Controlling Peak Power During Scan Testing - CiteSeerX

ControllingPeak Power DuringScan Testing Ranganathan Sankaralingam andNurA.Touba ComputerEngineeringResearch Center Departmentof Electrical andComputerEngineering University oTexas, f Austin,TX 78712 Abstract Thispaperpresentsaprocedureformodifyinga givensetofscanvectorssothatthepeakpower duringscantestingiskeptbelowaspecifiedlimit withoutreducingfaultcoverage.Theproposed approachworksforanyconventionalfull-scandesign --noextradesign-for-test(DFT)logicirsequired. If thepeakpowerinaclockcycleduringscantesting exceedsaspecifiedlimit(whichdependsonthe amountofpeakpowerthatcanbesafelyhandled withoutcausingafailurethatwouldnotoccurduring normalfunctionaloperation)thena"peakpower violation”occurs.Givenasetofscanvectors, simulationisdonetoidentifyandclassifythescan vectorsthatarecausingpeakpowerviolationsduring scantesting.Theproblemscanvectorsarethen modifiedinawaythateliminatesthepeakpower violations while preserving the fault coverage. Experimentalresultsindicatetheproposedprocedure isveryeffectiveincontrollingpeakpower.

Introduction 1. Thepeakpowerdrawninasingleclockcycle duringscantestingcanbemuchlargerthanduring normaloperation. Duringnormaloperation,typically arelativelysmallpercentageotfheflip-flopschange valueineachclockcycle. However,whenscanningin testvectors,typicallyamuchlargerpercentageothe f flip-flopswillchangevalueineachclockcycle resultinginexcessiveswitchingactivityinthecircuit Ifalargenumberoflip-flopsswitchsimultaneously inaclockcycle,thisresultsinalargecurrentspike. Achip’spowersupply/groundpinsanddistribution systemmaybedesignedforhandlingthepeakpower duringnormaloperation,anditmaynotbeableto handlethelargepeakpowerthatcouldoccurduring scantesting. If the peakpowerduringtestistoo large, thentherewillbeaV drop/ground bouncethatmay dd causeproblems(e.g.,memoryelementstolosetheir stateopr hase-lockloop(PLL)tomalfunction). With therapidincreaseinchipcomplexity,theproblemof excessivepeakpowerduringscantestingibsecoming animportantissue inindustry. Theaveragepowerdissipationduringscantesting can be controlled byreducing thescan clock

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frequency,however,thepeakpowerduringscan testingiisndependentofclockfrequencyandhenceis muchmoredifficulttocontrol. Moreover,controlling peakpowerrequiresensuringthatthepeakpower dissipationinanysingleclockcycledoesnotexceed thecapabilitiesofthechip,whichismoredifficult thansimplyreducingtheaveragepowerdissipation perclockcycle overthe entire testsession. Previousworkinlow powerscantestinghasmostly focusedontheproblemofcontrollingheatdissipation byreducingtheaveragepowerdissipation[Chou94], [Wang94,97ab,99],[Dabholkar98],[Girard99a], [Sankaralingam00,01],[Chandra01]. Somedesignfor-test(DFT)techniquesreducepeakpowerin additiontoaveragepower.In[Hertwig98]and [Gerstendörfer99],logiciasddedthooldthe outputof thescancellsaact onstantvalueduringscanshifting therebyreducingpowerdissipation.Thisapproach greatlyreducesaveragepower,andwillavoidpeak powerproblemsduringscanshifting,butwillnothelp withpeakpowerproblemsduringthecapturecycle (whichcanariseithe f circuitisplacedinastatet hat itcouldnotbeinduringfunctionaloperation). A drawbackofthisapproachisthatidt egradescircuit performancebecauseitaddsextralogicinthe functionalpaths. In[Whetsel00],anadaptedscan chainarchitecturethatsegmentssingle a scanchaint o minimizeswitchingactivityduringscanshiftingis proposed. Thisapproachalsogreatlyreducesaverage powerandcanavoidpeakpowerproblemsduring scanshifting,butitwillnotavoidpeakpower problemsduringthecapturecycleandrequires additionalDFThardware. In[Lee 00],amethodfor addingdelayelementstointerleavethecapturecycles formultiplescanchainsipsroposedforreducingpeak powerduringscancapture.In[Girard99b],a techniqueforreducingpeakpowerduringbuilt-in self-test (BIST) is proposed which involvespartitioning acircuitandperformingseparateBISTsessionson eachpartition.In[Corno99],atechniquefor modifyingtestsequencesforsequentialnon-scan testingiproposed s forreducingpeakpower. In this paper, parocedure iproposed s formodifying agivensetofscanvectorssothatthepeakpower duringscantestingiskeptbelowaspecifiedlimit

benchmarkcircuit.Duringeachclockcycle,the numberogate f transitionsthatoccurwasweightedby thenumberofanouts f oeach f gate. TheplotinFig.1 showsthenumberow f eightedgatetransitionsinthe x-axis,andthenumberofclockcycleswiththat numberofweightedgatetransitionsinthey-axis. Notethatitlookslikeanormalcurvewhichisa common characteristic for most circuits. The maximumpeakpowerduringscantestingoccursat thetailendofthecurveandhenceiscausedbya relativelysmallnumberoclock f cycles. Althoughthe fractionofproblemclockcyclesisverysmall,these clockcyclesmaybelongtoalargefractionofthe vectorsinthetotaltestset.Henceifthesimple approachodropping f problemvectorsirsesortedto,a large decrease infaultcoverage may result.

whilemaintainingthesamefaultcoverage. Noextra DFThardwareisrequired. Theproposedapproach worksforanyconventionalfull-scandesign.The basicideaisthatgivenasetofscanvectorsthat resultsinpeakpowerviolationsduringscantesting, simulationcanbedonetoidentifywhichscanvectors arecausingtheproblem.Theproposedprocedure thenmodifiestheproblemscanvectorsbryeassigning inputvaluesinawaythatreducestheamountof switchingactivity duringscantestingwhile preserving thefaultcoverage. By sdooing,peakpowerviolations duringscantestingcanbeliminated.

2. IdentifyingandCategorizingPeak Power Violations Thepeakpowerineachclockcycleduringscan testingcanbeestimatedthroughcyclebycycle simulationoftheswitchingactivitythatoccursinth circuit. Ifthepeakpowerinaclockcycleexceedsa specifiedlimit(whichdependsontheamountofpeak powerthatcanbesafelyhandledwithoutcausinga failurethatwouldnotoccurduringnormalfunctional operation)thena“peakpowerviolation”occursin thatclockcycle. Duringscantesting,eachscanvecto isscannedintothescanchain(s)andthenthereisa capturecycleinwhichtheoutputresponseofthe circuitiscapturedinthescanchain(s). Theoutput responseitshenscannedoutasthenextscanvectoris scannedin.Therecanbeapeakpowerviolation duringeitherscanshiftingodr uringscancapture. A peakpowerviolationcanoccurduringscancaptureif ascanvectorplacesthecircuitinastatethatitw nevergointoduringnormalfunctionaloperation,and thatstatecausedpeakpowerinexcessow f hatcould occurduringnormalfunctionaloperation.

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Figure1 Distribution . oPeak f PowerduringScan Testingof s9234BenchmarkCircuit Figure1showsaplotofthepeakpoweras measuredbythenumberow f eightedgatetransitions duringeachclockcyclewhenscantestingthe

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Figure2 Scan . vector response oscan f vector

tbeing scannedinand i ti-1beingscannedout

Notethatduringscanshiftingthescanchain containspartotfhescanvector, tibeing , scannedin andpartotfheoutputresponseotfhepreviousscan vector, ti-1which , isbeingscannedout(asillustrated inFig.2).Consequently,thenumberofflip-flop transitions(andhencetheamountofswitching activityinthecircuitasawhole)dependsonthe orderingofthescanvectors.Forfullscan,the orderingofthescanvectorsdoesnotaffectthefault coverage,andhencechangingtheorderingothe f scan vectorsisoneapproachthatcanbeusedtoreduce peakpower(thiswillbeexploredinSec.3). Another approachtoreducepeakpowerwouldbetoplacea “dummyvector”thathasfewornotransitions(e.g.,a scanvectoroall f 0’s)beforeoarftersomescanve ctor tso as to reduce the peak power t hat o ccurs w hen i scanningin tor i scanningoutitsoutputresponse. However,evenwithusingdummyvectorsbeforeor aftersomescanvector tthere stillmaybeapeak i powerviolationduringscanshifting. Notealsothat addingdummyvectorsmayresultinanunacceptable increase intestsetsize. Duringsimulationofscantesting,ifthereisa clockcycle inwhich peak a powerviolationoccurs,we willclassifythepeakpowerviolationinthefollowin g fourways: 1. Scancaptureproblem 2. Order dependent problem – scan shifting

problemwhichcanbesolvedbyusingdummy vectorsorreorderingthe scanvectors 3. Scan-inproblem –scanshiftingproblemwhile scanninginthevectorwhichcannotbesolvedby using dummy a vector 4. Scan-outproblem –scanshiftingproblemwhile scanningouttheoutputresponsewhichcannot be solvedbdyummy vectors Ifthepeakpowerviolationoccursduringscan capture,theniis tascancaptureproblem. Ifthepeak powerviolationoccursduringscanshifting,then furthersimulationisdoneusingdummyvectorsto classifyit. Considerthecasewherethepeakpower violationoccursduringscanshiftingwhenthescan chaincontainsapartofscanvector, tiwhich , ibseing scannedin,andapartotfheoutputresponseotfhe previousscanvector, ti-1which , isbeingscannedout (asillustratedinFig.2). Wefirstsimulatethecase wherescanvector tisishiftedintothescanchainand theoutputresponseothe f previousscanvector, ti-1is, adummyvectorthatcausesfewtransitionsasitis scannedout. Ifthereiaspeakpowerviolation,then weclassifyscanvector tiasa“scan-inproblem.” Thenwesimulatethecasewheretheoutputresponse ofscanvector, ti-1,isscannedout,andadummy vectorthatdoesnotcause any transitionsisscanned in (e.g.,avectorofall0’s). Ifthereisapeakpower violation,thenweclassifyscanvector ti-1 asa“scanoutproblem.” Ifthereins eitherscan-in a norsac anoutproblem,thenweclassifythescanvectorpair( ti, ti-1as )an“orderdependentproblem”because the peak power violation could be eliminated by either reorderingthescanvectorsoplacing r dummy a vector betweentestvectors tand t . i i-1 Sotheoverallprocedureistosimulatetheentire scantestandidentifyalltheclockcyclesinwhich peakpowerviolationsoccur. Wethenclassifyeach peakpowerviolationasdescribedabove.Theend resultisalistofindividualscanvectorsthatcause scancaptureproblems,scan-inproblems,andscan-out problems,andalistofscanvectorpairsthatcause orderdependentproblems. Thenextsectiondescribes theprocedureforhowwemodifythesetofscan vectorsto eliminate these peakpowerviolations.

Overview 3. of Procedurefor Eliminating Peak Power Violations After the peak power violations have been classified,thenextstepistoeliminatetheproblems. Theorderdependentproblemsaretheeasiesttosolve sincetheydonotnecessarilyrequiremodifyingthe scanvectorsthemselves. However,theotherproblem

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requiremodifyingthescanvectors. Thisneedstobe doneinawaythatpreservesthefaultcoverage. Bit-strippingisusedtointroduceunspecifiedvalues (i.e.,X’s)andthenthevaluesoftheX’sare reassignedinway a thatreducesthe peakpower. Bit-strippingisanoperationperformedonascan vector, t.Faultsimulationidsonetodropallthe faults thataredetectedbyallthescanvectorsinthetest set exceptfor Then t. isfault t simulatedtdoetermine the setofaults Fthat detectedby at ndbyno t areonly othervectorinthe testset. The firstbitin ischanged t toanXand3-valuedfaultsimulationisperformedto seeiall f thefaultsin Fare detected. Ifso,then t still thebitiskeptasanX,otherwiseiis treturnedtoit s previousvalue. Thisprocessisrepeatedforallthe bit s in t.The end result of the bit-stripping operation isthat na umberof X’sare introducedinto without t reducing theoverallfaultcoverageothe f testset. If the n umber ofX’sisstillnotsufficient,itcanbeincreasedb y partitioning Finto t t subsets,andthenbit-stripping withrespecttoonlyonesubsetof Fatttaime. If Fist partitionedinto nsubsets,then will t bereplacedby n different3-valuedvectors. Eachofthe nvectorswill havemoreX’sbecausethey aretargetingsmaller a set offaults,andbyincludingalloftheminthetestset, the overallfaultcoverage willremainthe same. Thestepsoftheprocedureforeliminatingpeak powerviolationsare afollows: s andclassify peakpowerviolations. Step1:Identify ThiswasdescribedinSec.2Simulation . isdoneto identifythepeakpowerviolations,andthenclassify themintoscancaptureproblems,scan-inproblems, scan-outproblems,andorder-dependentproblems. Step2: Eliminatescan-inproblems.Scan-in problemsoccurwhenascanvectorcausesexcessive switchingactivityinthecircuitasitis cannedin. Thiscanhappeniftherearealotoftransitionsin the scanvector. Ifthereisascan-inproblemforscan vector t,thenbit-strippingisdonetointroduceX’s intoThe t. X’sarethenreassignedtonewvaluesthat resultinfewertransitions(thiswillbedescribedin detailinSec.4.1). NotethatreassigningtheX’sin t toeliminateascan-inproblemmaycauseascan capture,scan-out,ororderdependentproblembecause tisnowadifferentvector. Sosimulationmustbe donetocheckif now t hasscan a capture,scan-out,or orderdependentproblem. Step3: Eliminatescancaptureproblems.Scan captureproblemsoccurbecauseascanvectormay placethecircuitintoastatethatwouldnotoccur duringnormalfunctionaloperation. Whentheoutput responseiscapturedinthescanchain,anexcessive numberoflip-floptransitionsmayoccurwhichcan

causeapeakpowerviolation. Scancaptureproblems aremuchlesslikelythanscan-inandscan-out problemsbecausethenumberoftransitionsduring scancapturewilltendtobesimilartowhatoccurs duringfunctionaloperation. Ingeneral,scancapture problemswillbearareoccurrence. Ifthereiassc an capture problem, then bit-stripping is done to introduceX’s,andthentheX’sarereassignedtonew valuestoreducepeakpowerduringscancapture withoutcausingascan-inproblem(thiswillbe describedindetailinSec.4.2). Simulationmustbe donetocheckifthemodifiedscanvectornowhasa scan-outororder-dependentproblem. Step4: Eliminatescan-outproblems.Scan-out problemsoccurwhentheoutputresponseofascan vectorcausesexcessive switchingactivity inthe circ uit asiistscannedout. Thiscanhappenithere f arela ot oftransitionsintheoutputresponseoascan f vector. Toreducethenumberoftransitionsintheoutput responseofascanvector,bit-strippingisdoneto introduceX’sintheinput,andthentheX’sare reassigned to newvalues that result in fewer transitionsintheoutputresponsewithoutcausinga scan-inoscan r captureproblem(thiswillbe described indetailinSec.4.3). AftertheX’sarereassigned, simulationmustbedonetocheckithe f modifiedscan vectornow causesany orderdependentproblems. Step5:Eliminateorderdependentproblems.Once allthescancapture,scan-in,andscan-outproblems havebeeneliminated,thelaststepitsoeliminatea ny orderdependentproblemsthatmayexist.Order dependentproblemsoccurforapairovf ectorswhere shiftingintestvector tconcurrently withshiftingout i theoutputresponseotest f vector ti-1resultsinapeak powerviolation. Firstanattemptismadetoeliminat e anorderdependentproblembysimplymovingscan vector tso afteradifferentscanvector i thatict omes whoseoutputresponsehasfewertransitionsand movingtestvector ti-1sothatict omesbeforeascan vectorthathasfewertransitions.Ifthisdoesnot eliminatetheproblem,thenadummyvectoris insertedinbetweentestvector ti-1and tto the isolve problem. Ifiis timportantnottoincreasethesize of thetestsetbyaddingadummyvector,thenanother optionistoreducethenumberotransitions f ineither testvector tor outputresponse otest f vector ti-1by ithe modifyingthevectorsinthesamewayafsorsolving scan-inoscan-out r problems. Thisgivesanoverviewoftheprocedurefor eliminatingpeakpowerviolations.Notethatthe numberosfcanvectorsinthetestsetmaybeslightl y increasedbtyhisprocedure. Ifthepeakpowerlimitis toolow,theprocedureins otguaranteedtobeableto

eliminateallpeakpowerviolations. Itispossibleth afaultcannotbedetectedduringscantestingwithout causingapeakpowerviolationithat f faultrequireda largepercentageofthescanelementstobeata specifiedvalueinordertobde etected. Ifsuchafault doesexist,peakpowerviolationscan onlybe eliminatedbydroppingcoverageforthatfault. Otherwise,the procedure willeliminate allpeakpower violationswhile preservingfaultcoverage.

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Reassigning 4. InputValuestoEliminate Peak Power Problems ThissectiondescribeshowtheX’sarereassigned toeliminateeachothe f typesopeak f powerproblems thatcanoccur.

4.1 EliminatingScan-In Problems Thefirstclassofpeakpowerproblemsthatare eliminatedarethescan-inproblems.Ascan-in problemoccursbecauseascanvectorhastoomany transitionsinitresultinginexcessiveswitching activity athe s vectorisscannediTo n.eliminate scaninproblems,thenumberotfransitionsinthevector needstobreeducedbyreassigninginputvalues. This isdonebyfirstbit-strippingthescanvectorto introduceX’s,andthendoinga minimumtransition fill(MT-fill) oftheX’s.MT-fillinvolvesfilling stringsofX’swiththesamevaluetominimizethe numberotfransitions. Forexample,whenfillingthe vector01XX10,iw t ouldbebesttofillthestringof X’swith1’s,i.e.,011110. ForeachstringoX’s f ina vector,ifthespecifiedbitsoneithersideothe f st ring havethesamevalue,thenthestringoX’s f shouldbe filledwiththatvaluetominimizethenumberof transitions.Iftheyhaveoppositevalues,thenit doesn’tmatterwhichvaluethestringofX’sifsilled with.Forexample,whenfilling0XX01X1X0,the firsttwoX’sshouldbefilledwith0’s,thethirdX shouldbfeilledwith1, aandthelastXcouldbfeilled witheither0o1. r Figure3showsanexampleoreducing f thenumber oftransitionsintheoriginalvectorbyfirstbitstrippingandthendoinganMT-filloftheX’s. After doingMT-fill,simulationisdonetocheckifthe resultingvectorstillcausesapeakpowerviolation whenitisscannedin. Ifthereisstillpeakpower violation,then reversebit-stripping isperformed which involves doing bit-stripping again, but processingtheinputsinthereverseorderfromthe originalbit-stripping. Thiswilltendtointroducea differentsetofX’sintothevector. Afterreverse bitstripping,MT-fillisdoneagain,andacheckim s ade toseeithere f istillapeakpowerviolation. Ift here isstillapeakpowerviolation,thenthesetofaults ,

Ftt,hatthescanvector, t,istargetingispartitioned intosubsetssothatbit-strippingwillproducemore X’s. Thescanvector is tbit-strippedwithrespectto eachsubsettoproduceamultiplicityofscanvectors eachofwhichhasmoreX’sthan twhiletogether providingthesamefaultcoverageas Ifthe t. original setoffaults, Fis , enough,thenbit-strippingcan tsmall bedonewithrespecttoeachindividualfaultin Ft. Otherwise, Ftcanberandomlydividedintotwo subsets,andthenrepeatedly dividedanecessary. s The numberofadditionalscanvectorsthatresultsfrom partitioningthefaultsetcanbereducedbystatic compaction. Thestaticcompactionprocedurecanbe constrainedsothatthenumberoftransitionsinthe merged vector never exceeds some threshold [Sankaralingam00]. Forexample,supposethatbitstrippingscanvector resulted t in a vector thathad100 transitionsinthe specifiedbits(the X’sare assumed to befilledwithMT-fill). However,bypartitioning Ft into 8 subsets, the resulting8 vectors after bit-stripping allhavefewerthan50transitionsinthespecifiedbit s. Atthispoint,staticcompactioncanbeusedtomerge someofthe8vectorsbacktogetherunderthe constraintthattheresultingvectorsaftermergingsti ll have nm o ore than50transitionsinthe specifiedbits. OriginalScanVector: (13transitions) AfterBit-Stripping: AfterMT-Fill: (7transitions)

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OriginalScanVector: OutputResponse: (13transitions)

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AfterReassigningInputs: 0 0 1 1 1 1 1 0 1 0 0 0 0 1 1 0 1 1 0 OutputResponse: 0 0 0 1 1 0 1 0 1 1 0 0 1 1 0 0 0 1 1 (7transitions)

Figure3 Example: . EliminatingScan-InProblem

4.2 EliminatingScan CaptureProblems Thesecondclassopf eakpowerproblemsthatare eliminatedarethescancaptureproblems.Ascan captureproblemoccursbecauseanexcessivenumber offlip-flopstransitionafterscancapture. Theflipflopsthattransitionafterscancapturearethosefor whichtheinputvaluediffersfromtheoutputvalue. Givenascanvector,fault-freesimulationcanbde one todetermineitscorrespondingoutputresponsevector. Toeliminatescancaptureproblems,thenumberof input/outputdifferencesinthevectorneedstobe reducedbyreassigninginputvalues. Thisids oneby firstbit-strippingthescanvectortointroduceX’s. ThebitpositionswithX’swillbereferredtoa“sfr inputs”becausetheycanbefreelyreassignedwithout affectingthefaultcoverage.Therestofthebit positions,whichdonothaveX’s,are“fixedinputs.” Oncethesetoffreeandfixedinputshasbeen determinedviabit-stripping,wegobacktooriginal fully specifiedscanvectorasourstartingpoint. Fig.4

showsanexample. Thegoalnowistoreassignsome ofthefreeinputssothatthenumberoifnput/output differencesirseduced. Ahillclimbingapproachcan beused. Weidentifyonefreeinputwhosevalueis differentfromthecorrespondingoutput. Thevalueof thefreeinputiscomplemented. Fault-freesimulation isdoneonthealteredinputvectortoobtainthenew outputresponse. Acheckim s ade tsoee ithe f number ofinput/outputdifferenceshasdecreased. Ifso,then thechangeiskept,inf otthenitisundone. Thisis repeatedforallthefreeinputs.Whilethisonly exploresasubsetoftheexponentialnumberof possiblefreeinputassignments,iht astheadvantage oftendingtominimizethenumberocfhangestothe originalscanvectorandtherebyreducingthechance ofcreatingascan-inproblem(sincetheoriginalscan vectordidnothaveascan-inproblem).Afterthe inputsarereassignedinthismanner,simulationis donetoseeifthescancaptureproblemhasbeen eliminatedandnoscan-inproblemhasbeencreated. Ifnot,thenreversebitstrippingcanbedoneto identify different a setoffreeinputsandthe procedure canbreepeated. Ifstillnosolution,thenbit-strippi withpartitioningthefaultsetcanbeusedtofurther increasethenumberofreeinputsuntilasolutionis found.

Figure 4.Example:Eliminating

Scan Capture Problem

4.3 EliminatingScan-OutProblems

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Thethirdclassofpeakpowerproblemsthatare eliminatedarethescan-outproblems.Ascan-out problemoccursbecausetheoutputresponseoasfcan vectorhastoomanytransitionsinitresultingin excessiveswitchingactivityasiitscannedout. To eliminatescan-outproblems,the numberof transitions inthescanvector’soutputresponseneedstobe reducedbyreassigninginputvalues. Thisids oneby firstbit-strippingthescanvectortointroduceX’sand therebyidentifytheseto“f freeinputs”whosevalue canbechangedwithoutaffectingthefaultcoverage. Oncethesetoffreeandfixedinputshasbeen determinedviabit-stripping,wegobacktooriginal fullyspecifiedscanvector.Thegoalnowisto reassignsomeothe f freeinputssothatthenumberof transitionsintheoutputresponseisreduced. Fig.5 showsanexample.

Akeycomponentthatweuseforreassigningthe inputsisalinejustificationprocedureliketheone usedinPODEM[Goel81]exceptwithamodification tothecontrollabilitycostfunction. Normally co a 1iassigned s forcontrollingeachothe f primary inputs toeitheralogic0orlogic1a, ndthenthecircuitis traversedfromtheprimaryinputstotheprimary outputstocomputethecontrollabilitycostfunctionat eachindividualnodebasedontheseinitialvaluesfor theprimaryinputs.Inourcase,sincewecannot changethevalueothe f fixedinputs,if we have fixed a inputatalogic1(0),thenwaessignacostofinfinity (zero)forcontrollingthatinputtoalogic0(1),and assignacostofzero(infinity)forcontrollingthat inputtoalogic1(0). Forthefreeinputs,weassigna costof1forcontrollingtheinputtoitsoriginalvalue andacostof10forcontrollingtheinputtothe complementof itsoriginalvalue.The reasonforthis tobiastheproceduretowardsreducingthenumberof changestotheoriginalscanvectorinordertoreduce thechanceocf reatinganewscan-inorscancapture problem(sincetheoriginalscanvectordidnothave any scan-inoscan r capture problemto startwith). Weuseahillclimbingprocedureforreducingthe numberoftransitionsintheoutputresponse. We look attheoutputresponseandidentify allthebitpositions wherethecontrollabilityvalueforcomplementingthe outputvalueislessthaninfinity.Thesearethe candidateoutputswhosevaluecanbechangedby reassigningfreeinputs.Weidentifythecandidate outputwiththelowestcontrollabilityvaluewhere complementingitsvaluewouldeliminateatransition. Thelinejustificationprocedureisusedtofindan assignmentofinputsthatcomplementsthisoutput’s value. Alltheinputassignmentstofixedinputsmade bythelinejustificationprocedurewillbethesameas theiroriginalvaluebydefinitionofourcontrollabili costfunction. Someotfheinputassignmentstothe freeinputswillbedifferentthantheiroriginalvalue andsomemaybethesame.Aftertheinput assignmentshavebeenmade,fault-freesimulationis donetobtainthenewoutputresponse. If the number oftransitionsintheoutputresponsedecreases,then theinputchangesarekept,otherwisethey areundone. Iftheinputchangesarekept,thenallfreeinputsthat requiredassignmentstojustifytheoutput(regardless ofwhethertheywerethesameodr ifferentthanthei original values) become fixed inputs. The controllabilityvaluesarethenrecalculatedandthe processrepeatsuntilpa ointisreachedwherenomore candidateoutputscanbereassignedtoreducethe numberoftransitions.Atthispoint,simulationis donetoseeifthescan-outproblemhasbeen eliminatedwhilenotcreatinganyscan-inorscan

captureproblems. Ifnot,thenreversebitstripping canbedonetoidentifyadifferentsetofreeinputs andtheprocedurecanbreepeated. If stillno solution, thenbit-strippingwithpartitioningthefaultsetcanbe usedtfourtherincrease the numberof free inputs.

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OriginalScanVector: OutputResponse: (13transitions) AfterBit-Stripping: FreeInputs

1 1 0 1 0 1 0 1 1 0 0 1 1 1 0 0 0 0 0 0 1 0 1 1 0 0 1 1 0 1 0 0 1 0 0 1 0 1

1 X X 1 0 1 X 1 1 0 X X X 1 0 0 X 0 X

AfterReassigningInputs:1 0 0 1 0 1 1 1 1 0 0 0 1 1 0 0 0 0 1 OutputResponse: 0 0 1 1 1 0 0 0 1 0 0 0 0 1 0 0 1 1 1 (7transitions)

Figure5 Example: . EliminatingScan-OutProblem , is

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Experimental 5. Results ExperimentswereperformedonthelargestISCAS 89benchmarkcircuits.Theprimaryinputsand primaryoutputswereincludedinthescanchain. A testsetwasgeneratedforeachcircuitusinga commercialATPGtoolwithmaximumcompression andMT-fillingoftheX’s.Foreachcircuit,scan testingwassimulatedusingthetestset. Foreachnew bitscannedin,thecircuitwasevaluatedtocalculate thenumberoifnternalgatesthatchangestate. Each gatetransitionwasweightedbythenumberoffanouts ofthegate. Afterallthebitsforonescanvector are scannedin,thescancapturecycleisimulated. The outputresponseitshenshiftedoutasthe nextvectoris scannedin. Thissimulationprocesswasusedtofind themaximumnumberow f eightedtransitionsinany cycle duringscantesting. Theproposedprocedurewasthenusedtomodify thescanvectortestsettoreducethemaximumpeak powerby 10%(showninTable1and ) b2y0%(shown inTable2). Thenumberosfcanvectorsthatcause eachtypeofpeakpowerviolationareshowninthe tablesfortherespectivemaximumpeakpowerlimits. Inourexperiments,wedidnotencounteranyscan captureproblems. Allofthepeakpowerviolations occurredduringscanshifting.The proposedprocedure modifiedthescanvectorstoeliminatethescan-inan d scan-out problems, and then reordered or addeddummy vectorstoeliminatethescandependentproblems. Theoriginalnumberofvectorsinthetestsetissho wn ineachtablefollowedbythefinalnumberovf ectors afterusingtheproposedprocedure. Thereiasslight increaseinthenumberofvectorsduetoadding dummyvectorsandbit-strippingvectorswithrespect to partitionedfaultsets(asdescribedinSec.4). Ascanbeseenfromtheresults,theprocedureis able tsoignificantly reduce peakpowerwithoutthe use ofDFThardware. Theonly costissmall a increase in the testsetsize.

Circuit Name ScanCells s9234 247 s13207 700 s15850 611 s38417 1664

Table1 Results . forReducingPeakPowerby 10% Numberof ScanVectorswithEachType oProbl f em Scan-In Scan-Out ScanCapture OrderDependent 5 4 0 27 154 2 8 0 16 240 2 11 0 0 118 0 0 0 2 96

Numberof Vectors Original Final 156 246 120 98

Circuit Name ScanCells s9234 247 s13207 700 s15850 611 S38417 1664

Table2 Results . forReducingPeakPowerby 20% Numberof ScanVectorswithEachType oProbl f em Scan-In Scan-Out ScanCapture OrderDependent 7 76 0 55 154 2 76 0 59 240 1 23 0 42 118 8 3 0 4 96

Numberof Vectors Original Final 164 247 128 100

6.Conclusions Thescanvectormodificationproceduredescribed herecanbeusedtokeepthepeakpowerduringscan testingunder sapecifiedlimit. The usercanadjustthe specifiedlimitasnecessarytoavoidfailuresdueto excessivepeakpowerduringscantestingthatwould notoccurduringnormalfunctionaloperation. The procedure may increase the testsetsize slightly. Notethattheproposedprocedurecanbeusedfor anysetofullyspecifiedscanvectors. Thereisno requirementforanyspecialATPGprocess. Normally duringATPG,randomfillingothe f X’sius sedtotry togeteachscanvectortocovermorefaults. Asimpl approachtoreducepoweristouseMT-fillingofthe X’s. Theproposedprocedurecanbeusedineither casetoreducepeakpower. Theexperimentalresults showedthatevenwithMT-fillingoftheX’sduring ATPG,theproposedprocedurecouldsignificantly reduce peakpowerbeyondthat.

e

Acknowledgements Thismaterialisbasedonworksupportedinpartby the National Science Foundation under Grant No. MIP9702236andinpartbytheTexasAdvancedTechnology ProgramunderGrantNo.003658-0644-1999.

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