2.11PacketTransmitOperation
2.11.1TransmitDMAHostConfiguration
2.11.2TransmitChannelTeardown
2.12ReceiveandTransmitLatency
Architecture
ThetransmitDMAisaneightchannelinterface.Prioritybetweentheeightqueuesmaybeeitherfixedor
round-robinasselectedbytheTXPTYPEbitintheMACcontrolregister(MACCONTROL).Ifthepriority
typeisfixed,thenchannel7hasthehighestpriorityandchannel0hasthelowestpriority.Round-robin
priorityproceedsfromchannel0tochannel7.
ToconfigurethetransmitDMAforoperationthehostmustperform:
•WritetheMACsourceaddresslowbytesregister(MACSRCADDRLO)andtheMACsourceaddress
highbytesregister(MACSRCADDRHI)(usedforpauseframesontransmit).
•InitializethetransmitchannelnDMAheaddescriptorpointerregisters(TXnHDP)to0.
•Enablethedesiredtransmitinterruptsusingthetransmitinterruptmasksetregister(TXINTMASKSET)
andthetransmitinterruptmaskclearregister(TXINTMASKCLEAR).
•SettheappropriateconfigurationbitsintheMACcontrolregister(MACCONTROL).
•Setupthetransmitchannel(s)bufferdescriptorsinhostmemory.
•EnablethetransmitDMAcontrollerbysettingtheTXENbitinthetransmitcontrolregister
(TXCONTROL).
•WritetheappropriateTXnHDPwiththepointertothefirstdescriptortostarttransmitoperations.
Thehostcommandsatransmitchannelteardownbywritingthechannelnumbertothetransmitteardown
register(TXTEARDOWN).Whenateardowncommandisissuedtoanenabledtransmitchannel,the
followingoccurs:
•Anyframecurrentlyintransmissioncompletesnormally.
•TheTDOWNCMPLTflagissetinthenextSOPbufferdescriptorinthechain,ifthereisone.
•Thechannelheaddescriptorpointerisclearedto0.
•Atransmitinterruptisissuedtoinformthehostofthechannelteardown.
•Thecorrespondingtransmitchannelncompletionpointerregister(TXnCP)containsthevalue
FFFFFFFCh.
•ThehostshouldacknowledgeateardowninterruptwithanFFFFFFFChacknowledgevalue.
Channelteardownmaybecommandedonanychannelatanytime.Thehostisinformedoftheteardown
completionbythesetteardowncomplete(TDOWNCMPLT)bufferdescriptorbit.TheEMACdoesnot
clearanychannelenablesduetoateardowncommand.Ateardowncommandtoaninactivechannel
issuesaninterruptthatsoftwareshouldacknowledgewithanFFFFFFFChacknowledgevaluetoTXnCP
(notethatthereisnobufferdescriptorinthiscase).Softwaremayreadtheinterruptacknowledgelocation
(TXnCP)todetermineiftheinterruptwasduetoacommandedteardown.ThereadvalueisFFFFFFFCh,
iftheinterruptwasduetoateardowncommand.
ThetransmitFIFOcontainstwenty-four64-bytecellsandthereceiveFIFOcontainssixty-eight64-byte
cells.TheEMACbeginstransmissionofapacketonthewireafterTXCELLTHRESHcells(configurable
throughtheFIFOcontrolregister,FIFOCONTROL)oracompletepacketareavailableintheFIFO.
TransmitunderruncannotoccurforpacketsizesofTXCELLTHRESH×64bytes(orless).Forlarger
packetsizes,transmitunderruncanoccurifthememorylatencyisgreaterthanthetimerequiredto
transmita64-bytecellonthewire;thisis0.512msin1Gbitmode,5.12msin100Mbpsmode,and
51.2msin10Mbpsmode.Thememorylatencytimeincludesallbufferdescriptorreadsfortheentirecell
data.TheEMACtransmitFIFOuses24cells;thus,underruncannothappenforanormalsizepacket(less
than1536packetbytes).Celltransmissioncanbeconfiguredtostartonlyafteranentirepacketis
containedintheFIFO;foramaximum-sizepacket,settheTXCELLTHRESHfieldtothemaximum
possiblevalueof24.
50EthernetMediaAccessController(EMAC)/ManagementDataInput/Output(MDIO)SPRUEQ6–December2007
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