Uncategories Logic Gates Functions

Logic Gates Functions

What are logic gates?

  • A logic gate is building block of a digital circuit.
  • It has one or more inputs and only one output.
  • The relationship between the inputs and output is based on certain logic.
  • Based on this logic the gates are named as AND, OR, NAND, NOR etc...
  • A terminal in this circuits is one of the binary conditions 0 (low) or 1 (high), represented by voltage levels as 0V (low) and 5V (high).

Primary Logic Gates:

  • There are three primary logic gates AND, OR AND NOT.
  • All other logic gates could be represented as a combination of these.
TypeDefinitionSymbolTruth Table
ANDAn electronic circuit that gives a high output(1) only if all inputs are high(1).
INPUT OUTPUT
A B A.B
0 0 0
0 1 0
1 0 0
1 1 1
ORAn electronic circuit that gives a high output (1) if one or more inputs are high.
INPUT OUTPUT
A B A+B
0 0 0
0 1 1
1 0 1
1 1 1
NOTAn Electronic circuit that produces an inverted version of the input at its output.
INPUT OUTPUT
A NOT A
0 1
1 0

Universal Logic Gates:

  • NAND and NOR gates are called universal logic gates.
  • NAND or NOR alone can be implemented to design all primary gates. So they are called universal gates.
TypeDefinitionSymbolTruth Table
NAND
  • NOT-AND gate i.e an AND gate followed by NOT gate.
  • Output is high (1) if any of the inputs are low. 
INPUT OUTPUT
A B NOT(A.B)
0 0 1
0 1 1
1 0 1
1 1 0
NOR
  • NOT-OR gate i.e an OR gate followed by NOT gate.
  • Output is low (0) if any of the inputs are high. 
INPUT OUTPUT
A B NOT(A+B)
0 0 1
0 1 0
1 0 0
1 1 0

Derived Logic Gates:

  • Derived logic gates are formed by combination of two or more primary logic gates.
  • XOR ( Exclusive OR) and XNOR (Exclusive NOR) are derived logic gates.
TypeDefinitionSymbolTruth Table
XOR
  • Exclusive OR gate i.e A OR B but not both.
  • Output is high (1) if either, but not both of its inputs are high. 
INPUT OUTPUT
A B A XOR B
0 0 0
0 1 1
1 0 1
1 1 0
XNOR
  • Exclusive NOR gate i.e opposite of XOR gate.
  • Output is low (0) if either, but not both of its inputs are high. 
INPUT OUTPUT
A B NOT(XOR)
0 0 1
0 1 0
1 0 0
1 1 1
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