high-level specification model based functional coverage generation · 2018-08-20 · 7 figure 2:...

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High-levelspecificationmodelbasedFunctionalcoveragegeneration

(WithcasestudyofUSBPowerdeliveryprotocollayercoverage)

Author:AnandShirahatti([email protected])

Intentistheseedofmanifestation


1 Introduction.................................................................................................................32 Howisthisapproachdifferent?.............................................................................4

3 Makingintentexecutable........................................................................................4

4 Benefits..........................................................................................................................55 Architecture..................................................................................................................65.1 Executablecoverageplan.............................................................................................75.2 High-levelspecificationmodeling.............................................................................75.3 Informationsource.........................................................................................................85.4 SystemVerilogCoverageAPIinPython....................................................................85.4.1 LimitationsofSystemVerilogCovergroup....................................................................85.4.2 CoverageAPILayering..........................................................................................................95.4.3 Implementation......................................................................................................................105.4.4 Structureofuserinterface.................................................................................................11

5.5 Sourceinformation.......................................................................................................126 Summary:...................................................................................................................137 USBPowerdeliveryprotocollayer–Casestudy...........................................147.1 Refresher..........................................................................................................................147.2 Protocollayerfunctionalcoveragechallenges...................................................167.3 Functionalcoverageitemsforcasestudy.............................................................167.4 USBpowerdeliveryspecificationinformationmodeling................................177.4.1 Messagemodeling.................................................................................................................187.4.2 Eventmodeling.......................................................................................................................197.4.3 Protocolsequencemodeling.............................................................................................20

7.5 Executablecoverageplan...........................................................................................247.5.1 Globalconfiguration.............................................................................................................247.5.2 Item1:Coveralltransmittedprotocolmessages....................................................257.5.3 Item2:Allreceivedprotocolmessages........................................................................267.5.4 Item3:Somekeyeventsduringmessagetransmission.......................................277.5.5 Item4:Foralltransmittedmessagesgettingallpossiblevalidresponses..297.5.6 Item5:Foralltransmittedmessagesunexpectedacknowledgement...........307.5.7 Item6:Powernegotiationsequence.............................................................................327.5.8 Item7:Timeouterrorinjectionwithinpowernegotiationsequence...........33

8 Conclusion..................................................................................................................35


1 IntroductionWewerepresentingourwhiteboxfunctionalandstatisticalcoveragegenerationsolution,oneoftheengineerasked,canittakestandardspecificationsasinputandgeneratethefunctionalcoveragefromit?

Figure1:Specificationtofunctionalcoveragemagicpossible?Ireplied“No”.Itcannot.Butthenafterthepresentation,questionedmyselfasto,whynot?No,nostillnotbraveenoughtoparsethestandardspecificationsusenaturallanguageprocessing(NLP)toextracttherequirementsandgeneratethefunctionalcoveragefromit.Butwehavetakenfirststepinthisdirection.It’sababystep.Maybesomeofyoumightlaughatit.Wearecallingitashighlevelspecificationmodelbasedfunctionalcoveragegeneration.Ithassomeremarkableadvantages.Aseverytime,Ifeltthisis“the”waytowritefunctionalcoveragefromnowonJIdeaisverysimple.Iamsuresomeofyoumighthavealreadydoingitaswell.Capturethespecificationinformofdatastructures.DefinebunchofAPIstofilter,transform,queryandtraversethedatastructures.CombinetheseexecutablespecificationswithourpythonAPIsforSystmVerilogfunctionalcoveragegeneration.Voila,poorman’sspecificationtofunctionalcoveragegenerationisready.Yes,youneedtolearnscriptinglanguage(pythoninthiscase)andre-implementsomeofthespecificationinformationinit.That’sbecauseSystemVerilogbyitselfdoesnothavenecessaryfirepowertogetitalldone.Scared?Turnedoff?Noproblem.Nothingmuchislost.Pleasestopreadingfromhereandsaveyourtime.Adventurersandexplorerssurvivingthishardfactpleasehopon.Iamsureyouwillfallinlovewithatleastonethingduringthisride.


2 Howisthisapproachdifferent?Howisthisapproachdifferentfrommanuallywritingcoveragemodel?ThisisaveryimportantquestionandwasraisedbyFaisalHaque.Therearemultipleadvantages,whichwewilldiscusslaterinthearticle.Inmyviewsinglebiggestadvantageismakingthecoverageintentexecutablebytrulyconnectingthehigh-levelmodelofspecificationstofunctionalcoverage.Nowearenottalkingaboutjustputtingspecificationsectionnumbersincoverageplanwearetalkingaboutreallycapturingthespecificationandusingitforgenerationoffunctionalcoverage.Letmesettheexpectationsright,thisapproachwillnotfigureoutyourintent.Theideaisaboutcapturingandpreservinghumanthoughtprocessbehindthefunctionalcoveragecreationinexecutableform.Sothatitcanbeeasilyrepeatedwhenthingschange.That’sall.It’sastartandfirststeptowardsspecificationstofunctionalcoveragegeneration.Typicallyfunctionalcoverageisimplementedassetofdiscreteindependentitems.Theintentanditsconnectiontospecificationsareweaktonon-existentinthistypeofimplementation.Mostoftheintentgetseitherleftbehindinthewordofexcelplanwhereitwaswrittenorintheformofcommentsinthecode,whichcannotexecute.

3 MakingintentexecutableWhycapturingintentinexecutableformisimportant?Werespectandvaluethehumanintelligence.Why?Isitonlyforthisemotionreason?No.Makinghumanintelligenceexecutableisfirststeptoartificialintelligence.Abilitytotranslatetherequirementsspecificationintocoverageplanishighlydependentontheexperiencesanddepthofspecificationunderstandingoftheengineeratthemomentofwritingit.Ifitsnotcapturedinthecoverageplanit’slost.Eventheengineerwhowrotethefunctionalcoverageplanmayfinditdifficulttorememberwhyexactlycertaincrosswasdefinedafter6months.Nowthiscanbecomerealchallengeduringtheevolutionandmaintenanceofthefunctionalcoverageplanastherequirementsspecificationsevolve.Engineerdoingincrementalupdatesmaynothaveluxuryofthetimeastheearlieronehad.Unlesstheintentisexecutablethequalityofthefunctionalcoveragewilldegradeoverperiodoftime.NowifyouaredoingthisdesignIPforonlyonechipandafterthatifyouarethrowingitawaythisfunctionalcoveragequalitydegradationmaynotbesuchabigconcern.


Let’sunderstandthislittlefurtherwithexample.IntheUSBPowerdeliverycasestudyyouwillbeabletofindmoredetailsaboutthisexample.USBpowerdeliverysupportsmultiplespecificationrevisions.Let’ssay,wewanttocoveralltransmittedpacketsforrevisionx.Inmanualapproachwewilldiscretelylistprotocoldataunitsvalidforrevisionx.Forthislistingyouscanthespecifications,identifythemandlistthem.Onlywaytoidentifythemincodeasbelongingtorevisionxiseitherthroughcovergroupnameorcommentinthecode.InthenewapproachyouwillbeabletooperateonalltheprotocoldataunitssupportedbyrevisionxasaunitthroughAPIs.Thisismuchmoremeaningfultoreadersandmakesyourintentexecutable.Aswecalledout,ourideaistomakecoverageintentexecutabletomakeitadaptable.Let’scontrastbothapproacheswithanotherexample.Forexample,let’ssayyouwanttocovertwoitems:

• Allpackettransmittedbydevicesupportingrevision2.0• Intermediateresetwhileallpackettransmittedbydevicesupporting

revision2.0

Ifyouweretowritediscretecoverage,youwouldhavesampledpackettypeandlistedallthevalidpackettypesofrevision2.0asbins.SincebinsarenotreusableinSystemVerilogyouwoulddocopyandpastethemacrossthesetwocovergorups.Nowimagine,ifyoumissedapackettypeduringinitialspecificationscanorerratacontainingonemorepackettypecameoutlater,youneedtogobackandaddthisnewtypeattwodifferentplaces.Butwiththisnewapproach,assoonasyouupdatethespecificationdatastructurewithnewtypeyouaredone.Allthequeriesrequestingrevisionxwillautomaticallygetupdatedinformation.Henceallthefunctionalcoveragetargetedtorevisionxwillbeautomaticallyupdated.Rememberinitiallyitmaybeeasytospottwoplaceswherethechangeisrequired.Butwhenyouhavehundredsofcovergroupsitwillbedifficulttoreflecttheincrementalchangestoallthediscretecovergroups.Itwillbeevenmoredifficultwhennewengineerhastodotheupdatewithoutsufficientbackgroundontheinitialimplementation.

4 BenefitsWhatarethebenefitsofthisapproach?Withhigh-levelspecificationmodelbasedfunctionalcoveragetheabstractionofthoughtprocessofwritingcoveragemovesupanditfreesupbrainbandwidthto


identifymoreitems.Thisadditionalbrainbandwidthcansignificantlyhelpimprovethequalityoffunctionalcoverageplanandhencetheoverallqualityoffunctionalverification.Benefitsofhigh-levelmodelbasedfunctionalcoveragegeneration:

• Intentgetscapturedinexecutableform.Makesiteasytomaintain,updateandreviewthefunctionalcoverage

• Executableintentmakesyourcoveragetrulytraceabletospecification.Itsmuchbetterthanjustincludingthespecificationsectionnumberswhichleadstomoreoverheadthanbenefit

• Itseasytomapthecoveragefromsinglespecificationfromdifferentcomponentspointsofview(Ex:USBdeviceorhostpointofvieworPCIerootcomplexorendpointorUSBPowerdeliverysourceorsinkpointofview)fromsinglespecificationmodel

• Easytodefineandcontrolthequalityofcoveragecontrolledbythelevelofdetailsinthecoveragerequiredforeachfeature(Ex:Coveranycategory,coverallcategoriesorcoverallitemsineachcategory)

• Easytosupportandmaintainmultipleversionsofthespecifications• Dynamicallyswitchtheviewofthecoverageimplementedbasedonthe

parameterstoeasetheanalysis(Ex:Perspeed,perrevisionorforspecificmode)

5 ArchitectureHowtogoaboutbuildinghigh-levelspecificationmodelbasedfunctionalcoverage?Firstlet’sunderstandthemajorcomponents.Followingistheblockdiagramofthehigh-levelspecificationmodelbasedfunctionalcoverage.Wewillbrieflydescriberoleandfunctionalityofeachoftheseblocks.Thisdiagramonlyshowsbasicbuildingblocks.Laterwewilllookatthecasestudieswherewewillseetheseblocksinactionmakingtheirexplanationsmoreclear.Itwillalsoguidehowtoimplementtheseblocksforyourprojectaswell.


Figure2:Blockdiagramofhigh-levelspecificationmodelbasedfunctionalcoveragegeneration

5.1 ExecutablecoverageplanExecutablecoverageplanistheblockthatactuallyhostsallthefunctionalcoverageitems.It’scoverageplananditsimplementationtogether.Itdoestheimplementationoffunctionalcoverageitemsbyconnectingthehigh-levelspecificationmodel,sourceofinformationandSVcoverageAPIs.TheAPIsutilized,specificationinformationaccessedandrelationsofvariousitemsutilizedpreservestheintentinexecutableform.Userstillspecifiestheintentofwhattocover.Itwon’treadyourmindbutyouwillbeabletoexpressyourthoughtsathigherlevelofabstractionsandmorecloserorspecificationsandinhighlyprogrammableenvironmentthatismuchmorepowerfulthatSystemVerilogalone.

5.2 High-levelspecificationmodelingThisblockiscombinationofsetofdatastructuresandAPIs.Datastructurescapturehigh-levelinformationfromthespecifications.Thesedatastructurescanbecapturinginformationaboutpropertiesofdifferentoperations,statetransitiontablesrepresentingthestatemachines,information


abouttimersastowhentheystart,stop,timeoutorgraphscapturingvariousformsofsequences.Ideahereiscapturetherelevantinformationaboutthespecificationthatisrequiredforthedefinitionandimplementationofthefunctionalcoverage.Choosetherightformofdatastructuresthatfitthepurpose.Thesedatastructureswillvaryfromdomaintodomain.APIsontheotherhandprocessthedatastructurestogeneratedifferentviewsoftheinformation.APIscanbedoingfiltering,combinations,permutationsorjusteaseaccesstotheinformationbyhidingthecomplexityofdatastructures.ThereissomelevelofreusepossiblefortheseAPIsacrossvariousdomains.UsingthesesetofdatastructuresandAPIsnowwearereadytotranslatethecoverageplantoimplementation.

5.3 InformationsourceSpecificationdatastructuresmaydefinethestructureofoperationsbuttocoverit,weneedtoknowhowtoidentifythecompletionofoperation,whatisthetypeoperationofoperationcompletedandcurrentvaluesofitspropertiesetc.InformationsourceprovidestheabstractiontobindthespecificationinformationtoeithertestbenchordesignRTLtoextracttheactualvaluesofthesespecificationstructures.Thisabstractionprovidestheflexibilitytoeasilyswitchthesourceofcoverageinformation.Bottomlinestoresinformationaboutsourcesthatareeithersampledforinformationorprovidestriggerstohelpdecidewhentosample.

5.4 SystemVerilogCoverageAPIinPythonWhydoweneedtheseAPIs,whycan’twejustdirectlywriteitinSystemVerilogitself?That’sbecauseSystemVerilogcovergrouphassomelimitations,whichpreventtheeaseofreuse.

5.4.1 LimitationsofSystemVerilogCovergroupSystemVerilog functional covergroup construct has some limitations, whichpreventsitseffectivereuse.Someofthekeylimitationsarefollowing:• Covergroupconstruct isnotcompletelyobjectoriented. Itdoesnotsupport

inheritance.Whatitmeansisyoucannotwriteacovergroupinbaseclassandadd,updateormodifyitsbehaviorthroughderivedclass.Thistypeoffeatureis very important when you want to share common functional coveragemodels across multiple configurations of DUT verified in different testbenchesandtosharethecommonfunctionalcoverageknowledge


• Withoutrightbinsdefinitionsthecoverpointsdon’tdomuchusefuljob.Thebins part of the coverpoint construct cannot be reused across multiplecoverpointseitherwithinthesamecovergrouporindifferentcovergroup

• Key configurations aredefined as crosses. In some cases youwould like toseedifferentscenariostakingplaceinallkeyconfigurations.Butthereisnocleanwaytoreusethecrossesacrosscovergroups

• Transition bin of coverpoints to get hit are expected to complete definedsequence on successive sampling events. There is no [!:$] type of supportwhere the transition at any point is considered as acceptable. This makestransitionbinimplementationdifficultonrelaxedsequences

5.4.2 CoverageAPILayering

At VerifSudha, we have implemented a Python layer that makes theSystemVerilog covergroup construct object oriented and addresses all of theabove limitations tomake the coveragewriting processmore productive. Alsothe power of python language itself opens up lot more configurability andprogrammability.Based on this reusable coverage foundationwe have also builtmany reusablehighlevelcoveragemodelsbundledwhichmakethecoveragewritingeasierandfaster.Greatpartisyoucanbuildlibraryofhigh-levelcoveragemodelsbasedonbest-knownverificationpracticesofyourorganization.These APIs allows highly programmable and configurable SystemVerilogfunctionalcoveragecodegeneration.FundamentalideabehindalltheseAPIsisverysimple.

Figure3:SVCoverageAPIlayering

WehaveimplementedtheseAPIsasmultiplelayersinpython.


Bottommostlayerisbasicpythonwrappersthroughwhichyoucangeneratethefunctionalcoveragealongwiththesupportforobjectorientation.Thisprovidesthefoundationforbuildingeasytoreuseandcustomizehigh-levelfunctionalcoveragemodels.Thisissufficientforthecurrentcasestudy.RTLelementscoveragemodelscovervariousstandardRTLlogicelementsfromsimpleexpressions,CDC,interruptstoAPPsforthestandardRTLelementsuchasFIFOs,arbiters,registerinterfaces,lowpowerlogic,clocks,sidebands.Genericfunctionalitycoveragemodelsarestructuredaroundsomeofthestandardhigh-levellogicstructures.Forexampledidinterrupttriggerwhenitwasmaskedforallpossibleinterruptsbeforeaggregation.Sometimesthistypeofcoveragemaynotbeclearfromthecodecoverage.Someofthesearealsobasedonthetypicalbugsfoundindifferentstandardlogicstructures.Athighest-levelaredomainspecificoveragemodel.Forexamplemanyhigh-speedserialIOshavesomecommonproblemsbeingsolvedespeciallyatphysicalandlinklayers.Thesecoveragemodelsattempttomodelthosecommonfeatures.Allthesecoveragemodelsareeasytoextendandcustomizeastheyarebuiltonobjectorientedparadigm.That’stheonlyreasontheyareuseful.Iftheywerenoteasytoextendandcustomizetheywouldhavebeenalmostuseless.

5.4.3 Implementation• BackboneoftheseAPIsisdatastructurefortheSystemVerilogcovergroups

modeledaslistofdictionaries.Eachofthecovergroupbeingadictionarymadeupoflistofcoverpointdictionariesandlistofcrossdictionaries.Eachofthecoverpointandcrossdictionariescontainlistofbindictionaries

• Thesedatastructuresarecombinedwithsimpletemplatedesignpatterntogeneratethefinalcoveragecode

• UsinglayerofAPIsonthesedatastructureadditionalfeaturesandlimitationsofSystemVerilogcovergroupareaddressed

• SetofAPIsprovidedtogeneratethereusablebintypes.ForexampleifyouwanttodivideanaddressrangebetweenNequalparts,youcandoitthroughtheseAPIsbyjustprovidingthestartaddress,endaddressandnumberofranges

• Therearealsobunchofobjecttypesrepresentinggenericcoveragemodels.Bydefiningtherequiredpropertiesfortheseobjecttypescovergroupscanbegenerated

• Usingpythoncontextmanagersthecovegroupmodelingiseasedofffortheuser

AnyuserdefinedSystemVerilogcodecanco-existwiththeseAPIs.ThisenableseasymixofgeneratedandmanuallywrittencodewhereAPIsfallshort.


Figure4:WhattoexpectfromAPIs

5.4.4 StructureofuserinterfaceAlltheAPIsessentiallyworkontheobject.Globalattributescanbethoughtofasapplicabletoentirecovergroup.Forexampleifyouspecifiedbinsatthegloballevelitwouldapplytoallthecoverpointsofthecovergroup.Notonlytheinformationrequiredforcoveragegenerationbutalsodescriptionandtrackinginformationcanbestoredinthecorrespondingobject.Thisadditionalinformationcanbebackannotatedtosimulatorgeneratedcoverageresultshelpingyoucorrelateyourhigh-levelpythondescriptionstofinalcoverageresultsfromregressionseasily.AlsotheAPIssupportmindmapsandExcelfilegenerationstomakeiteasytovisualizethecoverageplanforreviews.


Figure5:Structureofuserinterfaceforobjects

5.5 SourceinformationCovergroupsrequirewhattosampleandwhentosample.Thisistheblockwhereyoucapturethesourcesofinformationforwhattosampleandwhentosample.It’sbasedonverysimpleconceptlikeVerilogmacros.Allthecoverageimplementationwillusethesemacros,sothatitabstractsthecoveragefromstaticallybindingtosourceoftheinformation.Laterthesemacroscanbeinitializedwiththeappropriatesourceinformation.

Snippet1:SpecifyingsourceinformationThisflexibilityallowsusinginformationsourcefromeitherbetweentheRTLandtestbench.Easilybeabletoswitchbetweenthembasedonneed.Followingcodesnippetsshowcasehowcovergroupimplementationforsimpleread/writeandaddresscanbedoneusingeitherRTLdesignortestbenchtransactions.


Snippet2:CoveragegeneratedusingtestbenchtransactionCoverpointsinsnippet2aresamplingtheregisterreadwritetransactionobject(reg_rd_wr_tr_obj).Samplingiscalledoneverynewtransaction

Snippet3:CoveragegeneratedusingDUTsignalsCoverpointsinsnippet3aresamplingtheRTLsignalstoextracttheread/writeoperationandaddress.Samplingiscalledoneverynewclockqualifiedbyappropriatesignals.

6 Summary:Functional coverage is one of the last lines of defense for verification quality.Being able to repeatedly do a good job and do it productively will havesignificantimpactonyourqualityofverification.Initiallyitmayseemlikelotofwork,youneedtolearnascriptinglanguageandlearndifferenttechniquesofmodeling.Butpayoffwillnotonlyforthecurrentproject but throughout the lifetime of your project by easing themaintenanceandallowingyoutodeliverthehigherqualityconsistently.


7 USBPowerdeliveryprotocollayer–CasestudyBeforewejumpintoimplementationdetails,hereisaveryshortrefresherwithonlylimitedthedetailsrelevantforthiscasestudy.

7.1 RefresherDynamicnegotiablepowerisbasicideabehindtheUSB–powerdelivery.Obviouslyfornegotiationtotakeplaceweneedtwoentities.Theyaresourceandsink.

Provider(SOURCE)suppliespowerandConsumer(SINK),usesthesuppliedpower.Asimpleprovidercouldbewalloutletorlaptopandsimplesinkcouldbemobileortablet.Powernegotiatedcanbeupto100Watts.Thecableshavetobedesignedtosupportthat.Cablesareelectronicallymarkedaswelltospecifytheircapabilities.RefinedUSBconnectorcalledtype-cisgainingpopularity,usedfortheUSBpowerdeliverysupport.Theretwoadditionalwiresaddedintheconnectortosupporttheprotocol.OnthesetwoadditionalwiresUSBpowerdeliveryrunsaprotocolstackmadeupoffollowingthreelayers:

• Physicallayer• Protocollayer• Policyengine.

Wewilllookintobrieflyprotocollayeronly.


Nowprotocolitselfisverysimple.It’shalfduplex.Half,yes,that’sright.It’smadeupofbunchofmessages.Therecanbeonlyoneoutstandingmessagebetweenthepairofcommunicatingdevices.Everymessagehastobeacknowledgedforsuccessfulreception.Onlyafterthatnextmessageistransmitted.Therearethreecategoriesofmessages.TheyareCONTROL,DATAandEXTENDED.

Let’slookatthesimpleprotocolsequence.Inverysimpleterms,transmittersendsrequest,waitforacknowledgement.Receiversendsacknowledgement.Followedbythatreceiversendstheresponseandwaitsfortheacknowledgementfromrequesttransmitter.That’sit.

Differenttimersgovernmaximumwaittimeforthevariousacknowledgementandresponses.


7.2 ProtocollayerfunctionalcoveragechallengesTherearethreechallengeshere:

• Multiplespecificationrevisionsupport(Rev2.0andRev3.0)• Protocolhasmultipleparticipants:Source,Sink,Cableplug,Source/Sink

(DRP)Therearesignificantdifferencesbetweenrevision2.0and3.0.EXTENDEDcategoryofmessagesisaddedonlyinrevision3.0.WhenyouwritecoveragemodelSOURCEandSINKarelikemirrorimages.Whatiscoveredastransmitforoneneedstocoveredasreceivefortheother.

7.3 FunctionalcoverageitemsforcasestudyNowlet’spicksomeitemstocoverandseehowtheywouldlooklikeinhigh-levelmodelingbasedfunctionalcoverage.Pickingrainbowofsimpleatomicitems,simplesequenceandcomplexsequences:Sl.No. Coverageitemdescription1 Alltransmittedprotocolmessages2 Allreceivedprotocolmessages3 Somekeyeventsduringmessagetransmission4 Foralltransmittedmessagesgettingallpossiblevalidresponses5 Foralltransmittedmessagesunexpectedacknowledgementreceived6 Powernegotiationsequence7 Timeouterrorinjectionwithinpowernegotiationsequence

Table1:SelectedfunctionalcoverageitemsforcasestudyForalltheseitemslet’sdemonstratehowallthebenefitsofhigh-levelmodelbasedfunctionalcoveragepromisedcanbeachieved.Nowlet’sjumpintodetailsofimplementationstartingwiththespecificationmodelingoftheUSBpowerdeliveryfirst.


7.4 USBpowerdeliveryspecificationinformationmodelingFirstthingweneedtodoismodelthespecificationdetails.Weareshowcasinglimiteditemsmodelingheretobalancebetweenthedetailsandeaseofdemonstratingconcepts.Wewillfocusonlyonthedetailsofthespecificationrequiredforimplementingtheitemswehavepickedinsection7.3.Hereishowwewillgoaboutdescribingtheimplementationofspecificationmodeling.Wewillfirstshowsomemindmapsforhigh-levelviewofinformationcapturedandthenshowthecorrespondingimplementationinpythonforreference.High-levelspecificationinformationmodeledlooksasfollowing.

Mindmap1:Toplevelviewofhigh-levelspecificationinformationitemscapturedWehavecapturedtheallthethreecategoriesofmessages,someimportantevents,informationabouttimersandprotocolsequencesarecapturedasgraphs.Wewillkeepgoingdeeperineachofthesenodestoseehowtheyareimplemented.


7.4.1 MessagemodelingLet’sgofirstleveldeepintothemessagemodeling.Therearethreecategoriesofmessagesmodeled:control,dataandextended.Eachcategoryismodeledseparatelytomaintaintheclassificationseveninthegeneratedcode.Forexamplewhenweneedtocoverallthemessagetypeswecanhaveeithersinglecoverpointforallmessagesorthreecoverpointcoveringeachcategoryofmessages.Lateriseasywhenweanalyzetheresults.Althoughitmaybeadditionalefforttowriteitbutlet’snotforgetwewriteoncebutanalyzeresultsmultipletimes.

Mindmap2:AllthreetypesofmessagetypesandControlmessagestypesexpandedLetsexpandonelevelfurtherintooneofcontrolmessagetypecalledGotoMin.Notalltheinformationshownbelowismanuallyfilled.Someofitismanuallyfilledandsomeofitisautomaticallygenerated.

Mindmap3:GotoMinmessagestypesexpandedwithdifferentfields


Forexample,foreachmessagetypesdecodinginformationismanuallyfilledin“DECODE”buttheTX_SIGNAL_EXPRandRX_SIGNAL_EXPR,whichindicatehowtodecodetheeachmessagetype,arecreatedautomaticallyforallthemessagetypes.Sinceit’susedacrossmultiplecoverageitemsinsteadofcreatingeverytime,it’screatedoncestoredinthedatastructure.Nowlet’sgetintodetailsofhowtoimplementinpython.It’sasimpledictionary.Everymessagetypeiskey.It’spointingtoanotherdictionarycontainingfollowingkey-valuepairs.Thisinformationhastobefilledmanuallyreferringtothespecifications.Key DescriptionaboutvaluestoredDECODE ShowtohowtodecodethismessagetypeTX ListofwhichDUTtypescantransmitthistypeofmessageRX ListofwhichDUTtypescanreceivethistypeofmessageRESPONSE Ifitsrequesttype,listofallpossiblelegalresponsesREVISION ListofspecificationrevisionswhichsupportthismessagetypeTable2:Fieldsofthebasicmessagefieldmodeling

Snippet4:Pythonmodelingforthemessages

7.4.2 EventmodelingAllspecificationsidentifysetofkeyevents.It’sinterestingtocover,thesekeyeventstakingplaceatdifferentstates.Let’slookattheevents.Hereishigh-levelviewofsomeoftheeventscaptured.


Mindmap4:EventsimplementationexpandedIt’sanothersimpledictionary.Keysbeinguserdefinedeventnames.Itpointstoanotherdictionarycontainingtheinformationaboutevents.Key DescriptionaboutvaluestoredORIGIN LayerfromwhicheventisemittedEVENT Actualevent.Thinkofthoseweirdlookingstringnamesstarting

andendingwithdoubleunderscoreslikeVerilogmacroTable3:Fieldsofthebasiceventfieldmodeling

Snippet5:PythonmodelingfortheeventsThoseevents(EVENTfield)canbepointedtoRTLdesignsignalsortestbenchevents.

7.4.3 ProtocolsequencemodelingHerecomesajuicypart.Simpletreedatastructureisusedtorepresenttheprotocolsequenceasagraph.Beforewecangettothat,let’sbrieflyfamiliarizeourselveswiththepowernegotiationsequence.


Followingdiagramfromspecificationshowsthestepsofpowernegotiationsequence.Step1:SOURCEsendsitscapabilitiestoSINK

Step2:SINKselectsthepowerofferingthatsuitsit

Step3:SOURCEacceptstheSINKrequestforspecificofferingifitlikesit

Step4:SOURCEsendsnewpowerofferingisready.Dealisdone


Figure:4stepssuccessfulpowernegotiationsequenceAtStep3aboveshowsAcceptscenario,therearemultiplepossibleresponses.Ifwegobacktoourspecification-modelingtable,wecansee,RESPONSEindicatesmultiplepossibilitiestothissequence.Abovesequenceshowsonlyoneofthem.Buttocompletelymodelitweneedtocaptureallthepossibilitiesandgeneratethefunctionalcoverageforallpossiblesequences.

Snippet6:PythonmodelingforallpossibleRESPONSEtoRequestmessageLetslookathowthesimplepowernegotiationsequencewithallthesepossibilitiescanbecapturedinthetreedatastructure.ThisishowthetreeoftheprotocolsequenceswouldlooklikevisualizedinMindmap.LookattheorderingofMESSAGE_TYPEfieldsinthenodefirst.Theoneinmiddle(Mindmap6)isthecompletediagram.Theoneintop(Mindmap5)inzoomoffirsthalf(2/4steps)andoneinbottom(Mindmap7)isthezoomofsecondhalf(4/4steps)ofthegraph.

Mindmap5:Zoomoffirsthalfofpowernegotiationsequence(Seebelowforfullgraph)

Mindmap6:Graphofpowernegotiationprotocolpossibilities

Firsthalfzoom

Secondhalfzoom


Mindmap7:Zoomofsecondhalfofpowernegotiationsequence(seeaboveforfullgraph)Completegraphmaynotbeclearlyvisible.Usethatonlyforgettingthebigpicture.Fordetails,Ihavezoomedthemintwoparts.Firstpartisfromstarttothenodewhereitbranchesintothreeparts(firsttwosteps).Secondonecontainsthedetailsaboutallthethreepossibleoptions.Nowifweexpandthisgraphintosequencesitwillresultsinthreeuniquesequenceswiththefirsttwosteps(Step1,Step2fromFigure4))beingcommon.Justcatchthevisualviewwiththefollowingpicture.Iwillexplainbitofdetailsbelow.

Mindmap8:Possible3sequencesfromthepowernegotiationgraphThreesequencespossibleare:

• Sequence1:SourceCapabilities=>GoodCRC=>Request=>GoodCRC=>Reject=>GoodCRC(6nodes)

• Sequence2:SourceCapabilities=>GoodCRC=>Request=>GoodCRC=>Wait=>GoodCRC(6nodes)

• Sequence3:SourceCapabilities=>GoodCRC=>Request=>GoodCRC=>Accept=>GoodCRC=>PS_RDY=>GoodCRC(8nodes)

ImplementationofthesesequencesrequiresDUTtype(SOURCE,SINK)tobedefined.BasedontheDUTtypethedirectionofeachofthesemessagesgetsdecided.UsingtheDUTtypeappropriateeventispickedfromthenodeinformation.Key DescriptionaboutvaluestoredNODE_TYPE Typeofnode:ItcanbeMESSAGE,EVENTMESSAGE_TYPE IfthenodetypeisMESSAGEwhatisthetypeofMESSAGESINK ForSINKtypeofdevice,theeventtobeusedSOURCE ForSOURCEtypeofdevice,theeventtobeusedTable4:Fieldsusedinthenodesofthegraphs


Orderingofstepsofsequenceisextractedfromgraphsandeventinformationforimplementingeachstepisextractedfromthenodeinformationforfunctionalcoveragesequencesgeneration.Interestingpartofthesegraphsisnotjustabovesequencegeneration,butexpandingthesegraphstocovermanymorecasesalgorithmically.ForexamplewesawthateverymessagetransmittedrequiresaconfirmationmessagecalledGoodCRC.Nowweknowwhereverthereiswaitthereisgenerallyatimerassociatedtopreventinfinitewaiting.ForGoodCRCthereisCRCReceiveTimer.ForimportantsequenceswecanautomaticallyaddatimeoutnodewhereverthereisGoodCRCmessageisinvolved.Thisallowscontrollingthequalityofverificationonspecificsequencesbycontrollingtheverbosityofcoveragegenerated.Itsdemonstratedinsection7.5.8.

7.5 ExecutablecoverageplanNowthatwehavecapturedallthespecificationinformation,let’sjumpintothehowtousethespecificationmodelforfunctionalcoveragegeneration.Wewillimplementallthe7coverageitemsselectedinSection7.3.Inearlyitems,wewillputinlittlebitmoreexplanation.Butaswegoalong,wewillreduceitasyoustartgettingfamiliar.Foreachitemwewillbrieflyexplainwhattheitemissupposetocover,thenshowtheinputpythoncodesnippetforcoverageitemimplementationandgeneratedfunctionalcoverage.WewillbrieflyexplaintheAPIsusedintheinputandpointtoanythingspecifictobeobservedinthecovergroups.Ideahereisnottoexplaineverythingdetailofthecodebutatthesametimestaygroundedtogiveaflavorofcodetomakeconceptsconcretebyshowinghowyoucangoaboutimplementations.Covergroupsandcoverpointsneedadditionallyneedwhattosampleandwhentosamplethatneedstobeprovidedasseparateinput.ThiscaneitherbetappedfromtestbenchvariablesorfromRTL.Thispartisskippedaswehavealreadyexplaineditinsection5.5.

7.5.1 GlobalconfigurationTokeepitsimplethecoveragemodelismadeconfigurableatgloballevelacrossallcovergroupsforsomeparametersbutitcanbedoneeasilyatperitemlevelaswell.Foralltheexampleitemsweareusingfollowingconfiguration.Whatitmeans?


Covergroupswillbegeneratedfor• DUTsupportingonlyRevision2.0ofspecification(Selectedthisbecause

numberofmessagesarelesseranditseasyshowsnippets)• DUTTypeofSINK• Qualityofcoveragesettohighestlevel(Thisconceptwillbe

demonstratedinthesection7.5.4)

Snippet7:Globalconfiguration

7.5.2 Item1:CoveralltransmittedprotocolmessagesHerewewanttocoveralltransmittedmessagestypespossibleacrossallthreecategoriesforspecifiedDUT_TYPEandDUT_REV_SUPPORT.Input:Pythoncodeforcoverageitem

Snippet8:PythonimplementationtocoveralltransmittedprotocolmessagesSo,What’shappening?TheAPIget_bins_for_matching_msgsreturnsthebinspercategoryofmessageslisted(argument:[“CONTROL”,“DATA”,“EXTENDED”])thatcanbetransmitted(argument:“TX”)byDUTofthetypeSINK(Globalsetting:DUT_TYPE)andsupportingtheUSBpowerdeliveryrevision2.0(GlobalsettingDUT_REV_SUPPORT).TheAPIadd_cp_for_msg_typescratesandaddsthecoverpointsandbinstothecovergroup.Alongwithbinspercategory,ittakesinwhattobesampledandwhentobesampledinformationaswell.BothoftheseAPIsinternallyusetheobjectorientedfunctionalandstatisticalcoveragecodegenerationAPIsthatarepartofcuriosityframework.Ifyouseethegeneratedoutput,thereisonecovergroupwithtwocoverpoints.ThereiscoveporinteachforCONTROLandDATAmessagecategories.Encodings


andnamesofbinsinthegeneratedoutputarederivedfromthespecificationinformationcapturedinthesnippet4.Output:Covergroupgenerated

Snippet9:GeneratedSystemVerilogCovergroupforalltransmittedprotocolmessagesDidyounotice?Messagecategoriespassedincludes“CONTROL”,“DATA”and“EXTENDED”.Butinthegeneratedoutputwedon’tseeEXTENDEDmessages–Why?That’sbecausetheUSBPowerdeliveryrevision2.0doesnotsupporttheEXTENDEDmessagetypes.Ifweswitchthesupportedrevisiontypeto3.0thentheEXTENDEDmessagetypeswillbeaddedasanothercoverpoint.BasedontheDUTtypeandsupportedrevisionsthecoverpointsandbinswillautomaticallychangeforeachmessagecategories.Isn’tthatsomething?

7.5.3 Item2:AllreceivedprotocolmessagesThisitemselectedtodemonstratehoweasyitistoswitchdirectioninthecoverageimplementationfromalltransmittedtoallreceivedmessagescoverage.Input:Pythoncodeforcoverageitem

Snippet10:Pythonimplementationtocoverallreceivedprotocolmessages


Everythingissameascoveringalltransmittedmessagetypes,exceptdirectionsensitiveinformationchangedtoreceive.Whentosamplechangestoreceivemessageevent.ToAPIget_bins_for_matching_msgswechangethedirectionasRXTheAPIadd_cp_for_msg_typeschangestheinformationastobesampledtoqualifiedreceivedmessage.Output:Covergroupgenerated

Snippet11:GeneratedSystemVerilogCovergroupforallreceivedprotocolmessages

7.5.4 Item3:SomekeyeventsduringmessagetransmissionInthisitemwewanttocoversomekeyeventstakingplacewhiledifferentmessagestypesarebeingtransmitted.Manyoftheseeventsleadtosomeformabruptterminationofmessagebeingtransmittedfollowedbyrecovery.It’sinterestingtoseeifDUTcanrecovercorrectlyandoperatenormallyagain.Wewanttointroduceyoutoaninterestingconceptofqualityoffunctionalcoveragewiththisitem.Basicideaisdepthandwidthoffunctionalcoveragecanbeguidedbytheimportanceofparticularfeaturetocurrentproject.Nowyoucancoverthisitematthreelevelsofdetailsdependingonimportanceofthisfeatureforyourapplication:

• LEVEL3:Foreachofthemessageinallthreecategoriesofmessages.Resultsinverycomprehensivecoverage


• LEVEL2:Anyonemessageineachcategory.Herewearerelaxingthecoveragebutstillcoveringeachofcategory

• LEVEL1:Anyonemessageacrossallcategories.Completelyrelaxed.Thistypeofconfigurabilitycanalsohelpeasilychangeyourmindwhileclosingcoverage.OnecouldcloseonLEVEL1forinitialmilestoneandthengraduallyreachLEVEL3basedonscheduleandpriorities.Input:Pythoncodeforcoverageitem

Snippet12:PythonimplementationtocoverkeyeventsduringmessagetransmissionOutput:WithqualityLEVEL3Highestquality.Coverallthemessagesinallthecategories.

Snippet13:GeneratedSystemVerilogCovergroupforqualityLEVEL3


Mediumquality.Coveratleastonemessagepercategory.

Snippet14:GeneratedSystemVerilogCovergroupforqualityLEVEL2Lowestquality.Coveratleastonemessageacrossallthreecategories.

Snippet15:GeneratedSystemVerilogCovergroupforqualityLEVEL1

7.5.5 Item4:ForalltransmittedmessagesgettingallpossiblevalidresponsesInthespecificationtablewehadcapturedforallthemessageswhatarethepossiblevalidresponses.ForexamplechecktheSnippet6forinformationcapturedfortheRequestmessagetypeinfieldRESPONSE.ThisinformationisutilizedintheAPIsbelowtogeneratealltherequestandresponsepairs.Input:Pythoncodeforcoverageitem

Snippet16:Pythonimplementationtocoveralltransmittedmessagesgettingallpossiblevalidresponses


Output:Covergroupgenerated

Snippet17:GeneratedSystemVerilogCovergroupforalltransmittedmessagesgettingallpossiblevalidresponses

7.5.6 Item5:ForalltransmittedmessagesunexpectedacknowledgementTheoutputcoverageshownhereisofLEVEL3quality.Meaning,allpossibleunexpectedresponsesareconsidered.Onecoulddebatethatcrosseswithsomecomplexexclusionbinscouldalsobeused.Rememberwewritecoverageoncebutanalyzeresultsmultipletimes.Sowritingcoveragewhichmightbeelaboratetowritebuteasiertoanalyzetheresultsthefasterwillbewellworththeeffort.Howeverpleasenoteelaborateworkifyouaremanuallywritingitbutifyouaregeneratingyoucanmakeiteasilysuityourpreferences.Alsoanyitemsmissedorspecificationsmisinterpretedareeasytofixinthespecificationtables.Itautomaticallyreflectsingeneratedcoderatherthanhavetopainfullyreanalyzetheintenttofixthebinsandexclusionbinsofthecrosses.Input:Pythoncodeforcoverageitem

Snippet18:Pythonimplementationtocoveralltransmittedmessagesgettingunexpectedacknowledgement


Output:Covergroupgenerated

Snippet19:GeneratedSystemVerilogCovergroupforalltransmittedmessagesgettingunexpectedacknowledgement

(…)Line95to180:Binsofcp_CONTROL_msg_types


7.5.7 Item6:PowernegotiationsequenceWehavealreadyseenthepowernegotiationprotocolsequenceanditsgraphintheSection7.4.3.Herewewilllookatitspythonimplementation.Weareusingsimpletreedatastructuretocapturethegraph.Input:GraphofpowernegotiationscenariosThisisthetreedatastructurerepresentationoftheprotocolsequencegraphshowninmindmap6.

Snippet20:PythonimplementationforgraphofpowernegotiationprotocolpossibilitiesOutput:CovergroupspersequenceSequencescanbecoveredwiththetransitionbinsbutit’sveryrestrictiveintermsofsampling.Wecoverthesequencewiththeadditionalstatemachinelikegluelogicin-ordertousethecovergroupconstruct.SystemVerilogassertionsdoprovidesomelevelflexibilityforsequencecoveragebuttaxingwhiledebugging.Asimplestatemachineisgeneratedasgluelogictotrackeachstepsofthesequence.Thispieceofgeneratedcodeisnotincludedtokeepfocusonthecoverage.Binperstepallowsveryclearideaofhowfarthesequencehadprogressedinregression.ThistypeofinformationisdifficulttofindoutwiththeSystemVerilogassertionbasedcoverage.AlsothestateofstepstrackingstatemachinegeneratedcanbeaddedtothewavesalongwiththeRTLmakingthecoverageholesdebugeasieraswell.Threesequencesgeneratedare:

• power_nego_protocol_SINK_0(binsfor8steps):SourceCapabilities=>GoodCRC=>Request=>GoodCRC=>Accept=>GoodCRC=>PS_RDY=>GoodCRC(8steps)

• power_nego_protocol_SINK_1(binsfor6steps):SourceCapabilities=>GoodCRC=>Request=>GoodCRC=>Wait=>GoodCRC(6steps)

• power_nego_protocol_SINK_2(binsfor6steps):SourceCapabilities=>GoodCRC=>Request=>GoodCRC=>Reject=>GoodCRC


Snippet21:GeneratedSystemVerilogCovergroupfor3generatedsequenceofpowernegotiationprotocolpossibilities(observenumberofstepstocorrelate)

7.5.8 Item7:TimeouterrorinjectionwithinpowernegotiationsequenceAswehadseen,foreveryrequestorresponsemessagestheGoodCRCisexpectedtoconfirmitssuccessfulreceptionbypeer.ThereisCRCReceiveTimerdefinedbythespecificationswhichisstartedwhenrequestorresponsearetransmittedandstoppedwhenthecorrespondingGoodCRCacknowledgementisreceived.PowernegotiationsequenceisoneoftheveryimportantsequencesoftheUSBpowerdeliveryprotocol.Solet’ssaywedecidetocoverCRCReceiveTimertimeoutatallpossiblepointsinthissequencetoensurethissequenceisthoroughlyverified.Nowthistypeofcomprehensivecoveragemightbeveryeffortintensivetowritebutwithgraphsitcanbecreatedalgorithmicallywiththefollowingapproach.


Input:AlgorithmInthefollowingsnippetwearetraversingthegraphofthepowernegotiationsequenceandwherevertheGoodCRCisnodeisfoundweareaddinganadditionaltimeoutnodetoparentnodeofGoodCRCnode.

Snippet22:PythonimplementationtoalgorithmicallyaddCRCReceiveTimeoutforallGoodCRCmessagewaitsOutput:MindmapofmodifiedgraphNowyoucanseewitheveryGoodCRCnode,aparallelnodeofCRCReceiveTimertimeoutnode(RED)hasbeenadded.

Mindmap9:AfteraddingCRCReceiveTimertimeoutnodetopowernegotiationgraphFromthisnewgraphautomaticallynewfunctionalcoveragesequencecanbegenerated.

Mindmap10:SequencesgeneratedafteraddingCRCReceiveTimertimeoutnodetopowernegotiationgraphWearejustshowingthemindmapsbutcodeonthesimilarlinesassimplepowernegotiationsequencewillgetgenerated.YoucanseeinthetimeoutsequencegraphstherearethreesequencesnotendinginREDblockarenormalpower


negotiationsequencescenarioswithouttimeout(whichwehadseenforpreviouslyforpowernegotiationsequenceitem).Theonesendingwiththeredblockarenewonesaddedalgorithmicallyforcoveringtimeoutateachstagesofthepowernegotiationsequence.Nowof-coursebasedonyourqualitygoalsyoucandecidetoaddtimeouttoonlycertainbranchesoraftercertaindepthintheprotocol.Possibilitiesareunlimited.Forcomplexprotocolsthisprovidesalotofflexibilitytodynamicallycontrolthequalityofcoveragegenerated.

8 ConclusionUSBpowerdeliveryprotocolprovidesatimecapsule.ItsoverallcomplexityislesserthanstandardUSB2.0butstillrelevantinthecurrenttimes.Wechooseitforitssimplicityandrelevance.USBpowerdeliverydoesexemplifytheproblemsofsimilarnaturefacedathighercomplexityinotherhigh-speedserialIOprotocols.Specificationtofunctionalcoveragepossibilityhasbeendemonstratedwiththiscasestudy.Forcomplexdesignsthisapproachcanmakewritingcomprehensiveandmaintainablefunctionalcoveragemodelseasierandfaster.Alsobeingabletodynamicallydefinelevelofqualityofcoveragecanhelptuneittoyourprojectpriorities.Ingeneralbymakingcoverageintentexecutable,itwillhelpyouchurnhigherqualityofdesignIPrevisionsthroughoutitslifecycle.

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