Understanding and Eliminating EMI_NS

TL/DD12857UnderstandingandEliminatingEMIinMicrocontrollerApplicationsAN-1050NationalSemiconductorApplicationNote1050RobinGetzBobMoeckelAugust1996UnderstandingandEliminatingEMIinMicrocontrollerApplications1.0ABSTRACTIntoday'sworld,withincreasingnumbersofbothfixedandmobileelectronicdevices,electromagneticcompatibility(EMC)isbecomingacriticalissue.Disastrous,ifnotannoy-ing,resultsoccurifasystem,subsystemorcomponentin-terfereswithanotherthroughelectromagneticmeans.TheEMCproblemisfirstexplained,andthenthebasictheoryispresentedwithanexplanationonhowtouseittocontrolanEMCproblemduringtheinitialdesignphase.ThemethodsthatsomesiliconmanufacturersareusingtocontrolEMCandhowthesechangesaffectoursystemswillbeexam-ined.Atotalsystem(automobileradio)willbeexaminedindetailtoprovetheeffectivenessofaEMCreducedCOP8TM(8-bitmicrocontroller).OveralldesignguidelineswillalsobegivensothatthedesignercanminimizeEMIbeforeitbe-comesaproblem.2.0INTRODUCTIONANDBACKGROUNDElectromagneticInterferencewithPoweredWheelchairsAugust26,1994FDAisreceivinginquiriesaboutreportsthatelectromagnet-icinterference(EMI)cancausesomepower-drivenwheel-chairsandscooterstomoveunexpectedly.TheagencyhasinvestigatedthismatteranddeterminedthatEMIcancauseunexpectedmovementinsomepower-drivenwheelchairswhentheyareturnedon.Notallbrandsandmodelsofpow-erwheelchairsandscootershavethisproblem.SomehavegreaterimmunitylevelsthanothersthatmayprotectagainstEMI.CommonsourcesofEMIincludecellularphones,CBradios,TVandradiostations,amateur(HAM)radiosandpolice,fireandambulanceradios.Afterreceivingreportsofproblems,FDAtestedsamplewheelchairsandscootersinitslaboratoriesandobtainedinformationfrommanufacturersandusersaboutpossibleEMI-relatedincidents.Asaresultofitsreview,theagencylastMayaskedwheelchairmanufacturerstotakestepstoprotectpoweredwheelchairusersfromthepotentialhaz-ardsofEMI.FDArequiredallfirmstoclearlylabelwheel-chairsandscooterswiththeimmunitylevel,orelsestatethattheimmunitylevelisnotknown.Italsorequiredthemtolabelwheelchairsandscooterstowarnusersofthepoten-tialhazardsofEMI.Theagencyalsoaskedmanufacturersto:1.Establishaminimumrecommendedimmunitylevelof20Vpermeterforallnewmotorizedwheelchairsandscooters.Thiswouldprovideareasonabledegreeofpro-tectionagainstthemorecommonsourcesofEMI.2.UndertakeaneducationalcampaigntoinformusersandthosewhocareforthemabouttherisksofEMIandwaystoavoidit;and3.SolicitreportsofpossibleEMIincidentsandcontinuetomonitorthefullscopeoftheproblem.FDAfirstlearnedofapossibleproblemwithmotorizedwheelchairsfromacomplaintinJune1992.Asaresult,theagencybegantestingsamplewheelchairsinitslaboratories.Thosetests,onseveralbrandsofpower-drivenwheelchairsandscooters,revealedthatmostÐbutnotallÐofthosesampledweresusceptibletointerferenceatvariousradiofrequencies.Insometests,thebrakesreleased.Inothers,thewheelsmoveduncontrollably.Sometimesbothoccurredatonce.Theextenttowhichthishappensinactualuseisnotknown.Inearly1993,FDAbeganinspectingfacilitiesofalldomesticandforeignmotorizedwheelchairandscootermanufacturerstogatherinformationontheproblem.Theagencyreviewedwarrantyandcomplaintfilesandlookedatanymanufacturingpracticesthatcouldcontributetothistypeofproblem.Theinspectionsrevealednumerouscom-plaintsabouterratic,unintentionalwheelchairmovement,sometimesresultingininjuries.Itisunclear,however,exact-lywhatcausedthechairstomoveinthosecases.Inaddi-tiontoEMI,unexpectedmovementcanbecausedbyfailureofanelectroniccomponentorbyusererror.InMay1993,FDAsentletterstoallU.S.manufacturersask-ingthemtoprovideanyinformationtheyhadaboutpossibleproblemswithradiowaveinterference,includingcom-plaints,reportsofinjuriesandstudiesonpower-drivenwheelchairs.InJuly1993,theagencyalertedconsumergroupsthatrepresentwheelchairusersaboutthepossibleproblemandrequestedsimilarinformation.Anestimated10,000motorizedwheelchairsand50,000motorizedscoot-ersaresoldannuallyinthiscountry.ÐFDAReport,August1994ThecontrolandminimizationofEMCisatechnologythatis,outofnecessity,growingrapidly.Manufacturersandregula-toryagencieshavemadeeffortstocontrolthisproblembuttheissueofmeetingtheseevertighteningspecificationsislefttothesystemdesigner.EMCcannolongerbeafix,designedinatthelastmoment,butmustbeadirectedeffortbetweenthecircuitdesigner,layoutengineer,softwareengi-neerandpurchaser.ThispaperwillexaminewhatcanbedonetoreducethechanceofhavingaproductfailEMCapproval.COP8TMisatrademarkofNationalSemiconductorCorporation.C1996NationalSemiconductorCorporationRRD-B30M106/PrintedinU.S.A.(EM)theory.Tobegin,lookatwhatIcallthemapoftheEMCworld,TableI.TheEMCproblemisbetweenanemis-sionculpritandsusceptiblevictimwhicharecoupledthroughameans,eitherradiationthroughspaceorconduct-edthroughwires(Figure1).WecanmapouttheEMCprob-lemcaseswithcolumnsheadedbyradiationandconduc-tion,androwsheadedbyemissionandsusceptibility,togetfourcasesabbreviatedasRE,CE,RS,CS.TABLEI.ElectroMagneticC