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3.19HCRootHubARegister(HCRHDESCRIPTORA)
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Registers
TheHCroothubAregister(HCRHDESCRIPTORA)definesseveralaspectsoftheUSBhostcontroller
roothubfunctionality.HCRHDESCRIPTORAisshowninFigure20anddescribedinTable20.
Figure20.HCRootHubARegister(HCRHDESCRIPTORA)
312416
POTPGReserved
R/W-AhR-0
15131211109870
ReservedNOCPOCPMDTNPSPSMNDP
R-0R/W-1R/W-0R-0R/W-1R/W-0R-3h
LEGEND:R/W=Read/Write;R=Readonly;-n=valueafterreset
Table20.HCRootHubARegister(HCRHDESCRIPTORA)FieldDescriptions
BitFieldValueDescription
31-24POTPG0-FFhPower-ontopower-goodtime.Definestheminimumamountoftime(2ms×POTPG)betweentheUSB
hostcontrollerturningonpowertoadownstreamportandwhentheUSBhostcanaccessthe
downstreamdevice.ThisfieldhasnoeffectonUSBhostcontrolleroperation.Afterturningonpowerto
aport,theUSBhostcontrollerdrivermustdelaytheamountoftimeimpliedbyPOTPGbefore
attemptingtoresetanattacheddownstreamdevice.Therequiredamountoftimeis
implementation-specificandmustbecalculatedbasedontheamountoftimetheVBUSsupplytakesto
providevalidVBUStoaworst-casedownstreamUSBfunctioncontroller.Theimplementation-specific
valuemustbecomputedandthenwrittentothisregisterbeforetheUSBhostcontrollerdriveris
initialized.BecausethedevicedoesnotprovideadirectcontrolfromtheUSBhostcontrollertoswitch
VBUSonandoff,thisvaluemusttakeintoaccountanydelayscausedbyothermethodsofcontrolling
VBUSexternally.ThisfieldhasnorelationshiptotheOTGcontrollerregisterbitsthatrelatetoVBUS.
Systemsoftwarecanupdatethisregistertosimplifyhostcontrollerdriverand/orOTGdrivercoding.
23-13Reserved0Reserved
12NOCP1Noovercurrentprotection.Becausethedevicedoesnotprovidesignalstoallowconnectionofexternal
overcurrentindicationsignalstotheUSBhostcontroller,thisbitdefaultsto1thatindicatesthattheUSB
hostcontrollerdoesnotimplementovercurrentprotectioninputs.ThisbithasnorelationshiptotheOTG
controllerregisterbitsthatrelatetoVBUS.
11OCPM0Overcurrentprotectionmode.Becausethedevicedoesnotprovidehostcontrollerovercurrent
protectioninputsignals,thisbithasnoeffect.ThisbithasnorelationshiptotheOTGcontrollerregister
bitsthatrelatetoVBUS.
10DT0Devicetype.Thisbitisalways0,whichindicatesthattheUSBhostcontrollerimplementedisnota
compounddevice.
9NPS1Nopowerswitching.BecausethedevicedoesnotprovideconnectionsfromtheUSBhostcontrollerto
controlexternalVBUSswitching,thisbitdefaultsto1thatindicatesthatVBUSpowerswitchingisnot
supportedandthatpowerisavailabletoalldownstreamportswhentheUSBhostcontrollerispowered.
ThisbithasnorelationshiptotheOTGcontrollerregisterbitsthatrelatetoVBUS.Systemsoftwarecan
updatethisregistertosimplifyhostcontrollerdriverand/orOTGdrivercoding.
8PSM0Powerswitchingmode.BecausethedevicedoesnotprovidesignalsfromtheUSBhostcontrollerto
controlexternalVBUSswitching,thisbitdefaultsto0thatindicatesthatallportsarepoweredatthe
sametime.
7-0NDP0-FFhNumberofdownstreamports.TheUSBsignalmultiplexingmodeandtop-levelpinmultiplexingfeatures
canplacethedeviceinamodewhere0,1,2,or3oftheUSBhostcontrollerdownstreamportsare
usable.Thisregisterreportsthreeports,regardlessofUSBsignalmultiplexingmodeandtop-levelpin
multiplexingmode.
SPRUFM8–September2008UniversalSerialBusOHCIHostController27
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