Page 47 - Digital Electronics by harish

UNIT - II
COMBINATIONAL CIRCUITS
Combinational circuits

In digital systems, there are two types of circuits, 1) Combinational logic circuits and 2)
Sequential logic circuits. In combinational circuits, the output depends only on the present
input conditions. But, in sequential circuits, the output depends not only on the present input
conditions but also on the previous output conditions.

2.1 Binary Arithmetic
Arithmetic circuits

Arithmetic circuits are the combinational logic circuits used to perform arithmetic operations
such as addition, subtraction, multiplication and division on binary numbers.

2.1.1 Binary addition

In addition, the first number is called augend and the second number is called is addend. The
result is called the sum. Sometimes, we may get carry also.

Augend
+ Addend
Carry Sum

The following rules are used in binary addition.

0 0 1 1
+ 0 + 1 + 0 + 1
0 1 1 10 Sum = 0 and Carry = 1

1
1
+ 1
11 Sum = 1 and Carry = 1

In a binary number, the left most bit is called Most Significant Bit (MSB) and the rightmost
bit is called Least Significant Bit (LSB). Addition must be carried out bit by bit starting from
the LSB.
Examples :

1) Add the binary number 101 and 10. (Apply basic rules)

Answer :(111) 2

2) Add (10) 10 and (3) 10 in binary.

First convert the decimal numbers to binary.

Answer :(1101) 2 = (13) 10

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