数字电子技术,英文版,chapter09

CHAPTER9Counters第8章Problems:2，4，8，12，17，20，21，29，34SynchronousandAsynchronousCounters同步和异步计数器Flip-flopscanbeconnectedtogethertoperformcountingoperations.Suchagroupofflip-flopsisacounter.Countersareclassifiedintotwobroadcategoriesaccordingtothewaytheyareclocked:asynchronousandsynchronous.Thetermasynchronousreferstoeventsthatdonothaveafixedtimerelationshipwitheachotherand,generally,donotoccuratthesametime.Inasynchronouscounters,commonlycalledripplecounters,thefirstflip-flopisclockedbytheexternalclockpulseandtheneachsuccessiveflip-flopisclockedbytheoutputoftheprecedingflip-flop.Sotheflip-flopswithinanasynchronouscountersdonotchangestatesatexactlythesametimeInsynchronouscounters,theclockinputisconnectedtoalloftheflip-flopssothattheyareclockedsimultaneously.A2-BitAsynchronousBinaryCounterThiscircuitconsistsoftwoflip-flops.TheCLKisappliedtotheclockinputofthefirstflip-flop.Thesecondflip-flopistriggeredbytheoutputofthefirstflip-flop.CLK0Q)(0LSBQ)(1MSBQ0000001111A2-BitAsynchronousBinaryCounterThiscircuitisa2-bitcounterbecauseitexhibitsfourdifferentstates(00,01,10,11).Inotherwords,itgoesthroughabinarysequence(00,01,10,11).Itactuallycountsthenumberofclockpulsesuptothree,andonthefourthpulseitrecyclestoitsoriginalstate.A3-BitAsynchronousBinaryCounterA3-bitcountergoesthroughabinarysequence(000,001,010,011,100,101,110,111).A3-BitAsynchronousBinaryCounter1-digitoctalcountercounterA4-BitAsynchronousBinaryCounterAnAsynchronousDecadeCounterThemodulusofacounteristhenumberofuniquestatesthatthecounterwillsequencethrough.Themaximumpossiblenumberofstates(maximummodulus)ofacounteris2n,wherenisthenumberofflip-flopsinthecounter.Counterscanalsobedesignedtohaveanumberofstatesintheirsequencethatislessthan2n.Theresultingsequenceiscalledatruncatedsequence.Onecommonmodulusforcounterswithtruncatedsequencesisten(calledMOD10).Counterswithtenstatesintheirsequencearecalleddecadecounters.Adecadecounterwithacountsequenceof0through9isaBCDdecadecounter.Toobtainatruncatedsequence,itisnecessarytoforcethecountertorecyclebeforegoingthroughallofitspossiblestates.AnAsynchronousDecadeCounterglitch:afalseorspuriouselectronicsignal.AnAsynchronousModulus-12Counter•Thisexampleshowshowanasynchronouscountercanbeimplementedhavingamodulusof12withastraightbinarysequencefrom0000through1011.•Butthiscircuitisnotveryreliable.•So,itisimprovedbyaddingadditionallogic.74LS93A:4-BitAsynchronousBinaryCounter•Itcanbeusedasadivide-by-2deviceifonlythesingleFFisused.•Itcanbeusedasamodulus-8counterifonlythe3-bitcounterportionisused.74LS93A:4-BitAsynchronousBinaryCounter•Itcanbeusedasamodulus-16counterbyconnectingtheQ0outputtotheCLKBinput.•Itcanbeusedasadecadecounterbyusingthegatedresetinputsforpartialdecodingofcountten(1010).9-2SYNCHRONOUSCOUNTEROPERATION•SynchronousmeansthatalltheFFsinthecounterareclockedatthesametimebyacommonclockpulse.•FF0isalwaysconnectedfortoggleoperation.•WhentheleadingedgeofCLKoccurs,FF1togglesifQ0is1,otherwiseitremainsinitspresentstate.•ThereisapropagationdelayfromthetriggeringedgeoftheclockpulseuntiltheQoutputactuallymakesatransition.No–changeconditionToggleconditionA3-BitSynchronousBinaryCounter1022011001QQKJQKJKJ•FF0isheldinthetogglemodebyconstantHIGHsonitsdatainputs.•WhenQ0isa1FF1isinthetogglemode.•WhenBothQ0andQ1areHIGH,FF2isinthetogglemode.A3-BitSynchronousBinaryCounter1022011001QQKJQKJKJ•FF0isheldinthetogglemodebyconstantHIGHsonitsdatainputs.•WhenQ0isa1FF1isinthetogglemode.•WhenBothQ0andQ1areHIGH,FF2isinthetogglemode.A4-BitSynchronousBinaryCounter210331022011001QQQKJQQKJQKJKJ•FF0isheldinthetogglemodebyconstantHIGHsonitsdatainputs.•WhenQ0isa1FF1isinthetogglemode.•WhenBothQ0andQ1areHIGH,FF2isinthetogglemode.•WhenQ0,Q1andQ2areallHIGH,FF3isinthetogglemode.A4-BitSynchronousBinaryCounterASynchronousDecadeCounter302103310223011001QQQQQKJQQKJQQKJKJASynchronousDecadeCounter302103310223011001QQQQQKJQQKJQQKJKJ74HC163:4-BitSynchronousBinaryCounterLOAD=ParallelloadEP,ET=EnablecountRCO=RippleclockoutputTC=Terminalcount=1111FunctionTablefor74HC163TimingExamplefora74HC16374HC160:SynchronousBCDDecadeCounterLOAD=ParallelloadEP,ET=EnablecountRCO=RippleclockoutputTC=Terminalcount=1001FunctionTablefor74HC160TimingExamplefora74HC160ICCounterswithTruncatedSequences•Basically,onlymodulus-10andmodulus-16ICcountersaremanufactured.Wecanconvertmodulus-NICcounterintomodulus-Mcounterbyadditionallogic,whereMcanbeanynumberlessthanN.•Forexample,acertainapplicationrequiresamodulus-12counter.Thedifferencebetween16and12is4,whichisthenumberofstatesthatmustbedeletedfromthefull-modulussequence.Weuseatechniquethatistopresetthemodulus-16counterto4eachtimeitrecycles,sothatitwillcounterfrom4to15oneachcycle.Soeachfullcycleofthecounterconsistsof12states.ICCounterswithTruncatedSequences9-3UP/DOWNSYNCHRONOUSCOUNTERSAnup/downcounterisonethatiscapableofprogressingineitherdirectionthroughacertainsequence.Anup/downcounter,sometimescalledabidirectionalcounter,canhaveanyspecifiedsequenceofstates.A3-bitUp/DownCounterDOWNQQUPQQKJDOWNQUPQKJKJ