#Technical studies – Study 20

Introduction

Adders in digital system are the combinational logic circuits that perform an arithmetic  operation called addition. The binary addition rules are –

0+0=0, 0+1=1, 1+0=1, 1+1=10

In the four rules stated above, the first three rules add two bits and the result is a single bit. But, in last rule two bits are added and the result is also two bits. The single digit result is called as sum. When the result is more than one digit the LSB is called sum and the MSB is called carry.

A combinational logic circuit that performs the addition of two numbers is called a half adder.

 Similarly, a combinational logic circuit that performs the addition of three numbers is called full adder.

Half adder:

Half adder is a digital logic circuit that takes two bits in adds them and provides two outputs one sum and one carry. The block diagram and truth table is drawn below.

Block diagram of Half Adder
Truth table of Half Adder

According to the truth table the sum output is similar to the output of X-OR operation. Therefore

S=A⊕B

The carry output is similar to the output of AND operation. Therefore

C = AB

So the logic circuit for the half adder can be designed by using a X-OR gate and an AND gate.

Logic circuit of Half Adder

Half adder using AOI (AND OR INVERTER) logic:

 Using AOI logic we can design the logic circuit for the half adder.

Logic circuit of AOI logic Half Adder

Half adder using NAND logic:

Half adder logic circuit can be designed using NAND logic.

S=\overline{A}B+A\overline{B}=\overline{A}B+B\overline{B}+A\overline{B}+A\overline{A}

      =A(\overline{A}+\overline{B})+B(\overline{A}+\overline{B})

      =A.\overline{AB}+B.\overline{AB}

      =\overline{\overline{A.\overline{AB}}.\overline{B.\overline{AB}}}

And C=AB=\overline{\overline{AB}}      

Half Adder using NAND Logic

Half adder using NOR logic:

Half adder logic circuit can be designed using NOR logic.

S=\overline{A}B+A\overline{B}=\overline{A}B+B\overline{B}+A\overline{B}+A\overline{A}

    =A(\overline{A}+\overline{B})+B(\overline{A}+\overline{B})

    =(A+B)(\overline{A}+\overline{B})

    =\overline{\overline{A+B}+\overline{\overline{A}+\overline{B}}}

 

And   C=AB=\overline{\overline{AB}}=\overline{\overline{A}+\overline{B}}

Half Adder using NOR logic

 Full adder:

Full adder is a digital logic circuit that takes three bits in adds them and provides two outputs one sum and one carry. Out of the three input bits two are the variables and the third one is the previous carry. Below drawing shows the block diagram and truth table of full adder.

Block diagram of Full Adder
Truth Table of Full Adder

 According to the truth table: –

S=

And

C=

So we can design the logic circuit of full adder as below:

Logic circuit of Full Adder

Full adder using AOI (AND OR INVERTER) logic:

 Design of full adder logic circuit Using AOI logic.

Logic circuit of Full Adder using AOI logic

Full adder using NAND logic:

To design Full adder logic circuit using NAND logic we have to make the expression of full adder in NAND form.

Logic circuit of Full Adder using NAND logic

Full adder using NOR logic:

We can design Full adder logic circuit using NOR logic.

Logic circuit of Full Adder using NOR logic

Please write on the comment box if you have any questions.

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