Sequential Logic Basics
Unlike Combinational Logic circuits that change state depending upon the actual signals being applied to their inputs at that time, Sequential Logic circuits have some form of inherent "Memory" built in to them and they are able to take into account their previous input state as well as those actually present, a sort of "before" and "after" is involved. They are generally termed as Two State or Bistable devices which can have their output set in either of two basic states, a logic level "1" or a logic level "0" and will remain "latched" indefinitely in this current state or condition until some other input trigger pulse or signal is applied which will cause it to change its state once again.
Sequential Logic Circuit
The word "Sequential" means that things happen in a "sequence", one after another and in Sequential Logic circuits, the actual clock signal determines when things will happen next. Simple sequential logic circuits can be constructed from standard Bistable circuits such as Flip-flops, Latches or Counters and which themselves can be made by simply connecting together NAND Gates and/or NOR Gates in a particular combinational way to produce the required sequential circuit.
Sequential Logic circuits can be divided into 3 main categories:
1. Clock Driven - Synchronous Circuits that are Synchronised to a specific clock signal.
2. Event Driven - Asynchronous Circuits that react or change state when an external event occurs.
3. Pulse Driven - Which is a Combination of Synchronous and Asynchronous.
Classification of Sequential Logic
As well as the two logic states mentioned above logic level "1" and logic level "0", a third element is introduced that separates Sequential Logic circuits from their Combinational Logic counterparts, namely TIME. Sequential logic circuits that return back to their original state once reset, i.e. circuits with loops or feedback paths are said to be "Cyclic" in nature.