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PRODUCT PREVIEW
TMS320C6454
Fixed-PointDigitalSignalProcessor
SPRS311A–APRIL2006–REVISEDDECEMBER2006
•OtherInputPins:IftheIPU/IPDdoesnotmatchthedesiredvalue/state,useanexternal
pullup/pulldownresistortopullthesignaltotheoppositerail.
Forthedeviceconfigurationpins(listedinTable3-1),iftheyarebothroutedoutand3-stated(notdriven),
itisstronglyrecommendedthatanexternalpullup/pulldownresistorbeimplemented.Although,internal
pullup/pulldownresistorsexistonthesepinsandtheymaymatchthedesiredconfigurationvalue,
providingexternalconnectivitycanhelpensurethatvalidlogiclevelsarelatchedonthesedevice
configurationpins.Inaddition,applyingexternalpullup/pulldownresistorsonthedeviceconfigurationpins
addsconveniencetotheuserindebuggingandflexibilityinswitchingoperatingmodes.
Tipsforchoosinganexternalpullup/pulldownresistor:
•Considerthetotalamountofcurrentthatmaypassthroughthepulluporpulldownresistor.Makesure
toincludetheleakagecurrentsofallthedevicesconnectedtothenet,aswellasanyinternalpullupor
pulldownresistors.
•Decideatargetvalueforthenet.Forapulldownresistor,thisshouldbebelowthelowestV
IL
levelof
allinputsconnectedtothenet.Forapullupresistor,thisshouldbeabovethehighestV
IH
levelofall
inputsonthenet.AreasonablechoicewouldbetotargettheV
OL
orV
OH
levelsforthelogicfamilyof
thelimitingdevice;which,bydefinition,havemargintotheV
IL
andV
IH
levels.
•Selectapullup/pulldownresistorwiththelargestpossiblevalue;but,whichcanstillensurethatthenet
willreachthetargetpulledvaluewhenmaximumcurrentfromalldevicesonthenetisflowingthrough
theresistor.Thecurrenttobeconsideredincludesleakagecurrentplus,anyotherinternaland
externalpullup/pulldownresistorsonthenet.
•Forbidirectionalnets,thereisanadditionalconsiderationwhichsetsalowerlimitontheresistance
valueoftheexternalresistor.Verifythattheresistanceissmallenoughthattheweakestoutputbuffer
candrivethenettotheoppositelogiclevel(includingmargin).
•Remembertoincludetoleranceswhenselectingtheresistorvalue.
•Forpullupresistors,alsoremembertoincludetolerancesontheDV
DD
rail.
Formostsystems,a1-kΩresistorcanbeusedtoopposetheIPU/IPDwhilemeetingtheabovecriteria.
Usersshouldconfirmthisresistorvalueiscorrectfortheirspecificapplication.
Formostsystems,a20-kΩresistorcanbeusedtocomplimenttheIPU/IPDonthedeviceconfiguration
pinswhilemeetingtheabovecriteria.Usersshouldconfirmthisresistorvalueiscorrectfortheirspecific
application.
Formoredetailedinformationoninputcurrent(I
I
),andthelow-/high-levelinputvoltages(V
IL
andV
IH
)for
theC6454device,seeSection6.3,ElectricalCharacteristicsOverRecommendedRangesofSupply
VoltageandOperatingCaseTemperature.
TodeterminewhichpinsontheC6454deviceincludeinternalpullup/pulldownresistors,seeTable2-3,
TerminalFunctions.
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