Chapter 2: Combinational Circuits : Adders, Mux, De-Mux
2
COMBINATIONAL CIRCUITS :
ADDERS, MUX, DE-MUX,
Unit Structure
2.1 Combinational Circuits
2.2 Adders
2.3 Mux, De-Mux
2.1 Combinational Circuits
Combinational circuit is a circuit in which we combine the different gates in the
circuit.
A logic gate is a basic building block of any electronic circuit. The output of the
combinational circuit depends on the values at the input at any given time. The
circuits do not make use of any memory or storage device
Examples are : encoder, decoder, multiplexer and demultiplexer.
Combinational logic is used in computer circuits to perform Boolean algebra on
input signals and on stored data.Other circuits used in computers, such as half
adders, full adders, half subtractors, full subtractors, multiplexers, demultiplexers,
encoders and decoders are also made by using combinational logic.
Some of the characteristics of combinational circuits are following :
● The output of a combinational circuit at any instant of time, depends only on
the levels present at input terminals.
● It does not use any memory. The previous state of input does not have any
effect on the present state of the circuit.
● It can have an n number of inputs and m number of outputs.
21
, COMPUTER ORGANIZATION AND ARCHITECTURE
Why do we use combinational circuits?
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2.2 Adder
What is an Adder?
An adder is a circuit that can be integrated with many other circuits for a wide range
of applications. It is a kind of calculator used to add two binary numbers. There are
two kinds of adders;
1. Half adder
2. Full adder
Half Adder
With the help of half adder, we can design circuits that are capable of performing
simple addition with the help of logic gates.
Example of the addition of single bits.
0+0 = 0
0+1 = 1
1+0 = 1
1+1 = 10
These are the possible single-bit combinations. But the result for 1+1 is 10. Though
this problem can be solved with the help of an EXOR Gate, the sum result must be
re-written as a 2-bit output.
Thus the above equations can be written as:
0+0 = 00
0+1 = 01
1+0 = 01
1+1 = 10
22
2
COMBINATIONAL CIRCUITS :
ADDERS, MUX, DE-MUX,
Unit Structure
2.1 Combinational Circuits
2.2 Adders
2.3 Mux, De-Mux
2.1 Combinational Circuits
Combinational circuit is a circuit in which we combine the different gates in the
circuit.
A logic gate is a basic building block of any electronic circuit. The output of the
combinational circuit depends on the values at the input at any given time. The
circuits do not make use of any memory or storage device
Examples are : encoder, decoder, multiplexer and demultiplexer.
Combinational logic is used in computer circuits to perform Boolean algebra on
input signals and on stored data.Other circuits used in computers, such as half
adders, full adders, half subtractors, full subtractors, multiplexers, demultiplexers,
encoders and decoders are also made by using combinational logic.
Some of the characteristics of combinational circuits are following :
● The output of a combinational circuit at any instant of time, depends only on
the levels present at input terminals.
● It does not use any memory. The previous state of input does not have any
effect on the present state of the circuit.
● It can have an n number of inputs and m number of outputs.
21
, COMPUTER ORGANIZATION AND ARCHITECTURE
Why do we use combinational circuits?
_________________________________________________________________
_________________________________________________________________
_________________________________________________________________
_________________________________________________________________
_________________________________________________________________
_________________________________________________________________
2.2 Adder
What is an Adder?
An adder is a circuit that can be integrated with many other circuits for a wide range
of applications. It is a kind of calculator used to add two binary numbers. There are
two kinds of adders;
1. Half adder
2. Full adder
Half Adder
With the help of half adder, we can design circuits that are capable of performing
simple addition with the help of logic gates.
Example of the addition of single bits.
0+0 = 0
0+1 = 1
1+0 = 1
1+1 = 10
These are the possible single-bit combinations. But the result for 1+1 is 10. Though
this problem can be solved with the help of an EXOR Gate, the sum result must be
re-written as a 2-bit output.
Thus the above equations can be written as:
0+0 = 00
0+1 = 01
1+0 = 01
1+1 = 10
22
