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HPRI/BIN101112131234APPENDIX≥10/Z101/Z112/Z123/Z134/Z145/Z156/Z167/Z171011121314151617A18α15145V18ENα1α2α4α976IEEESTANDARDSYMBOLSTogetherwiththeAmericanNationalStandardsInstitute(ANSI),theInstituteofElectricalandElectronicEngineers(IEEE)hasdevelopedastandardsetoflogicsymbols.ThemostrecentrevisionofthestandardisANSI/IEEEStd91-1984,IEEEStandardGraphicSymbolsforLogicFunctions.Itiscompatiblewithstandard617oftheInternationalElectrotechnicalCommission(IEC),andmustbeusedinalllogicdiagramsdrawnfortheU.S.DepartmentofDefense.A.1

GENERALDEFINITIONS

ANSIIEEE

TheIEEEstandardsupportsthenotionofbubble-to-bubblelogicdesignwiththefollowingdeﬁnitions:

•Aninternallogicstateisalogicstateassumedtoexistinsideasymboloutlineataninputoranoutput.•Anexternallogicstateisalogicstateassumedtoexistoutsideasymboloutlineeither(1)onaninputlinepriortoanyexternalqualifyingsymbolatthatinput,or(2)onanoutputlinebeyondanyexternalqualifyingsymbolatthatoutput.

internallogicstateexternallogicstate

791

792IEEESTANDARDSYMBOLSAPP.A

qualifyingsymbol

internal1-stateinternal0-state

Aqualifyingsymbolisgraphicsortextaddedtothebasicoutlineofadevice’slogicsymboltodescribethephysicalorlogicalcharacteristicsofthedevice.The“externalqualifyingsymbol”mentionedaboveistypicallyaninversionbubble,whichdenotesa“negated”inputoroutput,forwhichtheexternal0-statecorre-spondstotheinternal1-state.ThisconceptisillustratedinFigureA–1.Whenthestandardsaysthatasignalisinitsinternal1-state,wewouldsaythatthesignalisasserted.Likewise,whenthestandardsaysthatasignalisinitsinternal0-state,wewouldsaythatthesignalisnegated.

FigureA–1

Internalandexternallogicstates.

externallogicstates

internallogicstatesexternallogic stateexternallogicstatesinternallogicstatesexternallogic statedistinctive-shapesymbols

rectangular-shapesymbols

TheIEEEstandardprovidestwodifferenttypesofsymbolsforlogicgates.Onetype,calleddistinctive-shapesymbols,iswhatwe’vebeenusingallalong.Theothertype,calledrectangular-shapesymbols,usesthesameshapeforallthegates,alongwithaninternallabeltoidentifythetypeofgate.FigureA–2comparesthetwotypes.AccordingtotheIEEEstandard,“thedistinctive-shapesymbolisnotpreferred.”Somepeoplethinkthisstatementmeansthatrectangular-shapesymbolsarepreferred.However,allthestandardreallysaysisthatitgivesnopreferencetodistinctive-shapesymbolscomparedtorectangular-shapesymbols.Ontheotherhand,sincemostdigitaldesigners,authors,andcomputer-aideddesignsystemspreferthedistinctive-shapesymbols,that’swhatweuseinthisbook.

BeforethepromulgationoftheIEEEstandard,logicsymbolsforlarger-scalelogicelementsweredrawninanadhocmanner;theonlystandardrulewastouserectangleswithinputsontheleftandoutputsontheright.Althoughthelogicsymbolmightcontainashortdescriptionoftheelement(e.g.,“3–8

ANOTHERKINDOFBUBBLE

Inadditiontothefamiliarbubble,theIEEEstandardalsoallowsanexternal,triangular“polaritysymbol”tobeusedtospecifyactive-lowinputsandoutputs,forwhichtheexternalLOWlevelcorrespondstotheinternal1-state.However,underapositive-logicconvention,thebubbleandthetriangularpolaritysymbolareequivalent,soweusethemoretraditionalbubbleinthisappendix.

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SEC.A.2DECODERS

&793

FigureA–2

Distinctive-andrectangular-shapelogicsymbols.

ANDNAND&BUFFER

1OR

≥1INVERTER

1NOR

≥1decoder,”“2–1multiplexer”),itwasusuallynecessarytorefertoaseparatetabletodeterminetheelement’slogicfunction.However,theIEEEstandardcontainsarichsetofconcepts,suchasbitgrouping,commoncontrolblocks,anddependencynotation,thatallowsomeorallofalarger-scalelogicelement’sfunctiontobedisplayedinthesymbolitself.We’llintroducetheseconceptsasappropriateaswecoverthesymbolsforvariouscategoriesofdevicesinthesectionsthatfollow.

A.2DECODERS

Chapter5used“traditional”logicsymbolsfordecodersandotherMSIlogicelements.Althoughtraditionalsymbolsshowtheactivelevelsoftheinputsandoutputs,theydonotindicatethelogicalfunctionofthedevice—youalreadyhavetoknowwhata74x138or74x139doeswhenyoureaditssymbol.

TheIEEEstandardforlogicsymbols,ontheotherhand,allowsadecoder’slogicfunctiontobedisplayedaspartofthesymbol,showninFigureA–3.Thesesymbolsuseseveralconceptsofthestandard:

•Internalqualifyingsymbols.Individualinputandoutputsignalsmaybelabeledwithqualifyingsymbolsinsidethelogic-symboloutlinetodescribethesignals’characteristics.Inthisbook,wecallsuchsymbolsqualifyinglabelsforshort.•Generalqualifyingsymbols.Thetopofalogicsymbolmaycontainanalphanumericlabeltodenotethegeneralfunctionperformedbythedevice.Decodersandencoders(calledcoders)usethegeneralqualifyingsymbolX/Ytoindicatethetypeofcodingperformed,whereXistheinputcode

internalqualifyingsymbol

qualifyinglabelgeneralqualifyingsymbolcoder

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794IEEESTANDARDSYMBOLS

(a)

74x138BIN / 1-OF-812151413121110971413APP.A

(b)

74x139BIN / 1-OF-412EN012301234567FigureA–3

IEEEstandardsymbolsforde-coders:(a)74x138;(b)74x139.

0123&EN45671223451132151211109ENandYistheoutputcode.Forexample,a3-to-8decodermaybelabeledBIN/1-OF-8.

internalvalue

•Internalvalues.Eachinputcombinationofacoderproducesan“internalvalue”thatisdisplayedbythecoder’soutputs.Theinternalvaluesfora3-to-8decoderare0–7.•Inputweights.Theinputsofacodermayhavequalifyinglabelsindicatingthenumericalweightsassociatedwiththoseinputs.Inthiscase,theinternalvalueatanytimeisthesumoftheweightsoftheassertedinputs.Theinputweightsfora3-to-8decoderare1,2,and4.•Outputvalues.Eachoutputmayhaveaqualifyinglabellistingtheinternalvaluesthatcausethatoutputtobeasserted.Inabinarydecoder,eachoutputisassertedforjustoneinternalvalue.•Enableinput.AnenableinputhasthequalifyinglabelENandpermitsactionwhenasserted.Whennegated,anenableinputimposestheexter-nalhigh-impedancestateonthree-stateoutputs,andthenegatedstateonotheroutputs.The74x138and74x139haveactive-lowoutputs,whicharetherefore1whentheenableinputisnegated.•Embeddedandabuttedelements.Theoutlinesofindividuallogicelementsmaybeembeddedorabuttedtoformalargercompositesymbol.Thereisatleastonecommonlogicconnectionwhenthedividinglinebetweentwooutlinesisperpendiculartothedirectionofsignalﬂow,asshowninFigureA–4.Forexample,the74x138symbolhasanembedded3-inputANDgatethatdrivestheinternalENinput.Thereisnoconnectionbetweentheelementswhenthedividinglineisinthedirectionofsignalﬂow,asshowninFigureA–5.Forexample,the74x139containstwoseparate2-to-4decoders.

DraftofJuly6,1999

inputweight

outputvalue

enableinput

embeddedelementabuttedelement

SEC.A.2DECODERS795

FigureA–4

Acompositesymbolwithoneormorelogicconnectionsbetweenitselements.

ORTheabilitytoembedindividuallogicelementsinalargersymbolisprobablythemostusefulfeatureoftheIEEEstandard.Forexample,FigureA–6showsthesymbolfora3-to-8decoderwithadifferentsetofenableinputsthanthe74x138.Theﬁctitious74x328decoderisenabledwhenpin6is0orbothpins4and5are1.

74x328

FigureA–5

Acompositesymbolwithno

connectionbetweenitselements.

FigureA–6

IEEEstandardsymbolforaﬁctitiousdecoder,the74x328.

174x328BIN / 1-OF-8012345671514131211109712234≥1&EN5Copyright

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796IEEESTANDARDSYMBOLS

1EN1EN1EN1ENAPP.A

(a)(b)(c)(d)

FigureA–7

IEEEstandardrectangular-shapesymbolsforthree-statebuffers:(a)noninverting,active-highenable;(b)noninverting,active-lowen-able;(c)inverting,active-highenable;(d)inverting,active-lowenable.

A.3THREE-STATEBUFFERS

Three-state-buffersymbolsuseonemorefeatureoftheIEEEstandard:

downward-pointingtriangle

•Downward-pointingtriangle.Thisdenotesathree-stateoutput.

Alsorecallthatanenableinput,labeledEN,isspeciﬁcallyunderstoodtoforceallaffectedoutputstoadisabledstate.Forthree-stateoutputs,thedisabledstateisHi-Z.Thus,thethree-statebuffersofFigure5–35aredrawnasshowninFigureA–7.AnotherimportantfeatureoftheIEEEstandardisintroducedinthesymbolsforMSIthree-statebuffers:

•Commoncontrolblock.Thisconcept,illustratedinFigureA–8,maybeusedwithanarrayofrelatedelements.Inputstothecommoncontrolblockareunderstoodtoaffectalltheelementsofthearray.

commoncontrolblock

FigureA–8

CommoncontrolblockinanIEEEstandardsymbol.

Thus,symbolsmaybedrawnforthe74x1and74x245asshowninFigureA–9.Theenableanddirectioninputs(pins1and19)applytoallelementsofthedevice.The’1and’245symbolsintroduceseveralotherfeaturesofthestandard:

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SEC.A.3THREE-STATEBUFFERS

74x24574x1119119797

FigureA–9

IEEEstandardlogicsymbolsforthe74x1and74x245.

&ENG33EN13EN21218223456718171615141312113456717161514131211•Hysteresissymbol.Inputsbearingthissymbolhavehysteresis.

•Right-pointingorleft-pointingtriangle.Thesesymbolsareusedtodenote“ampliﬁcation”;inthecaseofthree-statebuffers,theyindicateanoutputcircuitthathasmorefanoutandcandriveaheavierloadthananordinaryoutputcircuit.•Arrows.Thesedenotethedirectionofsignalﬂowwhenitisnotstrictlylefttoright,asinthe’245symbol.•Identicalelements.Onlytheﬁrstoftwoormoreidenticalelementsinanarraymustbedrawnindetail.Thus,thebottomsevenelementsofthe’245symbolareunderstoodtobeidenticaltothetopelement(whichinthiscasehappenstobedividedintotwosubelements,thethree-statebuffersforthetwodirections).The’245symbolalsointroducesdependencynotation,ameansofdisplay-ingsomeofthemorecommonlogicalrelationshipsamonginputandoutputsignals.Afewmoreconceptsareneededtomakethisnotationﬂy:

•Affectingsignals.Aninputsignal(or,occasionally,anoutputsignal)mayaffectotherinputsoroutputsinawaythatcanbedisplayedonthesymbol.SuchasignalhasaqualifyinglabelLi,whereLisaletterthatindicatesthetypeofrelationshiporeffect,andiisanintegerthatidentiﬁestheaffectedsignals.The’245internalsignallabeledG3issuchasignal.•Affectedsignals.InputsoroutputsthatareaffectedbyasignalLibearthe

hysteresissymbolright-pointingtriangleleft-pointingtrianglearrows

identicalelements

dependencynotation

affectingsignal

affectedsignal

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798IEEESTANDARDSYMBOLSAPP.A

WHOOPS!

Thediscussionof“affectedsignals”said“G3”whenitshouldhavesaid“thesignallabeledG3.”IntheIEEEstandard,G3isaqualifyinglabel,andothersignalsmayhavethesamelabel.Thestandarddoesnotspecifyuniquenamesforinternalandexternalsignals.However,intherestofthisappendix,we’lltakethelibertyofusinganinternalsignal’squalifyinglabelasitsnameifnoambiguityresults.

qualifyinglabeli.Ifanaffectedsignalrequiresaqualifyinglabelofitsown,theniisusedasapreﬁxtothatlabel.Thus,thesignallabeled3EN1inthe’245symbolaffectssignalslabeled1,andisitselfaffectedbyG3.

ANDdependencyGi•ANDdependency.ThisrelationshipisdenotedbyGi,andisasortofenablefunction.Affectedsignalsperformtheir“normal”functionsonlyifGiisasserted;otherwise,theyarenegated.Inthe’245symbol,inputs3EN1and3EN2can“dotheirthing”onlyifG3isasserted.FigureA–10showsanequivalentnotationforthedependentsignals.Noticethatdependencyisdeﬁnedintermsofinternal,notexternal,signalvalues.

G33 EN13 EN2ab&&EN1EN2FigureA–10

IllustrationofANDdependency.

enabledependency

ENi•Enabledependency.ThisdependencyisdenotedbyENi,andhasthesameeffectasENinputsdeﬁnedearlier.Thus,inthe’245symbol,theinternalsignal3EN1,whichisassertedonlyifpins1and19areLOW,enablesthethree-statedriversforpins2through9.Whewww!Youmightbethinkingthatthisisalotoftroubleforafewlousythree-statebuffers,butjustwaituntilyouseesomeoftheIEEE-standardcounterandshift-registersymbolslaterinthisappendix!A.4

PRIORITYENCODERS

FigureA–11showstheIEEEstandardsymbolfora74x148.Stillmorefeaturesofthestandardareusedinthissymbol:

solidus

•Solidus.Theslashinalabelsuchas0/Z10,calledasolidusinthestan-dard,separatesmultiplefunctionsofasingleinternalsignal.AnequivalentnotationisshowninFigureA–12.

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SEC.A.4PRIORITYENCODERS

74x148HPRI/BIN101112131234799

FigureA–11

IEEEstandardlogicsymbolforthe74x1488-inputpriorityencoder.

0/Z101/Z112/Z123/Z134/Z145/Z156/Z167/Z171011121314151617≥118α15141α5976V18ENα2α4α•ORdependency.ThisrelationshipisdenotedbyVi;theVisignalisOR’edwithaffectedsignals.Thus,affectedsignalsperformtheir“normal”func-tionsonlyiftheVisignalisnegated;otherwise,theyareasserted.•Virtualinputsandoutputs.Theseareinternalsignals,denotedbyahorizon-tallinegoingnowhere,suchastheoneslabeled10–17inthe’148symbol.Thesesignalshavenoexternalconnectionbutaffectorareaffectedbyothersignalsviadependencynotation.•Interconnectiondependency.ThisrelationshipisdenotedbyZi,andindi-catesaninternalconnection.AffectedsignalsequaltheZisignal,unlessmodiﬁedbyadditionaldependencynotation.ThinkoftheZasazig-zaginternalwire.•Greekletters.AGreeklettermaybeusedinsteadofanintegerinqualifyinglabelstoavoidambiguitywhentheaffectedsignalshaveanumericfunctionlabel,asintheinputsoroutputsofacoder.Ifyouunderstandtheseandthepreviouslyintroducedfeaturesofthestandard,youcan“read”thefunctionalbehaviorofa’148rightfromitssymbol.However,mostpeopledotheopposite—alreadyknowinghowa’148works,theytrytodeducehowthestandardworksfromthe’148symbol!

FigureA–12

Equivalentnotationforsolidi.

0Z10ORdependencyVivirtualinputvirtualoutput

interconnectiondependency

ZiGreekletters

a0 / Z10a

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800IEEESTANDARDSYMBOLSAPP.A

A.5MULTIPLEXERSANDDEMULTIPLEXERS

bit-groupingsymbolinternalvalue

rangenotation

TheIEEEstandardprovidesaspecialnotationformultiplexersanddemultiplex-ers.Forexample,FigureA–13showstheIEEEsymbolsformultiplexerICsthatwediscussedinChapter5.ThegeneralqualifyingsymbolMUXidentiﬁesamultiplexer.Thebracketiscalledabit-groupingsymbolandindicatesthatthegroupedinputsproduceaninternalvaluethatisaweightedsum.Theweightsaregivenbythequalifyinglabelsontheindividualinputs;iftheweightsareallpowersof2,theymaybereplacedbythecorrespondingexponents,andallbuttheﬁrstandlastexponentsmaybeomitted“ifnoconfusionislikely.”Thus,theweightsofpins9,10,and11ofthe74x151are22,21,and20,respectively;iftheinputsignalonthesepinsis110,theinternalvalueis6.

Theinternalvalueproducedbybitgroupingaffectsotherinternalvaluesoroutputsaccordingtothequalifyinglabelwrittentotherightofthegroupingsymbol.Inthe74x151multiplexersymbol,thenotationG0indicatesAND

dependencywithsignalswhoselabelsareintherange0–7.Inotherwords,inputiisselected(andtransferredtotheoutput)ifandonlyiftheinternalvalueisi.Therearetwooutputs,normallyequaltotheselectedinputanditscomplement.However,theseoutputsareassertedonlyiftheenableinputENisasserted.

The74x153symbolalsousesbitgroupingfortheselectinputs,anditusesacommoncontrolblocktoindicatethattheselectinputsaffectbothsectionsof

74x1531474x151MUX711109432115141312201G0374x157151EN0G20123456707165MUXEN07ENG123565615101423311MUX47119101413111213912FigureA–13IEEEstandardsymbolsformultiplexers.

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SEC.A.5MULTIPLEXERSANDDEMULTIPLEXERS801

themultiplexer.Sincethebottomhalfofthemultiplexerisidenticalinfunctiontothetop,itsqualifyinglabelsarenotrepeated.NoticethateachhalfhasanindependentENinput.

Thesymbolforthe74x157doesnotusebitgrouping,becauseithasonlyoneselectinput.Instead,theselectinputislabeledG1,indicatingthatithasanANDdependencywithsignalsbearingtheidentiﬁer“1.”Thus,pin3isselectedonlyifG1isasserted.Pin2,ontheotherhand,bearstheidentiﬁer“1”;theoverbarindicatesthatpin2isselectedonlyifG1isnegated.AllfoursectionsarecontrolledbyG1inthisway.

FigureA–14

IEEEstandardsymbolsfordemultiplexers.

74x13974x138DMUX123215141312111097151413174x15513301G03DMUX0310G30123456721DMUXG440123760G2&07012345671211109514159101112FigureA–14showstheIEEEstandarddemultiplexersymbolsfortheMSIdecoder/demultiplexerICsthatwediscussedinChapter5.Thegeneralquali-fyingsymbolDMUXidentiﬁesademultiplexer.ThenotationG0indicatesANDdependencywithoutputswhoselabelsareintherange0–7.Inotherwords,outputimaybeassertedonlyiftheinternalvalueisi.Inaddition,alloftheotherinputs(pins4–6)mustbeasserted.

Asimilarnotationisusedinthe74x139,whichcontainstwoindependentdemultiplexers.Sincetheseconddemultiplexerisidenticalinfunctiontotheﬁrst,thequalifyinglabelsarenotrepeated.

Thesymbolforthe74x155usesacommoncontrolblocktoshowthatthesameselectinputs(andinternalvalueproducedbybitgrouping)areusedforbothsectionsofthedemultiplexer.Also,theinputlabeled“4”hasanANDdependencyontheinputlabeled“G4,”sotheselectedoutputisassertedonlyifbothoftheseinputsareasserted.Now,isthatallclear?

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802IEEESTANDARDSYMBOLSAPP.A

A.6EXCLUSIVE-ORANDPARITYFUNCTIONS

TheIEEEstandardsymbolsforEXCLUSIVEORandEXCLUSIVENORgatesareshowninFigureA–15.Eitheroftwodifferentnotationsmaybeused,dependingonhowyou’rethinkingaboutthegate’sfunction.Thetopsymbols,with“=1”insidethesymboloutline,asserttheiroutputswhenexactlyoneoftheinputsisasserted.Thebottomsymbols,with“=”insidethesymboloutline,asserttheiroutputswhentheirinputsareequal.

(a)

=1=1(b)=1=1====FigureA–15

IEEEstandardsymbols:(a)EXCLUSIVEORgates;(b)EXCLUSIVENORgates.

TheIEEEstandardsymbolfor74x2809-bitparitygeneratorisshowninFigureA–16(a).Thegeneralqualifyingsymbol“2k”atthetopofthesymbolindicatesthattheoutputsareassertedif2koftheinputsareassertedforsomeintegerk.Thus,pin5,anactive-highoutput,andpin6,anactive-lowoutput,bothindicateevenparity.Thenatureofthefunction,ofcourse,issuchthatpin6couldinsteadbeviewedasanactive-highoutputthatdenotesoddparity.Thestandardsymbolforthisinterpretationofthedevicefunctionisshownin(b).

FigureA–16

IEEEstandardsymbolsforthe74x2809-bitparitygenerator:(a)normalsymbol;(b)bothoutputsactivehigh.

74x280(a)

101112131246574x280(b)

10111213124[ODD]62k2k[EVEN]5Copyright

DraftofJuly6,1999

SEC.A.7COMPARATORS803

A.7COMPARATORS

IEEEstandardsymbolsforMSIcomparatorsareshowninFigureA–17.Liketheselectinputsofmultiplexers,thedatainputshavequalifyinglabelsindicatingtheirarithmeticweightsinpowersof2(0–3correspondingto20–23inthe’85).Thecascadinginputsandoutputsarelabeledwiththeappropriatearithmeticfunction.Inthe’682symbol,theright-pointingtriangleindicatesthattheoutputshavehighfanoutcapability,andhysteresisisindicatedonthedatainputs.

FigureA–17

IEEEstandardsymbolsforMSIcomparators.

74x85COMP

1012131523

74x682COMP20

P3<

468111315170=4>

911141

P=Q6

PP=Q70P>Q190

35791214161811Q7A.8ADDERS

TheIEEEstandardusesthegeneralqualifyingsymbol“Σ”toidentifyanadderoradditionfunction.FigureA–18(a)showsthesymbolfora74x2834-bitadder.Thenumbersontheaddendinputsandsumoutputindicatetheweightofeachpin,asapowerof2.

TheIEEEsymbolfora74x1814-bitALUisshowninFigureA–18(b).Theﬁrstﬁveinputsofthecommoncontrolblockforma“modecontrol”word,whichthestandarddesignatesbytheletterM.Theweightsofthemodecontrolbitsareshownaspowersof2,andtheydesignateamodenumberintherange0–31.Accordingtothestandard,aseparatetableaccompaniesthelogicsymboltodeﬁnethefunctionsperformedineachmode.TheCP,CG,andCOoutputsareenabledinmodes0–15.ThefourindividualALUblocksarelabeledwith

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804IEEESTANDARDSYMBOLSAPP.A

theweightsofthebitstheyprocess.TheIEEEsymbolforthe74x182carrylookaheadcircuitismuchsimpler,andisshownin(c).

FigureA–18IEEEstandardsymbols:(a)74x283;(b)74x181;(c)74x182.74x283ΣP30Q3CICO974x1816380M031ALU(0 . . . 15)CP15175314126215117047CI04121232221201918(0 . . . 15)CG146 (P=Q) CO16(0 . . . 15)13CICG0CG1CG2CG3CP0CP1CP2CP374x182CPG3PQPQPQPQΣ31310[1][2][4][8]91145CO011CO19CO212101142156CGCP10713(a)(b)(c)

A.9LATCHES,FLIP-FLOPS,ANDREGISTERS

S(set)R(reset)

ThereareafewdifferencesbetweentraditionalandIEEEsymbolsforlatchesandﬂip-ﬂops.FigureA–19showsIEEEstandardsymbolsforSSIlatchesandﬂip-ﬂops.Amajordifferenceisthattheasynchronouspresetandclearinputsaredrawnontheleft,notonthetopandbottom.Thenamesfortheseinputsaredifferent,too:S(set)andR(reset).Also,aclockinputissimplynamedCi,whereiisanintegerandallotherinputslabeledwithi(e.g.,iD)arecontrolledbyCi;thisisjustanotherinstanceofdependencynotation:

•Controldependency.ThisrelationshipisdenotedbyCiand,likeenabledependency,isasortofenablefunction.Itisintendedtobeusedonlyforclockortiminginputs.AffectedsignalsareenabledwhentheCiinputisintheinternal1-stateor,iftheCiinputhasadynamicindicator,onthe0-to-1changeininternalstate.Thesymbolsforthe’74,’109,and’112followtheusualIEEEconventionthatonlytheﬁrstoftwoormoreidenticalelementsmustbedrawnindetailwhentheyareabuttedinanarray.Alternatively,thetwoindependentsectionsofeachSSIpackageinFigureA–19maybedrawnseparately,asareothertraditionalandIEEEsymbolsfordeviceswithindependentsections,suchasthe74x139.

controldependency

CiCopyright

DraftofJuly6,1999

SEC.A.9LATCHES,FLIP-FLOPS,ANDREGISTERS

74x1095805

FigureA–19

IEEEstandardsymbolsforSSIlatchesandﬂip-ﬂops.

41474x1124631721574x74SC11DR9524361S1JC11KRS1JC11KR574x3751D3256111013143216C1C272D911141281316910101113121479101112133D12C3C41DThisisespeciallyusefulifthelogicfunctionsperformedbythetwosectionsinaparticularapplicationareunrelated.

FigureA–20showstheIEEEstandardsymbolsforMSIlatchesandreg-isters.AllofthesesymbolsmakegooduseoftheIEEEnotationforcommoncontrolblocks.Inthe’373and’374symbols,thelabelC1ontheclockinputin-dicatesthatitcontrolsallinputsthatbearthelabel1,thatis,allofthe1Dinputs.Thedownward-pointingtrianglesindicatethree-stateoutputs;byconvention,aninputlabeledENenablessuchoutputs.

Inthe’377symbol,theinputlabeledG1isanenableforinputsbearingthelabel1,thatis,fortheclockinput1C2.Theclockinputinturncontrolsalloftheinputsbearingthelabel2,thatis,datainputs2D.Asusual,onlytheﬁrstoftheeightidenticalﬂip-ﬂopelementsisdrawnindetail.

74x37311174x37411174x377111C1EN1D256912151619C1EN1D256912151619G11C234783478131417183478131417182D25691215161913141718FigureA–20IEEEstandardsymbolsforMSIlatchesandregisters.

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806IEEESTANDARDSYMBOLSAPP.A

A.10COUNTERS

IEEEstandardsymbolsforpopularcountersareshowninFigureA–21.LiketheIEEEsymbolsforotherMSIdevices,thecountersymbolsmakegooduseofthecommon-control-blockandarrayfeaturesofthestandard.ThegeneralqualifyingsymbolCTRDIV16indicatesthatthedeviceisadivide-by-16counter,andlabels[1]–[8]indicatethearithmeticweightofeachcounterbit.However,theadditionalnotationusedwithinthecommoncontrolblockstodescribethedevices’functions,thoughprecise,ishardlyintuitive.We’llhavetodescribeafewmorefeaturesofthestandardtounderstandthesesymbols:

contentinputCT=minput

•Contentinput.WhenaninputsignalbearingthelabelCT=misasserted,thevaluemisloadedintothedevice.Inthecountersymbols,youmightread“CT”as“count,”butingeneralitmeans“content.”Theonlydifferencebetweenthe’161and’163symbolsinFigureA–21isthatthe163’s5CT=0hasacontroldependencyontheclockinput(C5),andisthereforeasynchronousclear;the’161hasanasynchronousclear.

contentoutputCT=moutput•Contentoutput.AnoutputbearingthelabelCT=misassertedwhenthecontentofthedeviceism.Inthe’161and’163symbols,theoutput3CT=15isassertedwhenthecounterisinstate15andG3isasserted.

modedependency

Mi•Modedependency.ThisdependencyisdenotedbyMiand,likeenabledependency,isasortofenablefunction.Affectedsignalsperformtheir

74x16974x16174x163CTRDIV165CT=09M1M215103CT=15G37G42C5/2,3,4+134569CTRDIV16CT=09M1M215103CT=15G37G42C5/2,3,4+1345611072CTRDIV16M1 [LOAD]M2 [COUNT]M3 [UP]M4 [DOWN]G53,5CT=15G,5CT=02,3,5,6+/C72,4,5,6–3456151,5D[1][2][4][8]141312111,5D[1][2][4][8]141312111,7D[1][2][4][8]14131211FigureA–21IEEEstandardsymbolsforcounters.

DraftofJuly6,1999

SEC.A.10“normal”functionsonlyifMiisasserted;otherwise,theaffectedsignalshavenoeffectonthedevice’sfunctionandareignored.

•Multipledependencies.Asignalmaybeaffectedbyseveralothersignals.Theidentiﬁersoftheaffectingsignalsarelisted,separatedbycommas,inthequalifyinglabeloftheaffectedsignal.Thedependenciesareappliedintheorderthattheyarewritten,fromlefttoright.Ifallofthedependenciesareofthesametype(e.g.,ANDdependency),thentheorderisirrelevant.•Multifunctionoutputs.Asingleexternaloutputsignal,suchaspin15inthe’169symbol,mayhaveseveralsetsofqualifyinglabelscorrespondingtomultiplemodesofoperation.Suchasignalmayberepresentedbymultipleoutputsthatareconnectedtogetherexternally.SuchoutputsnormallyhaveafunctionalORrelationship—theexternalsignalisassertedifanyinternalsignalisasserted.Inthe’161and’163symbols,thelabel1,5DindicatesthatthedatawillbestoredwhenM1andC5areasserted(butC5is“asserted”onlyonanedge,becauseofitsdynamicindicator).Inthe’169symbol,theoutput3,5CT=15isassertedifM3andG5areassertedandthecounterisinstate15;and4,5CT=0isassertedifM4andG5areassertedandthecounterisinstate0;theexternalsignal(pin15)isasserted(LOW)ifeitheroftheseinternalsignalsisasserted.

•Countinginputs.Whenasserted,aninputlabeledwitha+causesthedevicetocountuponce.Aninputlabeledwitha–causesthedevicetocountdownonce.Accordingtothe’161and’163symbols,thedevicecountsupontherisingedgeofthesignalonpin2ifM2,G3,andG4areasserted.The’169countsupifM2,M3,G5,andG6areasserted,anddownifM2,M4,G5,andG6areasserted.Ineachdevice,thequalifyinglabelsfortwoseparatefunctionsofpin2—loadandcountup—aredrawnonthesameinputline,separatedbyasolidus;theycouldalsohavebeendrawnontwoseparateinputlines,asweshowedinFigureA–12onpage799.

•Nonstandardizedinformation.Descriptivefunctionnamesandothernon-standardized(i.e.,helpful)informationmaybewritteninbracketsnexttothequalifyinglabelsinasymbol.The’169symbolhasfour“nonstandard”labelstodescribethetraditional/LDand/ENPinputs.Theoretically,youdon’tneedsuchlabelsifyouunderstandthestandard.Conversely,ifsignalsaregivenmeaningfulnames(asintraditionallogicsymbols),thenyoudon’tneedthestandard!

COUNTERS807

multiple

dependencies

multifunctionoutput

countinginput

+–nonstandardizedinformation

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808IEEESTANDARDSYMBOLSAPP.A

A.11SHIFTREGISTERS

FigureA–22showstheIEEEstandardsymbolsforpopularshiftregisters.ThegeneralqualifyingsymbolSRGndenotesann-bitshiftregister.We’veusedmostoftheothernotationinthesesymbolspreviously;there’sjustonenewthing:

shiftinginput→←

•Shiftinginputs.Whenasserted,aninputlabeledwitha→causesthedevicetoshiftitsinformationonepositioninthedirectionfromlefttorightorfromtoptobottom.Aninputlabeledwitha←causesashiftintheoppositedirection.

74x2999231FigureA–22

IEEEstandardsymbolsforshiftregisters.

SRG8R&3EN1374x16691912SRG8RM1 [SHIFT]M2 [LOAD]≥1C3/1→1274x1981574x194SRG4191011700M13C4/1→/2←1,4D3,4D5,133,4D6,138SRG8RC1/→1267R01M&1D3456101112131,3D2,3D32,3D451011121413110313Z5Z6C41→/2←2345676141,4D3,4D3,4D3,4D3,4D2,4D1514131251163,4D12,132,4DZ121718A.12PROSANDCONSOFIEEESTANDARDSYMBOLS

Theabsenceofastandardizedmethodofgivinguniquenamestopinsanddis-playingthenamesonthelogicsymbolisprobablythebiggestdefectoftheIEEEstandard.Forexample,howdowenametheinternalsignalsonpins4,5,and6ofthe74x328inFigureA–6?Worse,howcanwedistinguishbetweentheinternalsignalsonpins3and11,whicharebothlabeled“4”onthelogicsymbol?Apparently,thestandardmakersexpectedustorefertothesesignalsbypinnumberonly.Butthisisawkwardandinconvenient,notonlyintextbooks,

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SEC.A.12PROSANDCONSOFIEEESTANDARDSYMBOLS809

butalsoindesignanddebugging.Itisfarmorenaturaltorefertoasignalbyafunctionalnamethanbyapinnumber.(Indeed,inASICdesigntherearenopinnumbersforinternalsignals!)

Still,thestandardhasseveralstrengths:•It’sastandard.•It’sconsistent.

•Itsupports,indeed,deﬁnes,thesymbologyusedinbubble-to-bubbledesign.•Itisverycomplete.Inadditiontothebasicdevicescoveredinthisbook,thefullIEEEstandardcoversmanyotherlesscommonlyuseddevicesandstructures.•Inmostcases,thestandardallowsthefunctionofalogicdevicetobedeﬁnedunambiguouslybythedevicesymbol.Forexample,the’299sym-bolinFigureA–22conveysasmuchinformationasthefunctiontableinTable9–3,andmore.That’sthegoodnews;nowhere’smorebadnews,ifyouhadn’talreadynoticed:•Althoughthestandardallowsyoutoﬁgureout,withmoderateeffort,theprecisefunctionofanunfamiliarsymbol,itdoesnotprovideanystandardwaytoremindyouquicklyofthefunctionofafamiliarsymbol.Thatis,itdoesnotprovideanystandard,descriptivenamesforinternalsignals.Suchitemsarerelegatedtothestatusof“nonstandardinformation,”andtheiruseisnotencouraged.•ManysignalsinIEEEsymbolshavenoqualifyinglabels,whileothershaveduplicates.Thismakesitawkwardtotalkaboutsignalsduringdesignanddebugging(e.g.,“ConnectXtothethirdbitupfromthebottom...”).•Thestandardencouragestheomissionofqualifyinglabels“ifnoconfusionwillresult.”That’slikesaying“Don’tuseyourturnsignalifnobody’snearbytoseeit.”It’spreciselywhensomeoneisdrivinginyourblindspotthatthehabitofalwaysusingyourturnsignalcanavertanaccident.IEEEsymbolshavelotsof“blind”inputsandoutputsthatmaybehookedupincorrectlywhiledrawingaschematic,ormisappliedduringdebugging.•Manypossiblesymbolsexistformoderatelycomplexdevices.Forexample,inthe’194symbolinFigureA–22,pin11couldbehandledasshown,orcouldbedrawnasasingleinputlinewiththelabelC4/1→/2←orasthreelineswiththelabelsC4,1→,and2←.Ifused,thesevariationsmakeitevenmoredifﬁcultto“eyeball”thesymbolforafamiliardeviceandrecognizeitsfunctions.

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810IEEESTANDARDSYMBOLSAPP.A

Whatlogicdesignersreallyneedisastandardsetofdescriptive,functional,namingconventionsfortheinputsandoutputsofMSIandLSIdevices,onethatisconsistentlyfollowedbyalldatabooksandCADsystems.Forexample,it’sridiculousthatinthepresentenvironment,asinglemanufacturercanproduce2-,4-,and8-inputmultiplexerswithdescriptiveinputnamesrangingfromA–BtoC0–C3toD0–D7!Unfortunately,theindustryhastoomuchinvestedindocumen-tationandCADsoftwareforbothinconsistenttraditionalsymbolsandunhelpfulIEEEstandardsymbols,foraconsistent,helpfulnamingstandardtobedeployedanytimesoon.Atleast,today’sASICdesignerscanworktoensureamodicumofnamingconsistencyandhelpfulnessinthelogic“macrocells”thattheydeﬁneanduseintheirowndesigns.

EXERCISES

A.1DrawandexplaintheIEEEstandardsymbolfora74x49seven-segmentdecoder.A.2DrawtheIEEEstandardsymbolforthedeviceinExercise5.43.

A.3ExplainallofthenotationusedintheIEEEstandardsymbolfora74x148.(Some

researchisrequiredtoanswerthisone.)A.4DrawtheIEEEstandardsymbolforthe74x155usedasadual2-to-4decoder.A.5DrawtheIEEEstandardsymbolfora74x251multiplexer.A.6DrawtheIEEEstandardsymbolforthecircuitinExercise5.56.

A.7Howdoesthemeaningofthelabel1DdifferbetweenthetraditionalandtheIEEE

symbolsforregisterslikethe74x374?(Hint:Howmightpin4ofa’374belabeledintheIEEEstandard?)A.8DrawanIEEE-standardsymbolforthemodiﬁedmultiplexerofFigure10–15.

DraftofJuly6,1999

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