100Switches PHY Speed and Duplex Configuration

M9999-101404October2004ApplicationNote124PHYSpeedandDuplexConfigurationMicrel10/100Switches/PHYsIntroductionTheFastEthernetspeedandduplexconfigurationprocessisoftentakenforgranted.MostofthetimewhenanEthernetcableisconnectedbetweentwoEthernetdevices,alinkisautomaticallyestablishedwithmatchingspeedandduplexsettings.However,ifthereisamismatchwitheitherorbothofthesesettings,theresultiseithernolinkorpoornetworkperformance.Anunderstandingofthespeedandduplexdetectionmechanismswillhelptoresolvethesesettingconflicts.ThisapplicationnoteprovidesaquickoverviewofhowspeedandduplexsettingsaredeterminedandconfiguredforFastEthernetdevices.First,abriefintroductiontosomekeyconfigurationcomponents,autonegotiation,paralleldetection,andforcemode,willbepresented.Adiscussionofconfigurationmodeswillfollow.Andlast,linkresultsforallcombinationsofconfigurationmodesbetweentwoEthernetdeviceswillbeprovided.AllMicrelFastEthernetPHYsandswitcheswithintegratedPHYssupportautonegotiationandparalleldetection.Inaddition,allofthesedevices,exceptfortheKS8997,supportforcemode.ConfigurationComponentsFastEthernetspeedandduplexconfigurationsaredeterminedusingthreekeycomponents:autonegotiation,paralleldetection,andforcemode.AutoNegotiationTheauto-negotiationprotocol,definedinsection28oftheIEEE802.3uspecification,allowstwoEthernetdevicestoexchangeinformationabouttheircapabilitiesoveralinksegment.Speedandduplexcapabilitiesaretwoparametersconveyedusingtheprotocol.AnEthernetdeviceadvertisesitsspeedandduplexcapabilitiestoitslinkpartnerusingFastLinkPulse(FLP)signals.FLPsareburstsofNormalLinkPulse(NLP)signals.ANLPisusedforlinkdetectionin10BASE-Tsystems,andisshowninFigure1.TheprotocolenablesanEthernetdeviceanditslinkpartnertoselectthehighestprioritysettingamongtheircommoncapabilities.AllofMicrel’sFastEthernetSwitches/PHYssupportandadvertisethefollowingspeedandduplexcapabilities,listedfromhighestprioritytolowestpriority:100BASE-TX/FullDuplex100BASE-TX/HalfDuplex10BASE-T/FullDuplex10BASE-T/HalfDuplexIfthereisnocommoncapabilitydetectedateitherendofthelink,thennolinkwillbemade.Detailinformationontheauto-negotiationprocesscanbefoundintheIEEE802.3uspecification,inEthernettextbooks,andatnetworktechnologyandequipmentvendors’websites.ParallelDetectionTheparalleldetectionmechanismoperatesonthePHY’sreceivesideandworksinconjunctionwiththeauto-negotiationprotocoltodeterminethespeedoflinkpartnersthatdonotsupportautonegotiation.Speeddetectionisachievedbylisteningforthefollowingcharacteristicsignals:NormalLinkPulsesignals100BASE-TXIdlePatternNLPsignalsindicatethelinkpartnerisin10BASE-Tmode,andhencethelocalEthernetdevicewillsetitsportto10BASE-Tmodeandestablishlinkaccordingly.Figure1showstheNLPwaveform,whichismeasureddifferentiallyacrossa100Ωterminationatthenearend.Thepresenceof100BASE-TXIdlePatternindicatesthelinkpartnerisin100BASE-TXmode,andhencethelocalEthernetdevicewillsetitsportto100BASE-TXmodeandestablishlinkaccordingly.Figure2showsthe100BASE-TXIdlePatternwaveform,whichismeasureddifferentiallyacrossa100Ωterminationatthenearend.MicrelApplicationNote124Figure1.NormalLinkPulseFigure2.100BASE-TXIdlePatternM9999-1014042October2004MicrelApplicationNote124Paralleldetectionislimitedtoidentifyingonlythespeedoflinkpartnersthatdonotsupportautonegotiation.Ifanon-negotiating,fixedspeedlinkpartnerissettofullduplexmode,thelocalauto-negotiatingEthernetdevicewillbeunabletoobtainitslinkpartner’sduplexsetting.Here,theIEEEspecificationcallsfortheauto-negotiatingdevicetosetitsporttohalfduplexmode.Theduplexmismatchthatresultsfromthisscenariowillproduceframeerrors,latecollisions,anddroppedframes.Allofwhichcanhaveasevereimpactonnetworkperformance.TheautonegotiationandparalleldetectionprocessflowisdepictedinFigure3.ForceModeForcemode,alsoreferredtoasmanualconfiguration,providestheoptiontosetthespeedandduplexmodeofoperationmanuallytoapre-determinedconfiguration.Thiscanbebeneficialforthoseapplicationsthatrequirethebandwidthtobelimitedtoamaximumof10Mbps,ormaintainedat100Mbps.Here,forcemodeisusedtopreventalinkpartnerfromlinkingwiththelocalEthernetdeviceifthereisaspeeddifference.AllMicrelFastEthernetPHYsandSwitchessupportforcemode,excepttheKS8997.Forcemodeisenabledbydisablingautonegotiationandselectingthedesiredspeedandduplexmodeofoperation.Dependingonthedevice,forcemodecanbeconfiguredthroughstrappingpinsorviaregisters.RegistersareaccessedusinganEEPROMoroneofthemanagementinterfaces.Moredetailsontheprogrammingoptionscanbefoundineachdevice’sdatasheet.ConfigurationModesAlocaldeviceanditslinkpartnercaneachbesettoeitherauto-negotiation(AN)modeorforcemode.Overalinksegment,thisproducesthreedifferentcombinationsofconfigurationmodes:AN-AN,AN-force,andforce-force.AN-ANInthismode,boththelocaldeviceanditslinkpartnerareconfiguredasauto-negotiatingdevices.FLPsareusedtoconveyeachdevice’sspeedandduplexcapabilitiestotheother.Linkwillbesettothehighestprioritysettingamongtheircommoncapabilities.Ifthereisnocommoncapabilitybetweenthetwodevices,nolinkwillbemade.AN-ForceInthismode,thelocaldeviceanditslinkpartnerareeachconfi