Editing Sequential logic (section)

{{Short description|Type of logic circuit}}
{{redirect|Sequential circuit|the synthesizer company|Sequential Circuits}}
{{use dmy dates|date=May 2023|cs1-dates=y}}
{{use list-defined references|date=May 2023}}
In [[automata theory]], '''sequential logic''' is a type of [[logic circuit]] whose output depends on the present value of its input signals and on the [[sequence]] of past inputs, the input history.<ref name="Vai_2000"/><ref name="Cavanagh_2006"/><ref name="Lipiansky_2012"/><ref name="Dally-Harting_2012"/> This is in contrast to ''[[combinational logic]]'', whose output is a function of only the present input.  That is, sequential logic has ''[[State (computer science)|state]]'' (''memory'') while combinational logic does not.

Sequential logic is used to construct [[finite-state machine]]s, a basic building block in all digital circuitry. Virtually all circuits in practical digital devices are a mixture of combinational and sequential logic.

A familiar example of a device with sequential logic is a [[television set]] with "channel up" and "channel down" buttons.<ref name="Vai_2000"/> Pressing the "up" button gives the television an input telling it to switch to the next channel above the one it is currently receiving. If the television is on channel 5, pressing "up" switches it to receive channel 6. However, if the television is on channel 8, pressing "up" switches it to channel "9". In order for the channel selection to operate correctly, the television must be aware of which channel it is currently receiving, which was determined by past channel selections.<ref name="Vai_2000"/> The television stores the current channel as part of its ''[[state (computer science)|state]]''. When a "channel up" or "channel down" input is given to it, the sequential logic of the channel selection circuitry calculates the new channel from the input and the current channel.

Digital sequential logic circuits are divided into [[synchronous logic|synchronous]] and [[asynchronous logic|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.