What Is Sequential Logic?

In previous posts, we have talked about logic gates, what they are, how you physically build them, and how you can code them. These logic gates; NOT, OR, AND, NOR, NAND, XOR, XNOR, and their combinations are all examples of combinational logic. That is, logic that only depends on the current value of inputs based on what A, B through N inputs are you can get a logical output.

Sequential logic works differently. Sequential logic depends on not only the current value of the inputs, but also on the past values of the inputs. It relies on a register to store a current state. This current state is the result of a history of inputs. Based on this current state and inputs the next state can be found. A good example of this is a volume knob that turns continuously. Since you can turn the knob continuously, there are no set turn angles for each volume level. Rather the next volume level is determined by the current volume level and the amount you turn the knob.



If you wanted to turn a combinational logic into sequential logic, you could add a feedback loop. For example, say you have a two input and gate and you feedback the output to one of the inputs. Now the output depends not only on the input, but also the current state.

An and gate with added feedback, turning it into sequential logic.
An AND gate with added feedback, turning it into sequential logic.

Sequential logic is used to create something called a state machine. To read more about state machines, stay tuned for my next post.

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