数字电子技术(Floyd 第十版)课件Chapter.ppt

数字电子技术(Floyd 第十版)课件Chapter.ppt

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,DigitalFundamentalsTenthEditionFloyd,Chapter8,2008PearsonEducation,Asyouknow,thebinarycountsequencefollowsafamiliarpatternof0sand1sasdescribedinSection2-2ofthetext.,LSBchangesoneverynumber.,Thenextbitchangesoneveryothernumber.,000001010011100101110111,Thenextbitchangesoneveryfourthnumber.,Summary,CountinginBinary,Acountercanformthesamepatternof0sand1swithlogiclevels.Thefirststageinthecounterrepresentstheleastsignificantbitnoticethatthesewaveformsfollowthesamepatternascountinginbinary.,LSB,MSB,Summary,CountinginBinary,Inanasynchronouscounter,theclockisappliedonlytothefirststage.Subsequentstagesderivetheclockfromthepreviousstage.,Summary,ThreebitAsynchronousCounter,Thethree-bitasynchronouscountershownistypical.ItusesJ-Kflip-flopsinthetogglemode.,Waveformsareonthefollowingslide,Summary,ThreebitAsynchronousCounter,CLK,Q0,Q1,Q2,NoticethattheQ0outputistriggeredontheleadingedgeoftheclocksignal.ThefollowingstageistriggeredfromQ0.TheleadingedgeofQ0isequivalenttothetrailingedgeofQ0.Theresultingsequenceisthatofan3-bitbinaryupcounter.,Summary,PropagationDelay,Asynchronouscountersaresometimescalledripplecounters,becausethestagesdonotallchangetogether.Forcertainapplicationsrequiringhighclockrates,thisisamajordisadvantage.,Noticehowdelaysarecumulativeaseachstageinacounterisclockedlaterthanthepreviousstage.,CLK,Q0,Q1,Q2,Q0isdelayedby1propagationdelay,Q2by2delaysandQ3by3delays.,Summary,AsynchronousDecadeCounter,Thiscounterusespartialdecodingtorecyclethecountsequencetozeroafterthe1001state.Theflip-flopsaretrailing-edgetriggered,soclocksarederivedfromtheQoutputs.Othertruncatedsequencescanbeobtainedusingasimilartechnique.,Waveformsareonthefollowingslide,CLR,Summary,AsynchronousDecadeCounter,WhenQ1andQ3areHIGHtogether,thecounterisclearedbya“glitch”ontheCLRline.,CLK,Q0,Q1,Q2,Q3,CLR,Glitch,Glitch,Summary,AsynchronousCounterUsingDFlip-flops,Dflip-flopscanbesettotoggleandusedasasynchronouscountersbyconnectingQbacktoD.ThecounterinthisslideisaMultisimsimulationofonedescribedinthelabmanual.Canyoufigureoutthesequence?

Thenextslideshowsthescope,MSB,LSB,Summary,CLK,LSB,MSB,Notethatitismomentarilyinstate3whichcausesittoclear.,Summary,The74LS93AAsynchronousCounter,The74LS93AhasoneindependenttoggleJ-Kflip-flopdrivenbyCLKAandthreetoggleJ-Kflip-flopsthatformanasynchronouscounterdrivenbyCLKB.,CLKA,K0,J0,Q0,C,C,C,J1,J2,K1,K2,Q1,Q2,C,J3,K3,Q3,CLKB,Thecountercanbeextendedtoforma4-bitcounterbyconnectingQ0totheCLKBinput.Twoinputsareprovidedthatclearthecount.,RO

（1）,RO

（2）,AllJandKinputsareconnectedinternallyHIGH,Summary,SynchronousCounters,Inasynchronouscounterallflip-flopsareclockedtogetherwithacommonclockpulse.Synchronouscountersovercomethedisadvantageofaccumulatedpropagationdelays,butgenerallytheyrequiremorecircuitrytocontrolstateschanges.,K0,J0,Q0,C,C,C,J1,J2,K1,K2,Q0Q1,Q0,Q1,Q2,CLK,HIGH,This3-bitbinarysynchronouscounterhasthesamecountsequenceasthe3-bitasynchronouscountershownpreviously.,Thenextslideshowshowtoanalyzethiscounterbywritingthelogicequationsforeachinput.Noticetheinputstoeachflip-flop,Summary,AnalysisofSynchronousCounters,Atabulartechniqueforanalysisisillustratedforthecounteronthepreviousslide.Startbysettinguptheoutputsasshown,thenwritethelogicequationforeachinput.Thishasbeendoneforthecounter.,Outputs,Logicforinputs,1.Putthecounterinanarbitrarystate;thendeterminetheinputsforthisstate.,000,0,0,0,0,1,1,2.Usethenewinputstodeterminethenextstate:

Q2andQ1willlatchandQ0willtoggle.,001,0,0,1,1,1,1,3.Setupthenextgroupofinputsfromthecurrentoutput.,Continuelikethis,tocompletethetable.Thenextslideshowsthecompletedtable,010,4.Q2willlatchagainbutbothQ1andQ0willtoggle.,Summary,AnalysisofSynchronousCounters,Outputs,Logicforinputs,000,0,0,0,0,1,1,001,0,0,1,1,1,1,010,1,1,1,1,1,1,1,1,1,1,1,1,011,100,101,110,111,000,0,0,0,0,1,1,1,1,0,0,0,0,0,0,1,1,0,0,0,0,1,1,1,1,Atthispointsallstateshavebeenaccountedforandthecounterisreadytorecycle,Summary,A4-bitSynchronousBinaryCounter,Q0,Q1,Q2,Q3,The4-bitbinarycounterhasonemoreANDgatethanthe3-bitcounterjustdescribed.TheshadedareasshowwheretheANDgateoutputsareHIGHcausingthenextFFtotoggle.,Withsomeadditionallogic,abinarycountercanbeconvertedtoaBCDsynchronousdecadecounter.Afterreachingthecount1001,thecounterrecyclesto0000.,Thisgatedetects1001,andcausesFF3totoggleonthenextclockpulse.FF0togglesoneveryclockpulse.Thus,thecountstartsover