Sequential Logic

In digital circuit theory, sequential logic is a type of logic circuit whose output depends not only on the present value of its input signals but on the past history of its inputs. This is in contrast to combinational logic, whose output is a function of only the present input. That is, sequential logic has state (memory) while combinational logic does not. Or, in other words, sequential logic is combinational logic with memory.

Sequential logic is used to construct finite state machines, a basic building block in all digital circuitry, as well as memory circuits and other devices. Virtually all circuits in practical digital devices are a mixture of combinational and sequential logic.

Digital sequential logic circuits are divided into synchronous and asynchronous types. In synchronous sequential circuits, the state of the device changes only at discrete times in response to a clock signal. In asynchronous circuits the state of the device can change at any time in response to changing inputs.