Combinational circuits and systems produce an output based on input variables only. Sequential circuits use current input variables and previous input variables by storing the information and putting back into the circuit on the next clock (activation) cycle.
In the above diagram, shown how sequential circuits work.
There are two types of inputs to
combinational logic circuit:
External Inputs and
Internal Inputs.
External Inputs are given from the outside of circuit design and are not controlled by the
circuit.
Internal Inputs are called as Secondary inputs. Secondary Inputs are produced by
circuits previous output. And these inputs are also produce state variables produced by
storage elements, where secondary outputs are excitations for the storage elements.
Two Main types of Sequential Circuits:-
There are two types of sequential circuit, synchronous and asynchronous.
Synchronous types use pulsed or level inputs and a clock input to drive the circuit (with
restrictions on pulse width and circuit propagation).
Asynchronous sequential circuits do not use a clock signal as synchronous circuits do. Instead the circuit is driven by the pulses of the inputs. You will not need to know anymore about asynchronous circuits for this course
A pulsed output (as used in the block diagrams above) is an output that lasts for the
duration of a particular input pulse but can be less in some cases. For the clocked
sequential circuits, the output pulse is the same duration as the clock pulse.
A level output refers to an output that changes state at the start of an input pulse or
clock pulse and remains in that state until the next input or clock pulse.
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