What Are Logic Circuits?
Logic circuits are electronic circuits that perform some type of processing or controlling function. See logic device. Logic circuits use two different values of a physical quantity, usually voltage, to present the Boolean values true (or 1) and false (or 0). Logic circuits can have inputs and they have one or more outputs that are, at least partially, dependent on their inputs. In logic circuit diagram, connections from one circuit’s output to another circuit’s input are often shown with an arrowhead at the input end.
In terms of their behavior, logic circuits are much like programming language functions or methods. Their inputs are analogous to function parameters and their outputs are analogous to function returned values. However, a logic circuit can have multiple outputs.
There are two basic types of logic circuitry: combinational circuitry and state circuitry.
- Combinational circuitry behaves like a simple function. The output of combinational circuitry depends only on the current values of its input.
- State circuitry behaves more like an object method. The output of state circuitry does not just depend on its inputs — it also depends on the past history of its inputs. In other words, the circuitry has memory.
This is much like an object method whose value is dependent on the object’s state: its instance variables.
These two types of circuitry work together to make up a processor datapath.
Logic circuits include such devices as multiplexers, register, arithmetic logic units (ALUs), and computer memory, all the way up through complete microprocessors, which may contain more than 100 million gates. In modern practice, most gates are made from MOSFETs (metal-oxide-semiconductor field-effect transistors).
Compound logic gates AND-OR-Invert (AOI) and OR-AND-Invert (OAI) are often employed in circuit design because their construction using MOSFETs is simpler and more efficient than the sum of the individual gates.