Calculator for CAN Bit Timing Parameters - TI.com

1 1 SPRAC35 March2016 SubmitDocumentationFeedbackCopyright 2016,TexasInstrumentsIncorporatedCalcula torfor CANBit TimingParametersApplicationReportSPRAC35 March2016 CalculatorforCANBitTimingParametersAdamH aunABSTRACTC ontrollerAreaNetwork(CAN)nodesuse user-specifiedtimingparametersto samplethe asynchronousbitstreamand recoverthe typicallybasedon the frequencyof the severaloptionsavailablefor a givenfrequency,and someof themwillallowa creationof a bit timingcalculatorto maximizethe oscillatortolerancefor the CANmoduleson TI this applicationreportcanbe downloadedfromthe followingURL: a trademarkof othertrademarksare the propertyof the bit timingparametersrequiresat leastthreeinputs: The desiredbit rate,whichis commonacrossthe entirebus. An estimateof the propagationdelaybetweenthe mostdistantnodeson the bus The CANmoduleclockfrequency,whichis usedto derivethe bitstreamsamplingclockThe propagationdelaycomesfromtwo mainsources:the speedof electromagneticpropagationalongthebus wiringand extradelaydue to transceivers,isolation,or bit timingcalculatormustprovideat leastfive outputs: The baudrate prescaler(BRP),whichdeterminesthe singleperiodis referredto as a time quantum(Tq).

2 The propagationdelayexpressedas a numberof time quanta. The lengthof phasesegment1 (PhSeg1)in time additionto determiningthe bit rate,phasesegment1 acts as a bufferthat can be lengthenedto resynchronizewith the bitstream. The lengthof phasesergment2 (PhSeg2)in time is like phasesegment1,but it occursafterthe samplingpointinsteadof can be shortenedtoresynchronizewith the bit stream. The synchronizationjumpwidth(SJW)in time is the maximumtime by whichthe bitsamplingperiodmay be lengthenedor shortenedduringeachcycleto adjustfor calculatordescribedin this documentalso outputsthe numberof time quantaper bit, the oscillatortolerance,the samplingpoint,and timingregistervaluesfor the eCANand D_CANmodulesusedon theTMS320C2000 seriesof microcontrollers.()2 10 SJWdbit _ timef =()()()min PhSeg1,PhSeg22 13 bit _ time PhSegf2d - = March2016 SubmitDocumentationFeedbackCopyright 2016,TexasInstrumentsIncorporatedCalcula torfor CANBit TimingParameters2 ImplementationThe calculatorfirst computesthe propagationtime as a fractionof the bit bus length,aphysicalpropagationdelay,or bothmay be bus propagationspeedis assumedto be twothirdsof the speedof light,the standardvaluefor twistedpair resultingdelaytime is doubledtogive a roundtrip numberof time quantaper bit (knownas the bit time)can rangefrom8 to 25.

3 Thereis no analyticformulafor determiningthe optimumbit time in advance,so the calculatortries all of closestintegralbaudrate prescaleris calculatedfor BRPand Tq/bitvaluesare thenusedtocalculatethe actualsamplingbit differencefromthe desiredvalueis addedto the total propagationsegmentis determinedfromthe fractioncomputedabove,roundedup. The remainingtime is split betweenphasesegments1 and 2, with PhSeg2roundedup and synchronizationjumpwidthis limitedto 4 Tq at most,and cannotbe longerthanPhSeg1or ,the SJWis the minimumof derivedfromtwo first givesthe conditionfor correctevaluationof 13dominantbits in a row duringan errorcondition,wherethereis no re-synchronization:(1)The secondformulagivesthe conditionfor full re-synchronizationafter10 consecutivebits:(2)The factorof 2 in eachformulacomesfromconsideringtwo oscillatorson differentnodes,eachhavingtheworst-casefr equencyerrorin final oscillatortoleranceis the minimumof thesetwo valuesminusany errorin the tolerancesfor all possiblevaluesof Tq/bithavebeencalculated,the settingswith the besttoleranceare reportedin the outputsectionof the settingsare thenconvertedinto eCANand D_CANregistervalues,whichare also calculatorcan reporterrorsbasedon severalconditions: The CANclockfrequencymustbe at least8 timesthe bit rate to give the minimum8 Tq/bit.

4 The propagationdelaymustbe shortenoughto allowreasonablevaluesfor the phasesegments. The ratiobetweenthe CANclockand the bit rate mustbe closeenoughto an integerto calculatorreportsa warningwhenthe exactbit rate cannotbe derivedfromthe also reportsa warningwhenthe propagationtime is morethanhalf the bit time,sincethis impliesthatthe bus lengthis too long for the desiredbit Introductionto the ControllerAreaNetwork(CAN)(SLOA101) ControllerAreaNetworkPhysicalLayerRequir ements(SLLA270) BoschCANS pecification( )IMPORTANTNOTICEFORTIDESIGNINFORMATIONAN DRESOURCEST exasInstrumentsIncorporated( TI ) technical,applicationor otherdesignadvice,servicesor information,including,but not limitedto,referencedesignsand materialsrelatingto evaluationmodules,(collectively, TI Resources ) are intendedto assistdesignerswho aredevelopingapplicationsthat incorporateTI products.

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