I2C Bus Pull-Up Resistor Calculation

1 1 SLVA689 February2015 SubmitDocumentationFeedbackCopyright 2015,TexasInstrumentsIncorporatedI2C Bus PullupResistorCalculationApplicationRepo rtSLVA689 February2015I2 CBusPullupResistorCalculationRajanAroraA BSTRACTP ullupresistorcalculationfor I2C interfaceis a this applicationnoteweshowhow to use simpleequationsfor this of Figures1 ApplicationExampleShowingI2C CommunicationBetweenthe DifferentIC's on a Systemand WithPullupResistorson I2C [RP(min)]vs PullupReferenceVoltage(VCC)..33 MaximumPullupResistance[RP(max)]vs Bus Capacitance(Cb)..3 List of Tables1 ParametricsfromI2C communicationstandardis the mostlywidelyusedinter-chipcommunications tandardin today is an open-drain/open-collectorcommunicationst andardwhichimpliesintegratedcircuits(IC s) with differentvoltagesupplyrails can be connectedfor be connectedfromthe I2C linesto the supplyto enablecommunicationas shownin Figure1. Thepullupresistorspull the line high whenit is not drivenlow by the valueof thepullupresistoris an importantdesignconsiderationfor I2C systemsas an incorrectvaluecan lead this articlewe showthe simpleequationsfor the pullupresistorcalculationwhichthe systemdesignercan use to do quickcalculationsfor theirdesign.

2 ()rpbtR (max) C = 1r2pbt t - t t2R CP bILCCCCV VV1 e- = = - t1R CP bIHCCCCV VV1 e- = = - tRCCCV(t) V1 e- = - ()CCOLPOLVV(max)R (min)I-= February2015 SubmitDocumentationFeedbackCopyright 2015,TexasInstrumentsIncorporatedI2C Bus PullupResistorCalculationFigure1. ApplicationExampleShowingI2C CommunicationBetweenthe DifferentIC's on a SystemandWithPullupResistorson I2C Bus2 PullupResistorCalculationA strongpullup(smallresistor)preventsthe I2C pin on an IC frombeingable to drivelow. The VOLlevelthat can be readas a validlogicallow by the inputbuffersof an IC determinesthe minimumpullupresistance[RP(min)].RP(min) is a functionof VCC, VOL(max),and IOL:(1)The maximumpullupresistanceis limitedby the bus capacitance(Cb) due to I2C standardrise the pullupresistorvalueis too high,the I2C line may not rise to a logicalhigh beforeit ispulledlow. The responseof an RC circuitto a voltagestep of amplitudeVCC, startingat time t = 0 ischaracterizedby time constantRC.

3 The voltagewaveformcan be writtenas:(2)For VIH= VCC:(3)For VIL= VCC:(4)The rise time for the I2C bus can be writtenas:(5)The maximumpullupresistanceis a functionof the maximumrise time (tr):(6)whereparametricsfromI2C specificationsare listedin February2015 SubmitDocumentationFeedbackCopyright 2015,TexasInstrumentsIncorporatedI2C Bus PullupResistorCalculationTable1. ParametricsfromI2C specificationsParameterStandardMode(Max) FastMode(Max)FastModePlus(Max)UnittrRise time of bothSDAand SCLsignals1000300120nsCbCapacitiveload for eachbus line400400550pFVOLLow-leveloutputvoltage (at 3 mA currentsink,VCC> 2 V) (at 2 mA currentsink,VCC 2 V) VCCVThe RP(min)is plottedas a functionof VCCin Figure2. The RP(max)is plottedas a functionof CbinFigure3 for standard-modeand MinimumPullupResistance[RP(min)]vs PullupReferenceVoltage(VCC)Figure3. MaximumPullupResistance[RP(max)]vs Bus Capacitance(Cb)()() 0(min).4V V (max) 10---===W ()()()rPb9300 20(m x0)10a- ===W- February2015 SubmitDocumentationFeedbackCopyright 2015,TexasInstrumentsIncorporatedI2C Bus PullupResistorCalculation3 SpeedVersusPowerTrade-offOncethe minimumand maximumvalueof the pullupresistorhas beenselected,the decisionfor the valueof resistorcan be madebasedon trade-offbetweenthe speedand smallerresistorwillgive a higherspeedbecauseof smallerRC delay,and a largerresistorwill give Fast-modeI2C communicationwith the followingparameters,calculatethe 200 pF, VCC= VSolution:Takingthe valuesfromTable1:(7)(8)Therefore,we can selectany and.

4 The valueof thepullupresistorcan be selectedbasedon the trade-offfor the powerconsumptionand ( TI ) technical,applicationor otherdesignadvice,servicesor information,including,but not limitedto,referencedesignsand materialsrelatingto evaluationmodules,(collectively, TI Resources ) are intendedto assistdesignerswho aredevelopingapplicationsthat incorporateTI products;by downloading,accessingor usingany particularTI Resourcein any way,you(individuallyor, if you are actingon behalfof a company,yourcompany)agreeto use it solelyfor this purposeand subjectto the termsofthis s provisionof TI Resourcesdoesnot expandor otherwisealterTI s applicablepublishedwarrantiesor warrantydisclaimersfor TIproducts,and no additionalobligationsor liabilitiesarisefromTI providingsuchTI reservesthe rightto makecorrections,enhancements,improvement sand otherchangesto its TI understandand agreethat you remainresponsiblefor usingyourindependentanalysis,evaluationa nd judgmentin designingyourapplicationsand that you havefull and exclusiveresponsibilityto assurethe safetyof yourapplicationsand complianceof yourapplications(andof all TI productsusedin or for yourapplications)

5 With all applicableregulations,lawsand ,with respectto yourapplications,you haveall the necessaryexpertiseto createand implementsafeguardsthat (1)anticipatedangerousconsequencesof failures,(2) monitorfailuresand theirconsequences,and (3) lessenthe likelihoodof failuresthatmightcauseharmand take agreethat priorto usingor distributingany applicationsthat includeTI products,youwill thoroughlytest suchapplicationsand the functionalityof suchTI productsas usedin has not conductedanytestingotherthanthat specificallydescribedin the publisheddocumentationfor a particularTI are authorizedto use, copyand modifyany individualTI Resourceonly in connectionwith the developmentof applicationsthat includethe TI product(s)identifiedin suchTI OTHERLICENSE,EXPRESSOR IMPLIED,BY ESTOPPELOR OTHERWISETOANYOTHERTI INTELLECTUALPROPERTYRIGHT,ANDNO LICENSETO ANYTECHNOLOGYOR INTELLECTUALPROPERTYRIGHTOF TI OR ANYTHIRDPARTYIS GRANTEDHEREIN,includingbut not limitedto any patentright,copyright,maskworkright,orot herintellectualpropertyrightrelatingto any combination,machine,or processin whichTI productsor servicesare referencingthird-partyproductsor servicesdoesnot constitutea licenseto use suchproductsor services,or a of TI Resourcesmay requirea licensefroma thirdpartyunderthe patentsor otherintellectualpropertyof thethirdparty,or a licensefromTI underthe patentsor otherintellectualpropertyof RESOURCESAREPROVIDED AS IS ANDWITHALL DISCLAIMSALL OTHERWARRANTIESORREPRESENTATIONS,EXPRESS OR IMPLIED,REGARDINGTI RESOURCESOR USETHEREOF,INCLUDINGBUTNOTLIMITEDTOACCUR ACYOR COMPLETENESS,TITLE.

6 ANYEPIDEMICFAILUREWARRANTYANDANYIMPLIEDW ARRANTIESOFMERCHANTABILITY,FITNESSFORA PARTICULARPURPOSE,ANDNON-INFRINGEMENTOF SHALLNOTBE LIABLEFORANDSHALLNOTDEFENDOR INDEMNIFYYOUAGAINSTANYCLAIM,INCLUDINGBUT NOTLIMITEDTO ANYINFRINGEMENTCLAIMTHATRELATESTO OR IS BASEDON ANYCOMBINATIONOF PRODUCTSEVENIFDESCRIBEDIN TI RESOURCESOR NO EVENTSHALLTI BE LIABLEFORANYACTUAL,DIRECT,SPECIAL,COLLAT ERAL,INDIRECT,PUNITIVE,INCIDENTAL,CONSEQ UENTIALOR EXEMPLARYDAMAGESIN CONNECTIONWITHORARISINGOUTOF TI RESOURCESOR USETHEREOF,ANDREGARDLESSOF WHETHERTI HASBEENADVISEDOF THEPOSSIBILITYOF agreeto fully indemnifyTI and its representativesagainstany damages,costs,losses,and/orliabilitiesar isingout of yournon-compliancewith the termsand provisionsof this Noticeappliesto TI the use and purchaseof certaintypesof materials,TI productsand ;withoutlimitation,TI s standardtermsfor ), evaluationmodules, and samples( ).MailingAddress:TexasInstruments,PostOf ficeBox 655303,Dallas,Texas75265 Copyright 2018,TexasInstrumentsIncorporat