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2.9.2.6TransmitFlowControl
2.9.2.7Speed,Duplex,andPauseFrameSupport
PeripheralArchitecture
Incomingpauseframesareactedupon,whenenabled,topreventtheEMACfromtransmittinganyfurther
frames.IncomingpauseframesareonlyacteduponwhentheFULLDUPLEXandTXFLOWENbitsinthe
MACcontrolregister(MACCONTROL)areset.Pauseframesarenotacteduponinhalf-duplexmode.
Pauseframeactionistakenifenabled,butnormallytheframeisfilteredandnottransferredtomemory.
MACcontrolframesaretransferredtomemory,iftheRXCMFENbitinthereceive
multicast/broadcast/promiscuouschannelenableregister(RXMBPENABLE)isset.TheTXFLOWENand
FULLDUPLEXbitsaffectwhetherornotMACcontrolframesareactedupon,buttheyhavenoaffectupon
whetherornotMACcontrolframesaretransferredtomemoryorfiltered.
PauseframesareasubsetofMACcontrolframeswithanopcodefieldof0001h.Incomingpauseframes
areonlyacteduponbytheEMACif:
•TXFLOWENbitissetinMACCONTROL
•Theframe’slengthis64toRXMAXLENbytesinclusive
•TheframecontainsnoCRCerrororalign/codeerrors
Thepausetimevaluefromvalidframesisextractedfromthetwobytesfollowingtheopcode.Thepause
timeisloadedintotheEMACtransmitpausetimerandthetransmitpausetimeperiodbegins.Ifavalid
pauseframeisreceivedduringthetransmitpausetimeperiodofaprevioustransmitpauseframethen:
•Ifthedestinationaddressisnotequaltothereservedmulticastaddressoranyenabledordisabled
unicastaddress,thenthetransmitpausetimerimmediatelyexpires,or
•Ifthenewpausetimevalueis0,thenthetransmitpausetimerimmediatelyexpires,else
•TheEMACtransmitpausetimerimmediatelyissettothenewpauseframepausetimevalue.(Any
remainingpausetimefromthepreviouspauseframeisdiscarded).
IftheTXFLOWENbitinMACCONTROLiscleared,thenthepausetimerimmediatelyexpires.
TheEMACdoesnotstartthetransmissionofanewdataframeanysoonerthan512bit-timesaftera
pauseframewithanonzeropausetimehasfinishedbeingreceived(MRXDVgoinginactive).No
transmissionbeginsuntilthepausetimerhasexpired(theEMACmaytransmitpauseframesinorderto
initiateoutgoingflowcontrol).Anyframealreadyintransmissionwhenapauseframeisreceivedis
completedandunaffected.
Incomingpauseframesconsistof:
•A48-bitdestinationaddressequaltooneofthefollowing:
–Thereservedmulticastdestinationaddress01.80.C2.00.00.01h
–AnyEMAC48-bitunicastaddress.Pauseframesareaccepted,regardlessofwhetherthechannel
isenabledornot.
•The16-bitlength/typefieldcontainingthevalue88.08h.
•The48-bitsourceaddressofthetransmittingdevice.
•The16-bitpauseopcodeequalto00.01h.
•The16-bitpausetime.Apause-quantumis512bit-times.
•Paddingto64-bytedatalength.
•The32-bitframe-checksequence(CRCword).
Allquantitiesarehexadecimalandaretransmittedmost-significantbytefirst.Theleast-significantbit(LSB)
istransferredfirstineachbyte.
Thepaddingisrequiredtomakeuptheframetoaminimumof64bytes.Thestandardallowspause
frameslongerthan64bytestobediscardedorinterpretedasvalidpauseframes.TheEMACrecognizes
anypauseframebetween64bytesandRXMAXLENbytesinlength.
TheMACoperatesat10Mbpsor100Mbps,inhalf-duplexorfull-duplexmode,andwithorwithoutpause
framesupportasconfiguredbythehost.
38EthernetMediaAccessController(EMAC)/SPRU941A–April2007
ManagementDataInput/Output(MDIO)
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