Digital Circuits.pdf

Digital Circuits : Basics

1. DIGITAL ELECTRONICS
M.PRABU, M.E., (Ph.D).,
ASSISTANT PROFESSOR,
DEPARTMENT OF ECE,
MNM JAIN ENGINEERING COLLEGE,
CHENNAI-97.
EMAIL ID: [email protected]

2. Why digital electronics is important…..?
❖Anadvantageofdigitalcircuitswhencomparedtoanalogcircuitsisthatsignals
representeddigitallycanbetransmittedwithoutdegradationcausedbynoise.
❖Inadigitalsystem,amorepreciserepresentationofasignalcanbeobtainedby
usingmorebinarydigitstorepresentit.
❖Informationstoragecanbeeasierindigitalsystemsthaninanalogones.

3. 3
Thefollowingnumbersystemsarethemostcommonlyused.
❖DecimalNumber system
❖BinaryNumber system
❖Octal Number system
❖HexadecimalNumber system
Number System

4. 4
COMBINATIONAL CIRCUITS

5. 5
❖CombinationalcircuitsconsistofLogicgates.Thesecircuitsoperatewithbinary
values.Theoutputsofcombinationalcircuitdependsonthecombinationofpresent
inputs.Thefollowingfigureshowstheblockdiagramofcombinationalcircuit.
❖Thiscombinationalcircuithas‘n’inputvariablesand‘m’outputs.Eachcombination
ofinputvariableswillaffecttheoutputs.
COMBINATIONAL CIRCUITS

6. 6
BLOCK DIAGRAM OF COMBINATIONAL CIRCUIT

7. 7
Design procedure of Combinational circuits:
❖Findtherequirednumberofinputvariablesandoutputsfromgivenspecifications.
❖FormulatetheTruthtable.Ifthereare‘n’inputvariables,thentherewillbe2n
possiblecombinations.Foreachcombinationofinput,findtheoutputvalues.
❖FindtheBooleanexpressionsforeachoutput.Ifnecessary,simplifythose
expressions.
❖ImplementtheaboveBooleanexpressionscorrespondingtoeachoutputby
usingLogicgates.

8. 8
SEQUENTIAL CIRCUITS

9. 9
Thesequentialcircuitcontainsasetofinputsandoutputs.Theoutputsof
sequentialcircuitdependsnotonlyonthecombinationofpresentinputsbutalsoonthe
previousoutputs.Previousoutputisnothingbutthepresentstate.Therefore,sequential
circuitscontaincombinationalcircuitsalongwithmemorystorageelements.Some
sequentialcircuitsmaynotcontaincombinationalcircuits,butonlymemoryelements.
SEQUENTIAL CIRCUITS

10. 10

11. 11
Types of Triggering :
Followingarethetwopossibletypesoftriggeringthatareusedinsequentialcircuits.
❖Level triggering
❖Edge triggering
Level triggering :
Therearetwolevels,namelylogicHighandlogicLowinclocksignal.Followingarethe
twotypesofleveltriggering.
❖Positive level triggering
❖Negative level triggering

12. 12
Positive level triggering
Negative level triggering

13. 13
Followingarethetwotypesofedgetriggeringbasedonthetransitionsofclocksignal.
❖Positive edge triggering
❖Negative edge triggering
Positive edge triggering
Negative edge triggering

14. 14
Combinational Vs Sequential

15. 15
S.No Key Combinational Circuit Sequential Circuit
1 DefinitionCombinationalCircuitisthe
typeofcircuitinwhich
outputisindependentof
timeandonlyreliesonthe
inputpresentatthat
particularinstant.
OnotherhandSequential
circuitisthetypeofcircuit
whereoutputnotonlyrelies
onthecurrentinputbutalso
dependsontheprevious
output.
2 Feedback InCombinationalcircuitas
outputdoesnotdependon
thetimeinstant,nofeedback
isrequiredforitsnextoutput
generation.
Onotherhandincaseof
Sequentialcircuitoutputrelies
onitspreviousfeedbackso
outputofpreviousinputis
beingtransferredasfeedback
usedwithinputfornextoutput
generation.

16. 16
S.No Key Combinational Circuit Sequential Circuit
3PerformanceAstheinputofcurrent
instantisonlyrequiredin
caseofCombinational
circuit,itisfasterandbetter
inperformanceascompared
tothatofSequentialcircuit.
OnotherhandSequential
circuitarecomparatively
slowerandhaslow
performanceascomparedto
thatofCombinationalcircuit.
4ComplexityNo implementationof
feedback makes the
combinationalcircuitless
complexascomparedto
sequentialcircuit.
Howeveronotherhand
implementationoffeedback
makessequentialcircuit
morecomplexascompared
tocombinationalcircuit.

17. 17
S.No Key Combinational Circuit Sequential Circuit
5Elementary
Blocks
Elementarybuildingblocks
forcombinationalcircuitare
logicgates.
Onotherhandbuilding
blocksforsequentialcircuit
areflipflops..
6Operation Combinationalcircuitare
mainlyusedforarithmeticas
wellasBooleanoperations.
OnotherhandSequential
circuitismainlyusedfor
storingdata.

18. 18
Synchronous Vs Asynchronous Sequential Circuits

19. 19
S.No Key Synchronous Circuit Asynchronous Circuit
1 DefinitionSynchronous sequential
circuitsaredigitalsequential
circuitsinwhichthe
feedbacktotheinputfor
nextoutputgenerationis
governedbyclocksignals.
OnotherhandAsynchronous
sequentialcircuitsaredigital
sequentialcircuitsinwhichthe
feedbacktotheinputfornext
outputgenerationisnot
governedbyclocksignals.
2 Memory
Unit
InSynchronoussequential
circuits,thememoryunit
whichisbeinggetusedfor
governanceisclockedflip
flop.
Onotherhandunclockedflip
floportimedelayisusedas
memoryelementincaseof
Asynchronous sequential
circuits.

20. 20
S.No Key Synchronous Circuit Asynchronous Circuit
3 State ThestatesofSynchronous
sequentialcircuitsare
alwayspredictableandthus
reliable.
Onotherhandthereare
chancesfortheAsynchronous
circuitstoenterintoawrong
statebecauseofthetime
differencebetween the
arrivalsofinputs.Thisis
calledasracecondition.
4 ComplexityItiseasytodesign
Synchronous sequential
circuits
Howeveronotherhandthe
presenceoffeedbackamong
logicgatescausesinstability
issuesmakingthedesignof
Asynchronous sequential
circuitsdifficult.

21. 21
S.No Key Synchronous Circuit Asynchronous Circuit
5PerformanceDue to the propagation
delay of clock signal in
reaching all elements of the
circuit the Synchronous
sequential circuits are
slower in its operation speed
Since there is no clock signal
delay, these are fast
compared to the Synchronous
Sequential Circuits
6Example Synchronouscircuitsare
usedincounters,shift
registers,memoryunits.
OnotherhandAsynchronous
circuitsareusedinlowpower
andhighspeedoperations
such as simple
microprocessors,digitalsignal
processingunitsandin
communicationsystemsfor
emailapplications,internet
accessandnetworking.