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AppendixBWrite/ReadPerformance
B.1WritePerformance
AppendixB
Thefollowingsectionsdiscussthewriteversusreadperformanceandhowthethroughput(reador
write)shouldbecalculatedforagivendatawidthandserialclockfrequency.
Note:Thedataandthroughputcalculationsshownherearesamplecalculationsformost
idealsituations.Ingeneral,thedataratesdependonavarietyofotherfactors,such
asefficiencyofread/writebursttransactions,abilityofbufferingupread/writedata,
andhowbestitcanbeseriallyshiftedoutwithoutstallingadditionalread/writedata
burst,remoteandlocalcomponents,bothexternalandinternal(deviceoperations,
boardconsiderations,etc.).
Themaxwriteratedescribesthemaximumavailabledatarateoftheserialinterfacefortransmission,
takingintoconsiderationthe8b/10bencodingoverheads.Thisiscalculatedasfollows:
Maxwriterate=VLYNQSerialClock(MHz)×No.ofPins×8b/10bencodingoverhead
The8b/10bencodingoverheadessentiallyaccountsfor20%overhead,thustheactualdatathroughput
aftersubtractionoftheencodingoverheadgivesafactorof0.8.Forexample,iftheVLYNQclockis
runningat99MHzona4pinperdirectioninterface,therawdatais99×4or396Mbps.Afterthe8B10B
encodingisremoved,themaximumwriterateis396×0.8=316.8Mbps.
ThetotalthroughputontheVLYNQinterfaceincludesbothtransmitandreceivedirections.Therefore,for
theaboveconfiguration,aremotedevicecanalsobewritingtothelocaldeviceatthesamedatarates,
thenthetotalthroughputisthesumoftransmitandreceiverates,or633.6Mbps.
Inadditiontothe8b/10bencoding,thepacketstructureforread/writeoperationsalsoresultsinadditional
overheads.TheVLYNQmodulecantransfersingle32-bitwordsoraburstofuptosixteen32-bitwords.
Thepacketstructureofthewritesisshownbelow,hereeachcharacterrepresentsabyte.
Write32-caaaaddddT
WriteBurst-claaaaddddddddddddT
Where
T-EndOfPacket
d-data,ddddrepresentsadditional32-bitwordsinburst,upto16words.
a-address
c-command
l-length
Theexampleaboveillustratesthatsinglewritesrequire6bytesofoverhead,whileburstwritesrequire
8bytesofoverhead(duetotheadditionallengthofthefield).Fromthis,ascalingfactorcanbecalculated
(databytes/totalbytes),asshowinTableB-1.Theactualthroughputisthencalculatedasthe[scaling
factor]×[maxwriterate].
TableB-1.ScalingFactors
BurstSizein32-bitwordsDataBytesOverheadBytesScalingFactor
14640%
416769.56%
832782.05%
50Write/ReadPerformanceSPRUF89–October2007
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