Half Subtractor

Introduction

Half Subtractor is a combinational circuit that performs arithmetic subtraction of 2 bits. Half subtractor takes 2 inputs and produces 2 outputs. Let x, y be the inputs and D ( difference ), B ( borrow ) be the outputs.

Why we use two outputs?

The difference bit D tells us the difference between the two bits. Borrow bit B is used to check if a borrow is required or not. Borrow is only required if the result is negative.

For example, if we subtract 0 and 1 then the result is -1. In this case, the smaller number takes borrow. Since we are dealing with binary numbers only, talking borrow is equivalent to adding 2 to the number. Therefore, 0 becomes 2. The difference between 2 and 1 is 1. Thus, for “0-1” the difference bit D is 1, and the borrow bit B is also 1.

  • 0 – 0 = 0
  • 0 – 1 = 1 ( Borrow is required since the result is negative, B = 1 )
  • 1 – 0 = 1
  • 1 – 1 = 0

Truth Table

half subtractor truth table

K-Map

half subtractor k-map
K-Map for Difference
half subtractor k-map 2
K-Map for Borrow

Implementation of Half-Subtractor

half subtractor

Half Subtractor using AND, OR, NOT gate

half subtractor AND OR NOT gate

Half Subtractor using NAND gates

Minimum NAND gates required to design a half subtractor is 5.

half subtractor NAND gate

Half Subtractor using NOR gates

Minimum NOR gates required to design a half subtractor is 5.

half subtractor NOR gate

Half subtractor using 2×1 Multiplexer

half subtractor multiplexer
half subtractor 2x1 MUX

Half Subtractor using 2×4 decoder

half subtractor decoder

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