Filter
Top Products
www.ti.com
3.7Pullup/PulldownResistors
3.8ConfigurationExamples
SM320C6455-EP
FIXED-POINTDIGITALSIGNALPROCESSOR
SPRS462B–SEPTEMBER2007–REVISEDJANUARY2008
ProperboarddesignshouldensurethatinputpinstotheC6455devicealwaysbeatavalidlogicleveland
notfloating.Thismaybeachievedviapullup/pulldownresistors.TheC6455devicefeaturesinternalpullup
(IPU)andinternalpulldown(IPD)resistorsonmostpinstoeliminatetheneed,unlessotherwisenoted,for
externalpullup/pulldownresistors.
Anexternalpullup/pulldownresistorneedstobeusedinthefollowingsituations:
•DeviceConfigurationPins:Ifthepinisbothroutedoutand3-stated(notdriven),anexternal
pullup/pulldownresistormustbeused,eveniftheIPU/IPDmatchesthedesiredvalue/state.
•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,thereisanadditionalconsiderationthatsetsalowerlimitontheresistancevalue
oftheexternalresistor.Verifythattheresistanceissmallenoughthattheweakestoutputbuffercan
drivethenettotheoppositelogiclevel(includingmargin).
•Remembertoincludetoleranceswhenselectingtheresistorvalue.
•Forpullupresistors,alsoremembertoincludetolerancesontheDV
DD
rail.
Formostsystems,a1-kΩresistorcanbeusedtoopposetheIPU/IPDwhilemeetingtheabovecriteria.
Usersshouldconfirmthisresistorvalueiscorrectfortheirspecificapplication.
Formostsystems,a20-kΩresistorcanbeusedtocomplementtheIPU/IPDonthedeviceconfiguration
pinswhilemeetingtheabovecriteria.Usersshouldconfirmthisresistorvalueiscorrectfortheirspecific
application.
Formoredetailedinformationoninputcurrent(I
I
),andthelow-/high-levelinputvoltages(V
IL
andV
IH
)for
theC6455device,seeSection6.3,ElectricalCharacteristicsOverRecommendedRangesofSupply
VoltageandOperatingCaseTemperature.
TodeterminewhichpinsontheC6455deviceincludeinternalpullup/pulldownresistors,seeTable2-3,
TerminalFunctions.
Figure3-12andFigure3-13illustrateexamplesofperipheralselections/optionsthatareconfigurableon
theC6455device.
DeviceConfiguration 78SubmitDocumentationFeedback