IDT-Clock-interface-&Termination

©2009IntegratedDeviceTechnology,Inc.IDT–DistconferenceDecember2009SunnyLiangClockTermination&PCBLayoutGuidelineCONFIDENTIALPAGE2CONFIDENTIAL●Circuitbasic:Ohm’sLaw,KCL,KVL,VoltageDividingRule,VoltageDividingRuleThevenin’sTheorem,Norton’sTheorem●Termination:Single-endsignal:Differentialsignal:●LevelTranslating●PCBlayout●AddendumCONFIDENTIALPAGE3CONFIDENTIAL●Voltage:EnergyneedtoseparatetheunitCharge.v=dw/dq●Current:Currentisrefertothechargeflowspeed.i=dq/dt●Capacitor:Thecontainerofthecharge.TheStoredChargeundertheunitVoltage.C=dq/dvCONFIDENTIALPAGE4CONFIDENTIAL=V/IKCL(Kirchhoff`sCurrentLaw)Totalcurrentflowthroughapointequal0:0outinIIOhm’sLawKVL(Kirchhoff`sVoltageLaw)Totalvoltageinaclosedringis00iVCONFIDENTIALPAGE5CONFIDENTIALVoltageDividingRule2112RRRVVR2121RRRVVRBasicTheoremCONFIDENTIALPAGE6CONFIDENTIALCurrentDividingRuleXPTXRRIIRP：EquvalientresistenceofparallelcircuitRX：ThespecificresistorofIXBasicTheoremCONFIDENTIALPAGE7CONFIDENTIALVoltagesourceBasicTheoremCurrentsourceR0=0,idealvoltagesourceR0=0,idealcurrentsourceCONFIDENTIALPAGE8CONFIDENTIALVoltagesource-CurrentSourceConversions*Is=V0/RCONFIDENTIALPAGE9CONFIDENTIALThevenin`sTheoremVTH：VABwhenloadopenRTH：EquivalentresistencebetweenABwhenTakeofthevoltagesourceandCurrentsouce.(Vsource=short，Isource=open).BasicTheoremCONFIDENTIALPAGE10CONFIDENTIALThevenin`sequavilentexample:OriginalcircuitEquavilentVoltageEquavilentReisistorEquavilentResultCONFIDENTIALPAGE11CONFIDENTIALNorton`sTheoremBasicTheoremVTH：VABwhenloadopenRTH：EquivalentresistencebetweenABwhenTakeofthevoltagesourceandCurrentsouce.(Vsource=short，Isource=open).CONFIDENTIALPAGE12CONFIDENTIALNorton`sequavilentexample:OriginalcircuitTheveninEquavilentRortonEquavilent*INo=V0/R=3.75mA3.75mACONFIDENTIALPAGE13CONFIDENTIALThevenin-NortonConversionsTHNRRRRVITHNBasicTheoremCONFIDENTIALPAGE14CONFIDENTIAL●Circuitbasic:●Termination:Single-endsignal:Differentialsignal:●LevelTranslating●PCBlayout●AddendumCONFIDENTIALPAGE15CONFIDENTIALHighSpeedSignalDefinition:Trise4*Td*Td:TracedelaybetweenSourceandEndPCBTrace:MicroStrip:Td:140-180ps/InchCharacterImpedance:50ohmStripline:Td:180ps/InchCharacterImpedance:50ohmCONFIDENTIALPAGE16CONFIDENTIALWhyweneedTermination?PreventSignalreflectionFiltertheunwantedpulseoccuring.General:BesttoalwaysterminatethetraceNocircumstancetoterminationTd1/6*TriseTd:TracedelaybetweenSourceandEndTrise:ClockrisetimeCONFIDENTIALPAGE17CONFIDENTIAL:(addendum)SingleEndsingletermination:Rd:driverimpedance.Theloweroutputimpedance,Thefastersignlewilltransition.1.SourceTermination–Seriestermination.2.EndTermination:StandardTerminationParallelEndTerminationACTerminationCONFIDENTIALPAGE18CONFIDENTIAL=RS+R0=50W●Advantage:EasytoimplementDoesnotneedaconstantDCcurrentEasytocalculatethenecessaryresistorvalue.So,Customerusuallyadoptthistermination.●Disadvantage:Hasreflectiononthetrace.ExtendtheTrise/TfallingAbsorbthereflectedsignalRsasclosetothedriveraspossible–inordertoattenuatethereflectionThedelaybetweenthedrivertotheRsshouldbe1/10*Trise.VV=+V/2V/2V/2V/2CONFIDENTIALPAGE19CONFIDENTIAL●Advantage:GoodTrise/Tfalling,Littleornoreflection●Disadvantage:DifficulttogetVDDO/2,Lessopportunitytouse.Thefollowingconditionsneedtobemet:1.VoltageatAisclamptoVddo/22.Impedanceseenfromdriverbekeptcloseto50ohmforimpedancematching.CONFIDENTIALPAGE20CONFIDENTIAL●Advantage:Goodeffection.Littleornoreflection●DisadvantageButConstantDCcurrentontheR1&R2,powerdissipationCONFIDENTIALPAGE21CONFIDENTIAL=50OhmReceiverRo~7OhmLVCMOS_DriverR1Zo=50SetCsothattheRCtimeconstantismuchgreaterthanthatoftheperiodoftheclock:TypicalvalueforC1:10pF.Ensurethatthecapacitordoesn’tsignificantlydischargeduringthehighorlowportionsofourclockcycleR1=Z0●Advantage:DoesnotneedaconstantDCcurrent,So,Lesspowerdissipation●Disadvantage:TheCapwilldelayTrise/Tfalling,Doesn’tworkwellonthelongtrace.LessopportunitytouseCONFIDENTIALPAGE22CONFIDENTIAL●BothSideofthebusallcandrivethebus●Recommendtousethesourcetermination.●Bothside:Rs+R0=50ohmCONFIDENTIALPAGE23CONFIDENTIAL=R0-ZdTwoloadisverycloseeachotherThesublengthdelaymustbefasterthan6*TriseCONFIDENTIALPAGE24CONFIDENTIAL●ShortTrace:PulsewidthclosetoTracedelay,bettertousesoucetermination.●LongTrace:PulsewidthmuchshorterthanTracedelay,Loadterminationhasbettereffect.Morethanonesignaltransportingonthetrace,thereflectsignalmustbeattenuatedfromtheloadsideincasetointerferethesignal.*LongTrace:TracedelayTrise/Tfalling/2SingleEndtermination:CONFIDENTIALPAGE25CONFIDENTIAL