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Shared-Processing(SP)Mode
Eachsub-frameisindependentofeachother.Therearethreetypesofsub-frames.Thefirstsub-frame
startsthetrellisfromthezerostate.Thelastsub-frameendsthetrellisfromaknownstate.Theremaining
middlesubframesdonotstartorendfromaknownstate.
TheEDMA3transfersACNT*BCNTnumberofbytesinA-SyncModeandACNT*BCNT*CCNTnumberof
bytesinAB-SyncMode.Thetotalnumberofbytesforbothmodesshouldbeamultipleof8.Also,the
startingaddressofthefirstsub-framethattheEDMA3willtransferneedstobememory-mapped.
Inthesharedprocessingmode:
•Prologlengthmustbemultiplesof8
•StartingaddressforreadingextrinsicRAMmustbe:
RAMbaseaddress+middleandlastsubframesprologlength
•CRCisturnedoff
•SNRisturnedoff
•Prologreductionisturnedoff
•Extrinsicscalingisturnedoff
Theturbodecodingofthefullframeisperformedinseveralstepsasdescribedbelow:
•TheEDMA3sendstheinputbuffersforonesub-frame(theMAP0inputsaredescribedinFigure19).
•TheTCP2performstheMAP0forthecurrentsub-frame.
•TheEDMA3readstheMAPoutput(extrinsic)ofthecurrentsub-frameandwritesitintotheDSP
memory.
ThestepsfortheMAP0processarerepeatedforalltheothersub-frames.
Onceallthesub-framesMAP0havebeencomputed,thefullMAP0extrinsic(=apriori1)isthenavailable.
ThisallowstheDSPtointerleavetheextrinsicoutput1topreparethenextMAP(=MAP1).Oncethis
interleavingisdone,thesameprocessisapplied,inMAP1configuration:
•TheEDMA3sendstheinputbuffersforonesub-frame(theMAP0inputsaredescribedinFigure19).
•TheTCP2performstheMAP1forthecurrentsub-frame
•TheEDMA3readstheMAPoutput(extrinsic)ofthecurrentsub-frameandwritesitintotheDSP
memory.
ThestepsfortheMAP1processarerepeatedforalltheothersub-frames.
Onceallthesub-framesMAP1havebeencomputed,thefullextrinsic(=apriori2)isthenavailable.This
allowstheDSPtode-interleavetheextrinsicoutput2topreparethenextMAP(=MAP0).Oncethis
de-interleavingisdone,thesameprocessisapplied,inMAP0configuration.Steps1-4arethenrepeated
foralliterations.TheDSPisinchargeofanystoppingcriteriaalgorithmimplementationandcomputing
thefinalharddecisions.Figure19showsadescriptionoftheTCP2processingunitfunctionalblock
diagraminsharedprocessingmode.
SPRUGK1–March2009TMS320C6457Turbo-DecoderCoprocessor221
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