Digital Circuits/Gates

Logic gates are one of the fundamental units of a digital circuit. They can be made from only a handful of transistors each, and they can implement any logical function, and therefore build any digital device. At the most basic level, all digital circuits are combination of logic gates.

Logic Gates

AND, OR and NOT gates are the fundamental functions, out of which the other functions are built. The other functions available are

• NAND: an inversion of AND's output.
• NOR: an inversion of OR's output.
• XOR (exclusive-OR): outputs high only if just one input is high.
• XNOR: inversion of the output of XOR.

In total then, we have seven basic gates. Although there are sixteen possible outputs from two input logic gates, the other functions not represented here are usually made up of these seven. This idea can even be extended further - NOR or NAND are each capable of producing all the other functions and are sometimes called the "universal gates" (see NOR Logic and NAND Logic for more information). At the transistor level, many devices consist only of one of these gates. That said, by using the other gates, we make conceptualising and designing digital circuits much easier, and loses no generality, as the conversion to the relevant type of gate will be done by automatic tools if needed.

Only the NOT gate and the buffer (which just copies the input to the output) have one input. The other basic gates have two inputs, although we will later look at how to extend the concept to more inputs.

Name	Function	Symbol
		ANSI	IEC
Buffer	$Q=A$
NOT gate (Inverter)	$Q={\overline {A}}$
AND gate	$Q=A\cdot B$
NAND gate (NOT−AND)	$Q={\overline {A\cdot B}}$
OR gate	$Q=A+B$
NOR gate (NOT−OR)	$Q={\overline {A+B}}$
XOR gate (Exclusive-OR)	$Q=A\oplus B$
XNOR gate (NOT−exclusive−OR)	$Q={\overline {A\oplus B}}$

Truth Tables

The truth tables for the seven common logic gates are given below:

A	B		Logic Gate Output
			NOT A	A AND B	A NAND B	A OR B	A NOR B	A XOR B	A XNOR B
0	0		1	0	1	0	1	0	1
0	1		1	0	1	1	0	1	0
1	0		0	0	1	1	0	1	0
1	1		0	1	0	1	0	0	1

Logic gates with more than 2 inputs

We can separate and make a group of 2 and work on it. for eg:

Y=A+B+C then Y is also equal to X+C where X=A+B.

For further reading

• The Apollo Guidance Computer, the first computer ever to use integrated circuits, was built from 4,100 ICs, each containing a single 3-input NOR logic gate.

Digital Circuits

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