Synthesis of an 8051Like MicroController Tolerant to Transient Faults.docx
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Synthesis of an 8051Like MicroController Tolerant to Transient Faults.docx
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Synthesisofan8051LikeMicroControllerToleranttoTransientFaults
Synthesisofan8051-LikeMicro-ControllerToleranttoTransientFaults
Thispaperpresentstheimplementationofafaultdetectionandcorrectiontechniqueusedtodesignarobust8051micro-controllerwithrespecttoaparticulartransientfaultcalledSingleEventUpset(SEU).AspecificstudyregardingtheeffectsofaSEUinthemicro-controllerbehaviorwasperformed.Furthermore,afaulttoleranttechniquewasimplementedinaversionofthe8051.TheVHDLdescriptionofthefault-tolerantmicroprocessorwasprototypedinaFPGAenvironmentandresultsintermsofareaoverhead,levelofprotectionandperformancepenaltiesarediscussed.
1.Introduction
Theconstantimprovementsachievedinthemicroelectronicstechnologyallowthemanufacturingofverycomplexcircuits,substitutingboardsorevencomputersofthepast80’s.Nowadays,becauseofthemicroelectronicsadvances,traditionalapplicationsbecomecheaperandmorereliable,whilealargerangeofnewapplicationscantakeadvantageofintegrateddevicesbyusingtheso-calledembeddedsystems.Inallcases,architecturesarestronglybasedonsomekindofdataprocessor,suchasamicro-controlleroraDSPprocessingunit,forexample.
Thecontinuousdecreaseinthesemiconductordimensionsandinelectricalfeatures,leadstoanincreasingsensitivitytosomeeffectsoftheenvironment(ionizationduetoradiation,magneticperturbations,thermal,...)consideredminorornegligibleinthetechnologiesofthepast.Particularly,digitalcircuitsoperatinginspacearesubjecttodifferentkindsofradiation.However,someproblemshavealsobeenreportedforsomeEarthapplications,likeavionicssystems.
Radiationeffectscanbepermanentortransient.Permanentfaultsresultfromparticlestrappedatthesilicon/oxideinterfacesandappearonlyafterlongexposuretoradiation(TotalIonizationDose).Transientfaults(SingleEventEffects,SEE)maybecausedbytheimpactofasinglechargedparticleinsensitivezonesofthecircuit.
Dependingontheimpactlocation,twokindofSEEsaredistinguished:
SELs(SingleEventLatchups)andSEUs(SingleEventUpsets).SELsresultfromthetriggeringofparasiticthyristors(presentinCMOStechnologies)andprovokeshortcircuits,capabletodamagethecomponentbythermaleffectifthecircuitisnotpowered-offattime.SEUsareresponsiblefortransientchanges,calledupsetsorbitflips,inbitsofinformationstoredwithinanintegratedcircuit.Totalionizationdose(TID)andsingleeventlatch-up(SEL)effectscanbereducedtoacceptablelevelsusingsomeoftheexistingCMOStechnologies,forexampletheEpi-bulkCMOSprocess.However,SingleEventUpsets(SEUs)representradiationinducedhazards,whicharemoredifficulttoavoidinthespaceapplications,especiallyinhigh-densitysub-micronintegratedcircuits.Inthispaper,onlySEUfaultsarebeingconsidered.TheconsequencesofaSEUfaultdependonthenatureoftheperturbedinformation,rangingfromerroneousresultstosystemcrashes.ForcomplexcircuitslikeDSPprocessors,co-processors,microcontrollers,thesensitivitytoSEUcorrelatesstronglywiththeamountofinternalmemory(registers,memorybits,flip-flops,etc.)available.
Inthiscontext,itiscleartheneedforcircuitsimmunetoradiationeffects,mainlythoseworkinginspace,whereafaultcanimplythelostofmillionsofdollarsandyearsofwork.Moreover,itisextremelyimportanttoknowtheefficiencyofafault-toleranttechniquebeforethecircuitisinitsrealenvironment.
Thispaperaimsatinvestigatingtheefficiencyofafastprototypingdesignhardeningtechnique,whichfocusesongeneral-purposeprocessorarchitectures.Theproposedtechniqueismainlybasedontheinclusionoferrordetectingandcorrectingcapabilities.Areducedinstructionsetversionofawell-knownmicrocontroller,the8051fromIntel,waschosenasthetestvehiclefortheseresearches.Thischoicewasmotivatedbythefactthatthismicro-controlleriswidelyusedinspaceapplications.Thepaperisorganizedasfollows:
inSection2somerelatedworksarerevisited.InSection3theeffectsoftransientfaultsinamicro-controllerarepresentedalongwithatoolcapabletoemulatetherealprocessofaSEUfaultoccurrence.Theimplementationofahardened8051micro-controllerispresentedinSection4.Experimentalresults,concerningboththeperformanceintermsofareaoverheadandoperatingfrequency,andthesensitivitytotransientbitflips,aresummarizedinSection5.Section6bringssomeconsiderationsabouttheimplementationofaprototypeofthefault-tolerantcircuittobetestedinarealradiationenvironment.ConcludingremarksandfutureworkarediscussedinSection7.
2.RelatedWork
Solutionstoimplementafaulttolerantdevicewithrespecttotransientfaultscanbeconsideredatdifferentstepsofthedevicedevelopmentprocess.Themitigationsolutioncanbedividedin:
circuitlevel,whereaspecifictechnologyprocessforfabricationisused;designlevel,wherelogicstructuresaremodifiedtoachievetheSEUimmunity;systemlevel,wheremodificationsinthesoftwareareperformed.
InordertoavoidSEU,somemicroprocessormanufacturerssuchasIBM,areproposingmicroprocessorsintheSilicononInsulator(SOI)technology.However,thissolutionisstillveryexpensive.
Solutionsatthedesignlevel,liketriplemodularredundancy,arewidelyusedtocopewithtransienterrors,especiallyinrandomlogic.Thedrawbackofthissolutionsistheresultingareaoverhead.
Errordetectionandcorrectiontechniques(EDAC)havebeenusedinthelastfewyearstoincreasememoriesreliability.ExamplesofthesetechniquesareparitycheckandHammingCode.SomestudieshaveshownthecapabilitiesofusingerrordetectionandcorrectioninStateMachinesinsteadoftheuseofredundantflip-flopswithavoter.Nopreviousworkwasfoundonprotectingafullmicro-controllerusingEDACtechniques.Relatedworksrestricttotheuseofdetectionandcorrectiontechniquesonlyininternalmemories.
3.SEUEffectsinthe8051Microcontroller
Severalspaceapplicationsarebasedonthe8051microcontroller,becauseithasagoodtradeoffintermsofcost,areaoccupation,performanceandsoftwarecompliance.Inordertoimplementefficientfaulttolerancetechniques,averyaccuratemeasureaboutthelocalizationandtheeffectsofSEUfaultsinthecircuitisnecessary.Thismeasureisobtained,inthiswork,byrunningcontrolledfaultinjectionexperimentsonanexistinghardwarebuiltonastandard8051.Duringtheexperiments,themicro-controllerexecutesaprogramdesignedtoprovideworstcaseconditionsintermsofexposingthecircuittotheeffectsofSEUs.Indeed,theselectedprogram,a6×6matricesmultiplicationwithboththeoperandandresultmatricesresidentwithinthe128byteinternalSRAM,occupiesmostoftheinternalmemory,whichconstitutesthemaintargetofSEU.
3.1.BasicPrinciplesofTHESICTester
Testingintegratedcircuitsinasevereradiationenvironmentpriortotheiruseinoperationalsystemsisamandatorystepandwillhelptoreducetheprobabilityoffailuresinfutureapplications.Thesensitivityevaluationofacircuitwithrespecttoradiationcanbedone:
1)bytheanalysisofflightdataissuedfromspacecraftoperatingintheactualenvironment;2)bygroundtestingusingelectronbeams;or3)byfaultinjectioninthecircuit.
Screeningtestsaremandatorytopredicterrorratesinthefinalenvironment.Theyconsistonagroundtestbasedontheexposureofthestudiedpartstosimulatedradiationconditionsissued,forinstance,fromparticleaccelerators.
3.4.ObtainedFaultInjectionResults
Detailedstepsofthisexperimentcanbefoundin.Inthissection,theresumedresultsaredescribedasamotivationtotheprotectionofthemicroprocessor.Atotalof12245randomsingleCEUswereinjectedwhileexecutingthematricesmultiplicationprogram.Accordingtotheconsequencesofinjectedupsetsattheprogramexecutionlevel,obtainedresultswereclassifiedintothethreefollowingtypes:
toleratederrors;resulterrors;andlostofsequence.
Thefirstgroup,toleratederrors,correspondstothosebitflipsinjectedonmemoryelementswhosecontentisnotrelevantfortherestoftheprogramexecutionwhenthefaultoccurs.Forinstance,itcanbearegisternotusedafterthefaultoccurrenceoraregisterthatwillbewrittenafterthefaultoccurrence,thus“erasing”thefault.
Allinjectedfaultsforwhichexpectedandobtainedprogramresults(theresultingmatrixinourcase)differinatleastonebitareconsideredasleadingtoresulterrors.Finally,caseswhereafterfaultinjection,noanswerisgotfromtheprocessor,areclassifiedinthelostofsequencegroup.Thesemalfunctionsareunrecoverable,needingahardwareresettorestartprogramexecution.Tables1and2summarizetheexperimentalresults.Foreachtypeoferror,thecorrespondingpercentageoftheidentifiedconsequencesattheprogramexecutionlevelisgiven.
ReferringtotheTable1,onecannotethatnearlyhalf(47.24%)ofthetotalnumberofinjectedfaultscausederrorsontheresultsofthisapplication,whileonly2.8%ofthemcausedthelostofsequence.Fromthe5784faultsthatresultedinerrors,98.5%(5700)ofthemcausedbitflipsintheinternalmemory,whileonly1.5%(84faults)affectedtheSFRs.ThisdemonstratesthattheinternalmemoryshouldbeprotectedagainstSEUfaultstogetareliableoperation.Furthermore,despitethereducednumberofbitsonSFRs(SpecialFunctionRegisters)usedinthisapplication(88bits=108-bitregisters+PC)comparedtothoseusedinthei
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