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AnoverviewofSIissuesMu-ShuiZhangSun-YatSenUniv.1.ReflectiononasinglenetAnetismadeupofallthemetalconnectedtogetherinasystem.Forexample,theremaybeatracegoingfromaclockchip’soutputpintothreeotherchips.Eachpieceofmetalthatconnectsallfourofthesepinsisconsideredonenet.Inaddition,thenetincludesnotonlythesignalpathbutalsothereturnpathforthesignalcurrent.Signalqualityonasinglenetdependsasmuchonthephysicalfeaturesofthesignaltraceasonthereturnpath.Exampleoftwonetsonacircuitboard.Allmetalconnectedtogetherisconsideredonenet.Note:Onenethasasurface-mountresistorinseries.Anyfeaturethatchangesthecrosssectionorgeometricalshapeofthenetwillchangetheimpedancethesignalsees.Wecallanyfeaturethatchangestheimpedanceadiscontinuity.Everydiscontinuitywillcausethesignaltobedistortedfromitsoriginalpristineshape,tosomeextent.Forexample,someofthefeaturesthatwouldchangetheimpedancethesignalseesincludethefollowing:1.Aline-widthchange2.Alayerchangethroughavia3.Agapinreturn-pathplane4.Aconnector5.Abranch,tee,orstub6.TheendofanetExample:Ringinginanunterminatedlineandgoodsignalqualityinasource-seriesterminatedinterconnectline.ThePCBtraceisonlytwoincheslongineachcase.Scaleis1v/divand2nsec/div.SimulatedwithMentorGraphicsHyperlynx.ReflectionsimulationbyHyperlinxReflectioninSDRAMReflectioninPCIcontrolsignal2.CrosstalkintwolinesWhenonenetcarriesasignal,someofthisvoltageandcurrentcanpassovertoanadjacentquietnet,whichisjustsittingthere,mindingitsownbusiness.Eventhoughthesignalqualityonthefirstnet(theactivenet)isperfect,someofthesignalcancoupleoverandappearasunwantednoiseonthesecond,quietnet.Simultaneousswitchingnoise(SSN)Toptrace:measuredvoltageonactivelinesinamultilinebuss.Bottomtrace:measurednoiseononequietlineshowingtheswitchingnoiseduetomutualinductancebetweentheactiveandquietnetsinthepackage.3.PowernoiseNoiseisgenerated,andisaproblem,inmorethanjustthesignalpaths.Itcanalsobeadisasterinthepower-andground-distributionnetworkthatfeedseachchip.Whencurrentthroughthepower-andground-pathchanges,aswhenachipswitchesitsoutputsorcoregatesswitch,therewillbeavoltagedropacrosstheimpedanceofthepowerandgroundpaths.Thisvoltagedropwillmeanlessvoltagegetstothechip,whichwillseeadecrease,orcollapse,ofthevoltagebetweenthepowerandgroundrails.TIPThegoalindesigningthepowerandgrounddistributionistotrytominimizetheimpedanceofthepower-distributionsystem(PDS),sometimesalsocalledthepowerdeliverynetwork(PDN).MeasuredVccvoltageatthreepinlocationsonthepackageofamicroprocessorjustcomingfroma“stop-clock”state.Nominalvoltageissupposedtobe2.5V,butduetovoltagedropsinthepowerdistributionsystem,thedeliveredvoltagecollapsesbyalmost125mV.Ifweunderstandhowthephysicaldesignoftheinterconnectsaffectstheirimpedance,wecanoptimizethedesignofthePDNforlowimpedance.Aswewillsee,designingalow-impedancePDNmeansincludingfeaturessuchasthefollowing:1.Closelyspacedadjacentplanesforthepowerandgrounddistributionwithasthinadielectricbetweenthemaspossible,nearthesurfaceoftheboard2.Multiple,low-inductancedecouplingcapacitors3.Multiple,veryshortpowerandgroundleadsinpackages4.On-chipdecouplingcapacitanceMeasuredrailvoltagenoiseonasmalldigitalboard,withvariousmethodsofdecoupling.TheworstcaseisnodecouplingcapacitorsonFR4.Thebestcaseisthe3MC-Plymaterial,showingvirtuallynovoltagenoise.CourtesyofNationalCenterforManufacturingScience.Scaleis0.5v/divand5nsec/div.4.ElectromagneticInterference(EMI)Withboard-levelclockfrequenciesinthe100-MHzto500-MHzrange,thefirstfewharmonicsarewithinthecommoncommunicationsbandsofTV,FMradio,cellphone,andpersonalcommunicationsservices(PCS).Thismeansthereistheveryrealpossibilityofelectronicproductsinterferingwithcommunicationsunlesstheirelectromagneticemissionsarekeptbelowacceptablelevels.5.TwoImportantSignalIntegrityGeneralizationsTwoimportantgeneralizationsshouldbeclearfromlookingatthefoursignalintegrityproblemsabove.First,eachofthefourfamiliesofproblemsgetsworseasrisetimesdecrease.Allthesignal-integrityproblemsabovescalewithhowfastthecurrentchangesorwithhowfastthevoltagechanges.ThisisoftenreferredtoasdI/dtordV/dt.ShorterrisetimesmeanhigherdI/dtanddV/dt.Thesecondimportantgeneralizationisthateffectivesolutionstosignalintegrityproblemsarebasedheavilyonunderstandingtheimpedanceofinterconnects.Ifwehaveaclearintuitivesenseofimpedanceandcanrelatethephysicaldesignoftheinterconnectswiththeirimpedance,manysignal-integrityproblemscanbeeliminatedduringthedesignprocess.6.TrendsinElectronicProducts:towardever-higherclockfrequencyHistoricaltrendintheclockfrequencyofIntelprocessorsbasedonyearofintroduction.Thetrendisadoublinginclockfrequencyeverytwoyears.SourceisIntelCorp.RisetimeAstheclockfrequencyincreases,therisetimeofthesignalsmustalsodecrease.Eachgatethatreadseitherthedatalinesortheclocklinesneedsenoughtimewiththesignalbeingeitherinthehighstateorthelowstate,toreaditcorrectly.Thismeansonlyashorttimeisleftforthesignaltobeintransition.Weusuallymeasurethetransitiontime,eithertherisetimeorthefalltime,asthetimeittakestogofrom10%ofthefinalstateto90%ofthefinalstate.Thisiscalledthe10–90risetime.TIPAsallfabsm