable changes are synchronized with a universal clock signal. | The State Variables are not synchronized to change simultaneously and may change at anytime irrespective of each other to achieve the next Steady Internal State |
Since all the Internal State changes are in the strict control of a master clock source they are less prone to failure or to a race condition and hence are more reliable. | Since there is no such universal clock source, the internal state changes as soon as any of the inputs change and hence are more prone to a race condition. |
Timings of the internal state changes are in our control. | The changes in the internal state of an asynchronous circuit are not in our control. |