What are Sequential Circuits

Sequential Circuits are digital circuits in which the value of the output not only depends upon the present values of the input but also upon the past values of the outputs. In other words, the output of the circuit depends upon a sequential behavior. An example of such a circuit is a binary counter. In a binary counter, the output is incremented or decremented by 1 with each clock pulse and therefore the new output always depends upon the previous output of the circuit along with the clock pulse.

Sequential circuits are important because only combinational logic is not enough for synthesizing a sophisticated digital system. A combinational logic just takes the inputs and gives the corresponding outputs irrespective of the requirements of the system. By adding a sequential logic to it, one can break down the various functions of the system in to a sequence of operations and implement them at only the time at which they are required. This makes for a much more practical digital system then just using combinational logic to perform all sorts of possible operations at all times.

A sequential circuit is nothing but the conjunction of a combinational circuit along with a storage facility or memory element. The memory element stores the past values of the outputs of the circuit and thus provides a way to feed them back to the circuit as inputs. Thus a sequential circuit can also be referred to as a combinational circuit whose inputs are the past outputs of the circuit along with the present inputs. It is not necessary that all the past outputs should be involved in the feedback mechanism in a sequential circuit. The number of these outputs can be one or more, depending upon the design of the digital system.

Another feature of sequential circuit is the provision for a clock signal in the form of a pulse. If a clock signal is used, then the inputs to the sequential circuit are only registered at the presence of this signal. This is done by binding the various inputs with the clock signal line by using an array of AND gates. The inputs to the sequential circuit thus only pass through these AND gates when the clock signal is at level ‘1’.  This is not always the case though in more sophisticated techniques like leading edge or falling edge clock triggers where the inputs are registered only when the clock pulse is rising or falling respectively between the high and low level.

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