Practical Insights in Combinational Logic: A Lecture by a CSIR-CEERI Expert

This lecture on Combinational Logic Circuits is presented by Dr. G. S. Virdi, Ex-Chief Scientist at CSIR–Central Electronics Engineering Research Institute (CEERI), Pilani, who has over 45 years of R&D experience in microelectronics, semiconductor device fabrication, and digital circuit design.

The content provides a clear and practical understanding of:

Fundamentals of combinational logic

Design and implementation of adders, subtractors, decoders, and encoders

Binary data transformation and logic gate combinations

Applications of combinational circuits in modern digital systems

This lecture is specially structured for:

Engineering students learning digital electronics

Researchers working in VLSI and microelectronic design

Industry professionals engaged in electronics and automation

By combining theoretical foundations with industry-oriented insights, this lecture aims to strengthen knowledge in digital circuit design and enhance professional skills in electronic system development.

1. CombinationalCircuits
Dr.G.S.Virdi
Ex.Chief Scientist
CSIR -Central Electronics Engineering Research Institute
Pilani -333031,India
1

2. Combinationalcircuits
Acombinationallogiccircuitisoneinwhichthe
presentstateofthecombinationofthelogic
inputsdecidestheoutput.
Thetermcombinationlogicmeanscombining oftwo
ormorelogicgatestoformarequired function
wheretheoutputatagiventime dependsonlyon
theinput.
2

3. Therequiredoutputdataisobtainedfromthis
processbytransforming thebinary information
givenattheinput.
3

4. ClassificationofCombinationalLogic
4

5. Characterstics:
Theoutputofcombinationalcircuitatany
instantoftime,dependsonlyonthelevels
presentatinputterminals.
Thecombinationalcircuitdonotuseany memory.
Thepreviousstateofinputdoesnot haveany
effectonthepresentstateofthe circuit.
Acombinationalcircuitcanhaveannnumber of
inputsandmnumberofoutputs.
5

6. HalfAdder
•Halfadderisacombinationallogiccircuitwith two
inputsandtwooutputs.
Thehalfaddercircuitisdesignedtoaddtwo
singlebitbinarynumberAandB.
Itisthebasicbuildingblockforadditionof two
singlebitnumbers.
Thiscircuithastwooutputscarryandsum.
6

7. Blockdiagram:
TruthTable:
CircuitDiagram:
7

8. 11November2016 8
Full-Adder
•Acombinationalcircuitthatperformsthe
additionofthreebitsatatimeiscalled“Full-Adder
’’
•Blockdiagram&thetruthtableofFull-Adder.
8

9. 11November2016 9
Circuitdiagramsof full-adder
9

10. HalfSubtractor
Halfsubtractorisacombinationcircuitwithtwo
inputsand twooutputs(differenceandborrow).
Itproducesthedifferencebetweenthetwobinary
bitsatthe inputandalsoproducesaoutput
(Borrow)toindicateifa1 hasbeenborrowed.In
thesubtraction(A-B),Aiscalledas Minuendbitand
BiscalledasSubtrahendbit.
10

11. 11
Circuitdiagramsof half Subtractor

12. Full Subtractor
•Thefullsubtractorisacombinationalcircuit
withthreeinputsA,B,CandtwooutputD
andC'.Aistheminuend,Bissubtrahend,Cis
theborrowproducedbytheprevious
stage,DisthedifferenceoutputandC'is
theborrowoutput.
12

13. 13

14. 14
Circuitdiagramsof full Subtractor

15. •Adecoderisacombinationalcircuitthathasninput
andtoamaximumm=2noutputs.
•Decoderisidenticaltoademultiplexerwithoutany
datainput.
•Itperformsoperationswhichareexactlyopposite to
thoseofanencoder.
15
Decoder

16. 16
Decoder

17. 17
Circuitdiagramsof Decoder

18. Encoder
•Encoderisacombinationalcircuitwhichisdesigned
toperformtheinverseoperationofthedecoder.
•Anencoderhasnnumberofinputlinesandm
numberofoutputlines.
•Anencoderproducesanmbitbinarycode
correspondingtothedigitalinputnumber.
•Theencoderacceptsanninputdigitalwordand
convertsitintoanmbitanotherdigitalword.
18

19. PriorityEncoder
•Thisisaspecialtypeofencoder.Priorityisgiven to
theinputlines.
•Iftwoormoreinputlineare1atthesametime,
thentheinputlinewithhighestprioritywillbe
considered.
•TherearefourinputD
0,D
1,D
2,D
3andtwooutput Y
0,
Y
1.OutofthefourinputD
3hasthehighest priority
andD
0hasthelowestpriority.
•IfD
3=1thenY
1Y
1=11irrespectiveoftheother
inputs.SimilarlyifD
3=0andD
2=1thenY
1Y
0=10
irrespectiveoftheotherinputs.
19

20. 20
PriorityEncoder

21. 21
Circuitdiagramsof PriorityEncoder

22. 26-10-2025 22