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1.2VideoPortFIFO
1.2.1EDMAInterface
VideoPortFIFO
ThevideoportincludesaFIFOtostoredatacomingintooroutfromthevideoport.Thevideoport
operatesinconjunctionwithEDMAtransferstomovedatabetweenthevideoportFIFOandexternalor
on-chipmemory.YoucanprogramthresholdsettingssothatEDMAeventsgeneratewhenthevideoport
FIFOreachesacertainfullness(forcapture)orgoesbelowacertainfullness(fordisplay).Yousetup
EDMAChannelsthatarerequiredtoservicetheFIFOindependentlyandarekeytocorrectoperationof
thevideoport.TheFIFOsizeisrelativelylargetoallowtimeforEDMAChannelstoservicethetransfer
requests,sincethedevicetypicallyhasmanyperipheralinterfaces,includingfivevideoports.
ThefollowingsectionsbrieflydescribetheinteractionwiththeEDMAanddifferentFIFOconfigurations
thatareusedtosupportthevariousmodesofthevideoport.
VideoportdatatransferstakeplaceusingEDMAChannels.EDMArequestsarebasedonbuffer
thresholds.Sincethevideoportdoesnotdirectlysourcethetransfer,itcannotadjustthetransfersize
basedonbufferempty/fullstatus.ThismeanstheEDMAtransfersizeisessentiallyfixedinthe
user-programmedEDMAparametertable.Thepreferredtransfersizeisoftenoneentirelineofdata
becausethisallowsthemostflexibilityintermsofframebufferlinepitch(inRAM).Somemodesof
operationforthehighestdisplayratesmayrequiremorefrequentEDMArequests,suchasonahalfor
quarterlinebasis.
Allrequestsarebasedonbufferthresholds.EDMArequestsaremadewheneverthenumberofsamples
inthebufferreachesthethresholdvalueinvideocapturemode.Inordertoensurethatalldatafroma
capturefield/framegetsemptiedfromthebuffer,thetransfersizemustbeequaltothethresholdandthe
totalamountoffield/framedatamustbeamultipleofthetransfersize.
Forvideodisplayoperation,EDMArequestsaremadewheneverthereisatleastthethresholdnumberof
doublewordsfreeintheFIFO.Thismeansthatthetransfersizemustbeequalorsmallerthanthe
thresholdsothatitfitsintotheavailablespace.Thefield/framesizemuststillbeamultipleofthetransfer
sizeortherearepixelsleftinthebufferattheendofthefield(whichappearatthestartofthenextfield).
SPRUEM1–May2007Overview19
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