Introduction
Half adder is a combinational circuit that performs an arithmetic addition of two bits. Half adder takes two inputs and produces two outputs. Let x, y be the two inputs, and S ( sum ), C ( carry ) be the two outputs.
Why we need two outputs?
This is because if we add ‘1’ and ‘1’ then the result is a 2-bit number. Therefore, we need two bits to represent the sum. S ( sum ) is used to represent the least significant bit and C ( carry ) is used to represent the most significant bit.
- 0 + 0 = 00
- 0 + 1 = 01
- 1 + 0 = 01
- 1 + 1 = 10
Truth Table
From the truth table, we can directly write
K-map
Implementation of Half-adder
Half adder using AND, OR, NOT gate
Half adder using XOR and AND gate
Half adder using NAND gate
The minimum number of NAND gates required to design a half-adder is 5.
Half adder using NOR gate
The minimum number of NOR gates required to design a half-adder is 5.
Half adder using 2×1 Multiplexer
We need two 2×1 multiplexers to implement a half-adder. One for sum bit and one for carry bit.
Half adder using 2 to 4 Decoder
Note
The term “Half-Adder” stems from the fact that two half-adders can be used to implement a full-adder.
References