单片机类毕业设计外文翻译.docx

单片机类毕业设计外文翻译.docx

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单片机类毕业设计外文翻译

ADCOversamplingTechniquesforStellarisFamilyMicrocontrollers

Introduction

LuminaryMicroprovidesananalog-to-digitalconverter（ADC）moduleonsomemembersoftheStellarismicrocontrollerfamily.ThehardwareresolutionoftheADCis10bits;however,duetonoiseandotheraccuracy-diminishingfactors,thetrueaccuracyislessthan10bits.Thisapplicationnoteprovidesasoftware-basedoversamplingtechnique,resultinginanimprovedEffectiveNumberOfBits（ENOB）intheconversionresult.Thisdocumentdescribesmethodsofoversamplinganinputsignal,andtheimpactonprecisionandoverallsystemperformance.

Oversampling

Asthenameimplies,oversamplinggathersadditionalconversiondatafromaninputsignal.StandardconventionforsamplingananalogsignalindicatesthatthesamplingfrequencyfSshouldbeatleasttwicethatofthehighestfrequencycomponentfHoftheinputsignal.ThisisreferredtoastheNyquistTheorem（seeEquation1）.

Equation1.NyquistTheorem

fS=2fH

AnysamplingfrequencyselectedabovefSisconsideredtobeoversampling,andwhencombinedwithaveragingtechniques,improvestheENOB.Thisispossiblebecauseaveragingtheoversampledresultsalsoaveragesthequantizationnoise,thusimprovingtheSignal-to-NoiseRatio（SNR）,whichhasadirecteffectontheENOB.

Foreachbitofaccuracyimprovement,thesignalmustbeoversampledbyafactoroffour,meaningthattherelationshipbetweentheoversamplingfrequencyfOSandsamplingfrequencyfSisasshowninEquation2.

Equation2.OversamplingFrequency

fOS=4x*fS

wherexisthedesiredimprovementontheENOB（forexample,fortwobitsofimprovement,x=2）.

Figure1showshowoversamplingimprovestheaccuracyoftheconversionresult.Inthisdiagram,theinputsignalisoversampledby4（samplegroupsareshowningreenandpurple）andaveraged.Thesamplepointsshownillustratethedifferencebetweentheraw,noisysignalandtheaverage;thenoiseinthisexampleaffecting±3bitsofaccuracyonanindividualsample.Notethattheaveragedvalues（orangedots）aremuchclosertotheidealvaluethanmostofthesinglesamples.

Averaging

Averagingactsasalow-passfilterontheinputsignal,withthepassbandofthefilternarrowingasthesamplesizeincreases.Whenaveragingconversionresults,therearetwoapproachesthatcanbetaken:

normalaverageorrollingaverage.

NormalAverage

Takingnsamples,addingthem,anddividingtheresultbynisreferredtoasanormalaverage,andisshowninFigure1.Whenusingnormalaveraginginanoversamplingscenario,afterthetechniqueisapplied,thesampledatausedinthecalculationisdiscarded.Thisprocessisrepeatedeverytimetheapplicationneedsanewconversionresult.

Figure1.AveragedConversionResults

Inanapplication,thenormalaveragingapproachisideallyusedincaseswherethesamplingfrequencyislowcomparedtothesamplingrateoftheADC.

Important:

Whenoversamplingbyninanormalaveragingscenario,theeffectiveADCsamplerateisreducedbythatsamefactor.Forexample,oversamplinganinputsignalby4cutsthemaximumeffectiveADCsamplerateby4,meaningthata250K-samples/sADCeffectivelybecomesa62.5K-samples/sADC.

Figure2showsasituationwherenormalaveragingisusedtooversampleaninputsourceby4.Forthisexample,theapplicationrequiresthatanewconversionvaluebeready（averagingcomplete）ateachstepoft（t0,t1,t2,andsoon）.

Whenusingaveragingtechniques,thereisaslightdelayassociatedwiththecalculatedconversionresultsinceitcorrespondstotheaverageofthelastnsamples.ThedelaycanbecalculatedusingtheformulashowninEquation3.

Equation3.AveragedSampleDelay

tdelay=（tSn–tS0）/2+tprocess

wheretS0isthetimeatwhichthefirstsampleoftheaverageoccurs,andtSniswherethelastsampleoccurs.Thetimerequiredbytheinterrupthandlertoprocessthesampledataandcalculatetheaveragetprocesstosupplytotheapplicationisalsofactoredintotheequation.InFigure2,thedelayedconversionresultishighlightedinorange.

RollingAverage

Arollingaverageusesasamplebufferofthenmostrecentsamplesintheaveragingcalculation,allowingtheADCtosampleatitsmaximumrate（theADCsamplerateisnotreducedbynasinnormalaveraging）,makingitideallysuitedforapplicationsrequiringoversamplingandhighersamplerates.Thesamplebuffercanbeprefilledwithvalidsampledata（bytakingn–1samplespriortothefirst“real”datapoint）,orcanbeleftinanunknownstate,dependingontheapplication.Thedisadvantageofnotprefillingthebufferisthatthefirstn–1samplescontaininvaliddataandadverselyaffecttherollingaveragecalculation.Ifacceptablebytheapplication,bufferpaddingcanbeeliminatedifthesoftwarecanaccountforthepossibilityofthefirstn–1samplesbeingskewed.

Figure3showsanexampleofoversamplingwitharollingaverage.Thediagramshowsacasewheretheinputsignalisoversampledby4,meaningthatthesamplebufferusesthe4mostrecentsamplestocalculatetheaverage.Inthisexample,theapplicationrequiresanewsampleateachstepoft.Beforethefirstoversampledresultiscalculatedatt0,thesamplebuffercollectsthreesamplessothatthefirstdatasuppliedtotheapplicationisvalid.

Figure2.NormalAveraging

Whenusingarollingaverage,thesamesampledelaycalculatedbyEquation3applies.InFigure3,thedelayedvaluesfort0,t1andt2（shownasd0,d1andd2,respectively）arehighlightedinorange.

Important:

Usingarollingaverageaddsadditionalprocessingoverheadduetothesamplebuffermanipulationthatmustbeperformedduringeachinterrupt.

Implementation

LuminaryMicro’ssamplesequencerarchitectureintheADCmakesoversamplingsimplebyallowingforupto17uniquesamples（fromanyoftheanalogchannels）tobecollectedusingasingletrigger.Thisallowsforflexibilityinsoftwarebyprovidingthemeansforanapplicationtooversampleanumberofchannelsatanygiventime.

ThissectionshowsvariousimplementationsofoversamplingusingtheStellarismicrocontrollers.Therearenumerousmethodsthatworkusingcombinationsofsamplesequencerconfigurations,ADCtriggersandinterrupts,however,theexamplesshownherefocusontechniquesthataremostlikelytobeused.

AlltheexamplecodeusestheStellarisFamilyDriverLibraryADCfunctions.ThesourcecodefortheDriverLibraryandthesoftwareexamplesshowninthisapplicationnotecanbefoundontheLuminaryMicrowebsiteat.

Figure3.RollingAverage

OversamplingUpto8TimesUsingtheDriverLibraryFunctions

TheStellarisDriverLibraryhasbuilt-infunctionsthatallowoversamplingupto8times.Formostapplications,thislevelofoversamplingissufficientsincetheimprovementontheENOBisapproximately1.4bits.

UsingtheDriverLibraryoversamplingfunctionsistheeasiestwaytooversampletheinputsignal.Themaindifferencebetweenconfiguringa“typical”ADCconversionandanoversampledconversionisthefunctioncalls.TheoversamplingfunctionshaveanADCSoftwareOversampleprefix,andareeasilydistinguishedfromthestandardADCfunctions.

OncetheparametersfortheADCconversionprocessaredetermined（samplefrequency,triggersource,channel,andsoon）,writingthecodeisstraight-forward.Forexample,thecodetosetupa10-msperiodicconversion（triggeredbyatimer）thatisoversampledby8consistsofthecodesegmentsshowninExample1.

Example1.8xOversamplingwiththeDriverLibraryFunctions

CodeSegment1.a.ADCConfiguration–DriverLibraryFunctions

//

//InitializetheADCtooversamplechannel1by8xusingsequencer0.

//Sequencerwillbetriggeredbyoneofthegeneral-purposetimers.

//

ADCSequenceConfigure（ADC_BASE,0,ADC_TRIGGER_TIMER,0）;

ADCSoftwareOversampleConfigure（ADC_BASE,0,8）;

ADCSoftwareOversampleStepConfigure（ADC_BASE,0,0,（ADC_CTL_CH1\

|ADC_CTL_IE|ADC_CTL_END））;

//

//InitializeTimer0totriggeranADCconversiononceevery10milliseconds

//

TimerConfigure（TIMER0_BASE,TIMER_CFG_32_BIT_PER）;

TimerLoadSet（TIMER0_BASE,TIMER_A,SysCtlClockGet（）/100）;

TimerControlTrigger（TIMER0_BASE,TIMER_A,true）;

TheADCconfigurationshowninCodeSegment1.adictatesthataninterruptoccurwhensamplingcompletes,meaningthataninterrupthandlermustbeimplemented（seeCodeSegment1.b）.SincetheDriverLibraryoversamplingfunctionsautomaticallyaveragethesampleddata,theinterrupthandlerfunctionisrelativelybasic.Keepinmindthathavingtheaveragecalculatedduringeachinterruptaddscomputationaloverheadtotheinterrupthandler.

CodeSegment1.b.ADCInterruptHandler

void

ADCIntHandler（void）

{

longlStatus;

//

//CleartheADCinterrupt

//

ADCIntClear（ADC_BASE,0）;

//

//GetaverageddatafromtheADC

//

lStatus=ADCSoftwareOversampleDataGet（ADC_BASE,0,&g_ulAverage）;

//

//PlaceholderforADCprocessingcode

//

}

Withtheconfigurationstepsandinterrupthandlerinplace,theconversionprocessisinitiated.Beforethetimeristurnedon（beginscounting）,theADCsequencerandinterruptmustbeenabled（seeCodeSegment1.c）.

CodeSegment1.c.EnablingtheADCandInterrupts

//

//EnableADCsequencer0anditsinterrupt（inboththeADCandNVIC）

//

ADCSequenceEnable（ADC_BASE,0）;

ADCIntEnable（ADC_BASE,0）;

IntEnable（INT_ADC0）;

//

//Enablethetimerandstartconversionprocess

//

TimerEnable（TIMER0_BASE,TIMER_A）;

OversamplingMoreThan8TimesUsingMultipleSequencersoraTimer

TheDriverLibraryoversamplingfunctionsarelimitedtooversamplingby8times