• Usha Saha

Adders and Subtractors

Updated: Nov 28, 2020

Modern-day is the era of digital electronics, digital systems, and moreover the digital world. What do these digital systems comprise of, how do they operate and many similar questions are there? Today let's answer some of these questions with the help of some digital circuit elements. Through this article, we would cover the two crucial #arithmetic circuits, those are #adders and subtractors.


Adders and #subtractor come under the category of combinational circuits. Combinational Logic Circuits as the name suggest consisting of a combination of logic gates whose output at any instant of time is calculated by the present combination of inputs only. Combinational circuits, therefore, are also called a memoryless system as the output is only dependent on the present values of input.


Adders

The Half-Adder:- The half adder is an arithmetic circuit used to perform two single-bit addition. The input to the half adder is a two-bit data while the output comprises sum and carry outputs. For better understanding purposes let us have a look at the block diagram of a half adder circuit where A and B be the two inputs, and S(sum) and C(carry) are the two outputs.

Sum=A'B+AB' (From the K-Map).

Carry=AB (From the K-Map).


Half Adder elements
Half Adder elements

Full Adder:- Similiar to half adder a full adder is also an #arithmetic circuit, used to perform the addition of three input bits. There are three input lines (assume A, B and, C) and two output lines namely sum and carry/Cout. Let us take a look at the block diagram, truth table, and k-map of a full adder for a better concept.


Full Adder Representation.
Full Adder Representation.

Substituting the sum and carry value in the K-map for simplification purpose, we get the K-map as below.

Full Adder K-map
Full Adder K-map

By solving the K-map

Sum= A'B'C+A'BC'+AB'C'+ABC

Carry= AB+BC+CA

The sum and carry can also be written in terms of minterms

Sum= Summation(1,2,4,7)

Carry=Summation(3,5,6,7)


Binary Parallel adder:- Another type of adder is a binary adder that adds two binary numbers in parallel form. It constitutes full adders connected in cascade, where the output carry from each full adder is connected to the input carry of the next adder.

Hence 'n' binary parallel adder can be implemented by:-

  • n full adders.

  • (n-1) full adder and a half adder

  • (n-1)[2 half adder + 1 OR gate] and a half adder.

  • (2n-1) half adder and (n-1) OR gates.

Key point:- In 'n' bit parallel adder minimum delay to produce the final output is 2nt(pd).

here t(pd) is the propagation delay of each adder. This propagation delay makes the parallel adder very slow, to overcome this another adder is used known as The look-ahead carry adder.


Look-ahead carry adder:- A look ahead eliminates the propagation delay of the binary adder and speeds up the process. It examines all the input bits simultaneously and also generates the carry bit for all the stages simultaneously.


Additional information

  • A half adder can be implemented using universal gates, that is by NAND or NOR gates. A total of 5 NAND/NOR gate is required to implement a half adder.

  • A full adder can be implemented using universal gates, that is by NAND or NOR gates. A total of 9 NAND/NOR gate is required to implement a full adder.

Subtractors

A half-subtractor:- A half subtractor is another #combinational circuit that performs one-bit subtraction to generate a difference. The block diagram of a half subtractor is shown below. There are two input lines namely A and B, and two output lines namely difference(d) and borrow(b).


Half Subtractor Representation
Half Subtractor Representation

By solving the outputs of half subtractor using K-map the difference and borrow are as follows.


Half Subtractor K-map
Half Subtractor K-map

Difference= A'B+AB'

Borrow= A'B


The Full subtractor:- The full subtractor as the name signifies performs the arithmetic subtraction of the three input data line and gives an output. The block diagram of a full subtractor is as below. It comprises three input lines A, B, and, C and two output lines d (difference) and b (borrow).

Full Subtractor Representation
Full Subtractor Representation

The difference and borrow as determined by the truth table and the k-map is as follows.

Difference= A'B'C+A'BC'+AB'C'+ABC

Borrow=A'C+A'B+BC

Full Subtractor K-map
Full Subtractor K-map

Key points

  • A half subtractor can be implemented using universal gates, that is by NAND or NOR gates. A total of 5 NAND/NOR gate is required to implement a half subtractor.

  • A full subtractor can be implemented using universal gates, that is by NAND or NOR gates. A total of 9 NAND/NOR gate is required to implement a full subtractor.

  • A minimum of 2x1 MUX is required to implement a half adder/half subtractor.

  • Half Adder+Inverter=Half subtractor

  • Full Adder=2 Half Adder+ OR gate.

  • Full Subtractor=2 Half Subtractor+ OR gate.



See Also

What is electronics?

MUX and DEMUX

Analog vs Digital

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