Tkool Electronics

I.Description74LS138isa3-lineto8-linedecoder/demultiplexer.Thechipisdesignedtobeusedinhigh-performancememory-decodingordata-routingapplications,requiringveryshortpropagationdelaytimes.Inhighperformancememorysystemsthesedecoderscanbeusedtominimizetheeffectsofsystemdecoding.Thethreeenablepinsofchip(inwhichTwoactive-lowandoneactive-high)reducetheneedforexternalgatesorinverterswhenexpanding.CatalogI.DescriptionII.DigitalVoltmeterCircuitFAQOrdering&QuantityII.DigitalVoltmeterCircuitWeuseAD574andAT89C2051toformahighprecisiondigitalvoltmeter.TheschematicdiagramisshowninFigure1.AD574isa12-bitsuccessivecomparisonA/Dconverterwith12datalinesintotal.P1ofAT89C2051isdirectlyconnectedtothehigh8-bitdatalineofAD574.Thelow4-bitdatalineofAD574isdirectlyconnectedwiththeupperhalf4-bitp1.4-p1.7ofsingle-chipmicrocomputer.Datareadingisbasedonthecontrollineofsinglechipmicrocomputer.P3.5isconnectedtoAD574byteshortperiodcontrolline(A0).P3.4isconnectedtoreadconversiondatacontrolpin.AndP3.7isdirectlyconnectedwiththeterminalofindicatingworkingstatus(STS).Suchstructuredeterminesthatitcanonlybe8-bitoutput,sothedatamodeselectionendcanbedirectlygrounded.AT89C2051hasonly15I/Oportwires,11ofwhichareusedabove,andonly4ofthemareleft.Theoutputdataisoutputthroughtheserialportofthesingle-chipmicrocomputer,andanexternal74LS164(serialinandparallelout)decoderisconnectedforexpansion.Atthesametime,thedatadisplayedis4bits,andtheremaining2portlinesstillcannotmeettherequirements.A74LS138decoderisneededtogatetheaddressofthedisplayLED.Hereweusetheinputmodeof10Vrange.Pin13ofAD574istheinputterminalofthemeasuredvoltage.BecauseonlyoneAD574conversionchipisused,theCSterminalcanbedirectlygrounded.Theconverteruses12Vpowersupplyvoltageandtheworkingvoltageis+5V.74LS164isaserialinputandparalleloutputdecoder.TheBCDserialcodeoutputbyAT89C2051throughtheserialportisdecodedby74LS164andoutputasaseven-segmentBCDcode,whichisdirectlyconnectedtoa-goftheLED,andthedatalinesofthefourLEDsareconnectedonebyone.LEDdigitaltubeusescommonanodetype.Theaddresscodeoutputby74LS138isconnectedtothecommonterminalofLEDviaatransistor2SA1015(PNP).Thedisplayofthefour-digitLEDistime-sharingstrobethroughtheaddressline,whichisourcommonlyuseddynamicscanningdisplaymethod.Itisworthmentioningthatinthedynamicscanningdisplaymode,thefrequencyofdynamicscanninghascertainrequirements.Ifthefrequencyistoolow,theLEDwillflicker.Ifthefrequencyistoohigh,thelightingtimeofeachLEDistooshort,andthebrightnessoftheLEDistoolow.Itcantbeseenclearlywithnakedeye.Soitisgenerallyappropriatetotakeabout10ms.Thisrequiresthatwhenwritingaprogram,acertainLEDshouldbeonandkeptforacertainperiodoftime.Theprogramoftenusesthecalldelaysubroutine.FAQWhatisthedifferencebetween74hc138and74LS138?Bothhavethesamefunction.74HC138ismadeofhigh-speedCMOSprocess,withlowpowerconsumption,highoutput,lowlevelandwiderange.74LS138adoptstheearlybipolarprocess,anditsdrivingcapabilityisrelativelylarger.Whatisthefunctionaldifferencebetween74ls138decoderand74ls148?74ls138isa3-8wiredecoder/multiplexer,74ls148isan8-3wireoctalpriorityencoder.Oneisdecodingandtheotherisencoding.OppositeeffectWhatsthedifferencebetween74LS138Dand74LS138N?Thosetwoarethesamechip,DisSOPpackage,NisDIPpackage.Whataretheoutputcharacteristicsof74LS138decoder?UnderthepremisethattheenableterminalsS1(activehigh),S2(activelow),andS3(activelow)arevalidatthesametime,onlyoneoutputterminalislowatatime(therestarehigh);Iftheenableterminalisinvalid,theoutputisallhighlevel.Whatdothelettersandnumbersin74ls138standfor?74ls138isa3-8-linedecoder.Thenumber74representsthe74seriesofthe54/74series,andthe74serieshasanoperatingtemperatureof0degreesto70degrees.LSisaseries,representingthelow-powerSchottkyseries.138isthevarietycode.Whatstheworkingprincipleof74ls138?74LS138workingprinciple①Whenonestrobeterminal(E1)ishighlevel,andtheothertwostrobeterminals(E2)and(E3)arelowlevel,attheoutputterminalscorrespondingtoY0toY7,thebinarycodeofaddressterminals(A0,A1,A2)canbedecodedatlowlevel.Forexample:whenA2A1A0=110,theY6outputterminaloutputsalow-levelsignal.②UsingE1,E2andE3,itcanbecascadedtoexpandintoa24-linedecoder;ifanexternalinverterisconnected,itcanalsobecascadedtoexpandintoa32-linedecoder.③Ifoneofthestrobeterminalsisusedasadatainputterminal,74LS138canalsobeusedasadatadistributor.④Itcanbeusedin8086decodingcircuittoexpandmemory.

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