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2.11PacketTransmitOperation
2.11.1TransmitDMAHostConfiguration
2.11.2TransmitChannelTeardown
2.12ReceiveandTransmitLatency
EMACFunctionalArchitecture
ThetransmitDMAisaneightchannelinterface.Prioritybetweentheeightqueuesmaybeeitherfixedor
roundrobinasselectedbytheTXPTYPEbitintheMACCONTROLregister.Iftheprioritytypeisfixed,
thenchannel7hasthehighestpriorityandchannel0hasthelowestpriority.Roundrobinpriority
proceedsfromchannel0tochannel7.
ToconfigurethetransmitDMAforoperation,thehostmustperformthefollowing:
•WritetheMACSRCADDRLOandMACSRCADDRHIregisters(usedforpauseframesontransmit).
•InitializetheTXnHDPregisterstozero.
•EnablethedesiredtransmitinterruptsusingtheTXINTMASKSETandTXINTMASKCLEARregisters.
•SettheappropriateconfigurationbitsintheMACCONTROLregister.
•Setupthetransmitchannel(s)bufferdescriptorsinhostmemory.
•EnablethetransmitDMAcontrollerbysettingtheTXENbitintheTXCONTROLregister.
•WritetheappropriateTXnHDPregisterswiththepointertothefirstdescriptortostarttransmit
operations.
ThehostcommandsatransmitchannelteardownbywritingthechannelnumbertotheTXTEARDOWN
register.Whenateardowncommandisissuedtoanenabledtransmitchannel,thefollowingoccurs:
•Anyframecurrentlyintransmissioncompletesnormally.
•TheTDOWNCMPLTflagissetinthenextSOPbufferdescriptorinthechain,ifthereisone.
•Thechannelheaddescriptorpointeriscleared.
•Atransmitinterruptisissued,informingthehostofthechannelteardown.
•ThecorrespondingTXnCPregistercontainsthevalueFFFFFFFCh.
•ThehostshouldacknowledgeateardowninterruptwithanFFFFFFFChacknowledgevalue.
Channelteardownmaybecommandedonanychannelatanytime.Thehostisinformedoftheteardown
completionbythesetteardowncompletebufferdescriptorbit(TDOWNCMPLT).TheEMACdoesnot
clearanychannelenablesduetoateardowncommand.Ateardowncommandtoaninactivechannel
issuesaninterruptthatsoftwareshouldacknowledgewithanFFFFFFFChacknowledgevaluetoTXnCP
(notethatthereisnobufferdescriptor).Softwaremayreadtheinterruptacknowledgelocation(TXnCP)to
determineiftheinterruptwasduetoacommandedteardown.ThereadvalueisFFFFFFFChifthe
interruptwasduetoateardowncommand.
ThetransmitFIFOcontainstwentyfour64-bytecells,andthereceiveFIFOcontainssixtyeight64-byte
cells.TheEMACbeginstransmissionofapacketonthewireafterTXCELLTHRESHcells(configurable
throughtheFIFOCONTROLregister)oracompletepacketareavailableintheFIFO.
Transmitunder-runcannotoccurforpacketsizesofTXCELLTHRESHtimes64bytes(orless).Forlarger
packetsizes,transmitunder-runcanoccurifthememorylatencyisgreaterthanthetimerequiredto
transmita64-bytecellonthewire;thisis0.512µsin1Gbitmode,5.12µsin100Mbpsmode,and51.2
µsin10Mbpsmode.Thememorylatencytimeincludesallbufferdescriptorreadsfortheentirecelldata.
TheEMACtransmitFIFOuses24cells;thus,under-runcannothappenforanormalsizepacket(less
than1536packetbytes).Celltransmissioncanbeconfiguredtostartonlyafteranentirepacketis
containedintheFIFO;foramaximum-sizepacket,settheTXCELLTHRESHfieldtothemaximum
possiblevalueof24.
Receiveoverrunispreventedifthereceivememorycelllatencyislessthanthetimerequiredtotransmita
64-bytecellonthewire(0.512µsin1Gbpsmode,5.12µsin100Mbpsmode,or51.2µsin10Mbps
mode).Thelatencytimeincludesanyrequiredbufferdescriptorreadsforthecelldata.
SPRU975B–August2006EthernetMediaAccessController(EMAC)/ManagementDataInput/Output(MDIO)55
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