resistor_ecn

USBENGINEERINGCHANGENOTICETitle:Pull-up/pull-downresistorsAppliesto:UniversalSerialBusSpecificationRevision2.0SummaryofECN:ThisECNchangestherangeofthepull-upandpull-downresistorsusedtobiasthebus.Newvaluesforthepull-downresistorsare14.25kΩto24.80kΩ.Forthepull-upresistors,thevaluesare900Ωto1575Ωwhenthebusisidleand1425Ωto3090Ωwhentheupstreamdeviceistransmitting.ReasonsforECN:ThischangeallowstheresistorstobemovedontothedeviceorhubSiliconandnotrequirelasertrimmingtoachievetherequiredtolerances.Thiscanresultinasignificantsavingsincost,especiallyforahub.ItalsohasasignificantimpactonanOnTheGo,dual-roledevicethathastoswitchboththepull-upandpull-downresistors.Impactonexistingperipheralsandsystems:ThereisnoimpacttocurrentdevicesthatarecompatiblewiththecurrentversionsoftheUSBspecification.Thechangehasnoimpactoninteroperability.Hardwareimplications:1)Increasingthepull-downresistorincreasesthedischargetimeofthebusafteradisconnect.Worst-casedischargetimeincreasesfrom15.75µsto20µs.Thisincreasehasnooperationalimplications.2)Decreasingthepull-upresistorvalueincreasestheamountofcurrentusedforbusbiasingbyasmallpercentage(from218µΑmaxto238µΑor9%).Onaverage,theamountofcurrentusedforthebiasingisreducedasthepull-downresistorvaluehasthelargestimpactonthetotalcurrent.Iftheincreaseinbiascurrentisaproblemforadevice,itisfreetousetheoldertolerance(i.e.ifadevicewouldbeaffectedthenitdoesnothavetoimplementthechangeandthewidertoleranceontheconnecteddevicewillnothaveanegativeimpact).3)Theoriginalvaluesforthepull-downresistorsassumedthattheoutputleakagecurrentofanoffoutputwouldbeabout10µΑ.Withboththehubanddeviceintheidlestate,thepull-downonthelowdataline(D-forhighandfull-speed,D+forlow-speed)wouldbe20µΑ*15.75kΩ=315mV.Tokeeptheidlevoltageatthesamelevel,anyhub/hostthatusesthehighervalueresistancehastoreduceitsleakagecurrentproportionately.If,forexample,ahubhasa24.80kΩpull-down,thentoallowforthelegacydeviceleakageof10µΑ,theleakagecurrentonthehubhastobenomorethan2.7µΑ.ThisiswellwithinthecapabilitiesofcontemporarySiprocessingandtesting.4)Thepull-upresistorschangefromlowerresistancetohigherresistancewhenthebusgoesfromidletoactive.Thisensuresthatexistinghub/hostSicandrivethedownstreamdevicewithoutincreasingjitterorotherwisedegradingsignalingmargin.5)Devicesthatusethisschemefortheirpull-downresistorsmustbeawarethatthepull-upresistoronaLSdevicemaybetiedtoVbus.Theincreaseinthepull-downresistancecancausetheidlevoltageonthebustoriseashighas4.1V.Iftheprocessuseddoesnotallowthissustainedvoltageonthebus,thenthestandardvaluesofresistanceshouldbeused.Softwareimplications:None.Compliancetestingimplications:None.SpecificationChanges:NochangesoutsideofChapter7.InChapter7,changealloccurrencesintextandindrawingsof:“15kΩ±5%”to“14.25kΩto24.80kΩ”InChapter7,changealloccurrencesintextandindrawingsof:“1.5kΩ±5%”to:“1.5kΩ(nominal)”Note:Nochangestotestloadsareproposed.Alltestingistobedoneusingthe“nominal”valuesforpull-up(1.5K)andpull-down(15K)ascurrentlyspecified.Changesections7.1.5.1,7.1.5.2asfollows:7.1.5.1Low/Full-speedDeviceSpeedIdentificationTheUSBisterminatedatthehubandfunctionendsasshowninFigure7-20andFigure7-21.Full-speedandlow-speeddevicesaredifferentiatedbythepositionofthepull-upresistoronthedownstreamendofthecable:•Full-speeddevicesareterminatedasshowninFigure7-20withthepull-upresistorontheD+line.•Low-speeddevicesareterminatedasshowninFigure7-21withthepull-upresistorontheD-line.•Thepull-downterminatorsondownstreamportsareresistorsof14.25kΩto24.80kΩ15kΩ±5%connectedtoground.Thedesignofthepull-upresistormustensurethatthesignallevelssatisfytherequirementsspecifiedinTable7-2.Inordertofacilitatebusstateevaluationthatmaybeperformedattheendofareset,thedesignmustbeabletopull-upD+orD-from0VtoVIH(min)withintheminimumresetrelaxationtimeof2.5µs.Adevicethathasadetachablecablemustmayusea1.5kΩ±5%resistortiedtoavoltagesourcebetween3.0Vand3.6V(VTERM)tosatisfytheserequirements.Deviceswithcaptivecablesmayusealternativeterminationmeans;.Hhowever,theTheveninresistanceofanyterminationmustbenolessthan900Ωandtheopen-circuitvoltageofthepullupmustnotexceedVBUS.Thesealternativemeansmustproduceanidlevoltage(VIHZ)onD-orD+ofbetween2.7Vand3.6Vwhenterminatedwithapull-downresistanceof15kΩ±5%toground.Note:Theveninresistanceofterminationdoesnotincludethe14.25kΩto24.80kΩ15kΩ±5%resistoronthehost/hub.AlternativeimplementationsforRPUareallowedonupstreamfacingportsondeviceswithdetachablecableswiththefollowingrestrictions:•WhenthebusisintheIdlestate(includingSuspend)ortheupstreamdeviceisassertingSE0,thepull-upresistancemustbebetween900Ωand1575Ω.The1575ΩmaximumvalueensuresthattheIdlevoltageisat2.7Vorhigherevenwhenaminimumvalueofpulldown(14.25kΩ)ispresent.•Whentheupstreamdeviceistransmittingdata,thepull-upresistanceshallswitchtoavaluebetween1425Ωand3090Ω.The1425Ωminimumvalueensuresthattheloadpresentedbytheswitchedresistanceisnogreaterthantheloadpresentedbya1.5kΩ±5%.The3090Ωmaximumvalueissufficienttoensurethatthedatalineisheldinavalidhighstateifthereceiverhasinterpretednoiseonthelineasthestartofapacket.•Wh