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2SRIOFunctionalDescription
2.1Overview
SRIOFunctionalDescription
2.1.1PeripheralDataFlow
ThisperipheralisdesignedtobeanexternalslavemodulethatiscapableofmasteringtheinternalDMA.
Thismeansthatanexternaldevicecanpush(burstwrite)datatotheDSPasneeded,withouthavingto
generateaninterrupttotheCPU.Thishastwobenefits.Itcutsdownonthetotalnumberofinterrupts,and
itreduceshandshaking(latency)associatedwithread-onlyperipherals.
SRIOspecifiesdatapacketswithpayloadsupto256bytes.Manytimes,transactionswillspanacross
multiplepackets.RapidIOspecifiesamaximumof16transactionspermessage.Althougharequestis
generatedforeachpackettransactionsothattheDMAcantransferthedatatoL2memory,aninterruptis
onlygeneratedafterthefinalpacketofthemessage.ThisinterruptnotifiestheCPUthatdataisavailable
inL2Memoryforprocessing.
Asanendpointdevice,theperipheralacceptspacketsbasedonthedestinationID.Twooptionsexistfor
packetacceptanceandaremodeselectable.ThefirstoptionistoonlyacceptpacketswhoseDestIDs
matchthelocaldeviceIDin0x0080.Thisprovidesalevelofsecurity.Thesecondoptionistoaccept
incomingpacketsmatchingthedeviceIDineither0x0080or0x0084.Thisallowsforsystemmulticast
operations.
Dataflowthroughtheperipheralcanbeexplainedusingthehigh-levelblockdiagramshowninFigure4.
High-speeddataentersfromthedevicepinsintotheRXblockoftheSERDESmacro.TheRXblockisa
differentialreceiverexpectingaminimumof175mVpeak-to-peakdifferentialinputvoltage(Vid).Level
shiftingisperformedintheRXblock,suchthattheoutputissingleendedCMOS.Theserialdataisthen
fedtotheSERDESclockrecoveryblock.Thesolepurposeofthisblockistoextractaclocksignalfrom
thedatastream.Todothis,alow-frequencyreferenceclockisrequired,1/10
th
or½0
th
thedatarate.For
example,for3.125Gbpsdata,areferenceclockof312.5Mhzor156.25Mhzisneeded.Typically,this
clockcomesfromanoff-chipstablecrystaloscillatorandisaLVDSdeviceinputseparatetotheSERDES.
ThisclockisdistributedtotheSERDESPLLblockwhichmultipliesthatfrequencyuptothatofthedata
rate.Eightphasesofthishigh-speedclockarecreatedandroutedtotheclockrecoveryblocks.Theclock
recoveryblockfurtherinterpolateseighttimesbetweentheseclockphases.Thisprovidesclockedge
resolutionof1/96
th
theUnitInterval(UI).Theclockrecoveryblocksamplestheincomingdataand
monitorstherelativepositionsofthedataedges.Withthisinformation,itcanprovidethedataanda
center-alignedclocktotheS2Pblock.TheS2Pblockusesthenewlyrecoveredclocktodemuxthedata
into10-bitwords.Atthispoint,thedataleavestheSERDESmacroat1/10ththepindatarate,
accompaniedbyanalignedbyteclock.
SPRU976–March2006SerialRapidIO(SRIO)19
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