Reducing Analog Input Noise in Touch Screen Systems (Rev. A

1 ApplicationReportSBAA155A July2007 ,theprintedcircuitboardlayoutandgroundin garethekeys; ,severalsimple, ' , July2007 +, X , Y+, Y--I Cor SPIBus2 TMTMI nterruptResistiveTouch ScreenTouch ScreenControllerHost ProcessorAnalog InterfaceAnalog Input CircuitryDigital InterfaceIntroductionInpracticalapplicat ions, ,suchasatouchscreencontroller(TSC). 'sADS7846 TouchScreenControllerInaTSCsystem,thetou chpanelisusuallyassembledontopofanLCDdis play,andtherefore, ,asthehumanandmachine(computer)interface device, ,aswellasaresultofelectrostaticdischarge (ESD)andelectromagneticpulses(EMP) , July2007 ,including: Attentiontoprintedcircuitboard(PCB)layou t; Additionalde-couplingcapacitors; Implementingaveragingandfilters;and ,inadditiontoeasingotherdesignrestrictio ns,reducingdesign/debugcosts, ,additionalcareshouldbegiventotheconnect ionbetweentheTSCandthetouchscreen;thatis , (100 to2000 ), (ADC)inmostresistiveTSCdevicesisasuccess ive-approximationresponse(SAR) ,groundconnections, ,duringanysingleconversionforann-bitSARc onverter, ,nearbydigitallogic, ,layout, ,powertotheTSCshouldbecleanandwell-bypas sed, , , ,cost,size,andweight; ,noisecanbeamajorsourceoferrorintouchscr eenapplications(forexample,applicationst hatrequireaback-litLCDpanel).

2 Thiselectromagneticinference(EMI) , , ,addingseveraldecouplingcapacitors,fromY +,Y ,X+,andX toground, ,however,thatusingthesecapacitorsincreas esscreensettlingtimeandrequireslongerpan elvoltagestabilizationtimes, July2007 PAIRS/SEC vs PANEL RESISTANCE vs FILTER CAPACITANCE0100 200 300 400 500 600 700 800 900 1000 Panel ResistanceCoordinate Pairs per Fm1 Fm4 SimpleSoftwareAverage/FilterAlgorithmsX( k) =bx(k n)*-n N 1-n = 0(1) ,inputcircuitcapacitance, ,000X/Ycoordinate-pairspersecondona300- ,inareal-worldapplication, ,firststartwithaddingasmallcapacitor(for example, F),placedonlyonthenegative(X orY ) (seetheTSCdiagraminFigure2).Thefirstplac eisatthehostprocessor,inthehostsoftware; theother(andabetter)locationisattheTSC,i ntheTSCbuilt-inhardware( , ). ,considerselectingandusingaTSCdevicewith abuilt-infilteringfunction, ,thedataaveragingorfilteringcanbeexpress edasafiniteimpulseresponse(FIR)filter,as showninEquation1:Where: X(k)istherefinedtouchdata x(k)istherawtouchdata bnisthecoefficientorweightfactorofthefil ter Nisthenumberofthemultiplesamplesforeacht ouchcoordinate(N 1istheorderofthefilter)Thissectionpresen tsseveralsimpleaverage/filteralgorithms, , July2007 =n1N(2)X(k) =x(k n) / 4 =- 3n = 0[ x(k) + x(k 1) + x(k 2) + x(k 3) ]---4(3) =n0 for the first one or more data x(k)1/M for the later x(k)(4)SimpleSoftwareAverage/FilterAlgor ithmsAveragingcanbeconsideredasEquation1 withEquation2:Equation3showsthisexample: sample4 Xdata,N=4andbn=1/N, **//#defineN4//multiplesamplingdataN=4ti mesunsignedintX;//therefined/averagedXun signedintx[N];//the4multiplesamplesrawto uchdatabytei;//aloopcounter//storethe4ra wsampledataintox[4]arraybeforerunthefoll owingcode////**FindtheaveragevalueXofNsa mplesinx[4]array**////X=x[0].

3 //sumtheNrawdatafor(i=1;i<N;i++){X+=x[i];}X=X>>2;//dividebyN(hereN=4)//**//Inatouchscr eensystem, ,whenthehostsamplesacoordinate(forexampl e,X), ;useEquation4inEquation1:Where: Misthenumberofaveragedmultipledata M<NForexample,sample4 Xdata,N=4;anddiscardthefirsttwosamples,thenM= July2007 =n1 for middle value x(k)0 for all other x(k)(5) **//#defineN4//multiplesamplingdataN=4timesunsignedintX;//therefined/averagedXunsignedintx[N];//the4multiplesamplesrawtouchdatabytei;//aloopcounter//storethe4rawsampledataintox[4]arraybeforerunthefollowingcode////**FindtheweightedaveragevalueXofNsamplesinx[4]array**////X=x[2];//sumtheMrawdatafor(i=2;i<N;i++){X+=x[i];}X=X>>1;//dividebyM(hereM=2)//**//Themiddleva lueofmultiplesamplescanbeconsideredasEqu ation1withEquation5:Obviously, (forexample,atN=3)is: Samplethreetouchdatapoints;x(k),x(k 1),andx(k 2); Comparex(k)andx(k 1),andthencomparethelargerone(xH)tox(k 2); IfxHissmallerthanx(k 2),thenX=xHandthecalculationisdone. Else:comparex(k 2)andnonxH(thatis,xL);thelargeroneisX, July2007 =n1/2 for 2x data with values closer to each other0 for 1x with values not close to each other (outlying)(6) **//unsignedintX;//themiddlevalueXunsign edintx[3];//the3multiplesamplesrawtouchd ata//storethe3rawsampledataintox[3]array beforerunthefollowingcode////**Findthemi ddlevalueXamong3samplesx[0].

4 X[1]andx[2]**////if(x[0]>=x[1]){//compar ex(k)andx(k-1)xH=x[0];xL=x[1];}else{xH=x [1];xL=x[0];}if(xH<=x[2]){//largeronexHcomparetox(k-2);X=xH;//ifxHsmallerthanx(k-2):thenX=xH}else{//else:comparex(k-2)andnonxH( )if(x[2]>=xL){//thelargeroneisXX=x[2];}else{X=xL; }}//**//Thereisanothercommonaveragealgor ithmthatusesonlythedatapointsthatareclos etoeachother, ,TakeXdatathreetimes(again,forN=3), :Theexamplealgorithmis: Samplethreetouchdata,sayx(k),x(k 1),andx(k 2); Gettheirabsolutedifferentials:d1=|x(k) x(k-1)|,d2=|x(k 1) x(k 2)|andd3=|x(k 2) x(k)|, Findthesmallestamongd1,d2,andd3 SBAA155A July2007 **//unsignedintX;//themiddlevalueXunsign edintx[3];//the3multiplesamplesrawtouchd ataintd[3];//thedifferentialsofrawtouchd atabytei;//aloopcounter//storethe3rawsam pledataintox[3]arraybeforerunthefollowin gcode////**FindtheaveragevalueXof2close- valuedsamplesinx[0].x[1]andx[2]**////d[0 ]=x[0] x[1];//getthedifferentialsd[1]=x[1] x[2];d[2]=x[2] x[0];for(i=0;i<3;i++){//gettheabsolutedifferentialsif(d[i]<0){d(i)= d(i);}}if(d[0]<d[1]){//whend[0]<d[1]if(d[0]<d[2]){//compared[0]tod[2]X=x[0]+x[1];//d[0]isthesmallestifd[0]<d[2]also}else{X=x[0]+x[2];//d[2]isthesmallestifd[2]<=d[0]<d[1]}}else{//otherwise, :whend[1]<=d[0]if(d[1]<d[2]){//compared[1]tod[2]X=x[1]+x[2];//d[1]isthesmallestifd[1]<d[2]also}else{X=x[0]+x[2];//d[2]isthesmallestifd[2]<=d[1]<=d[0]}}X>>=1;//**//Anotherwaytoreducenoiseandelim inateunreliabledata,usingsoftware, ,thehostdoesnotwantthetouchdatasetwhenth epanelhasnotbeentouched(nopressure),orwh enithasnotbeenfirmlytouched(verylightpre ssure).

5 July2007 X-PositionTouchX+Y+X-Y-VD+-ABX-Positiona )Add panel driving power to X+/X pins, and read X-position from Y+ )Add panel driving power to Y+/X pins, and read Z -position from X+ pin -position from Y pin-Measure Z -Position1 TouchX+Y+Y-X-VD+-Z -Position1Z -Position2 ITOUCHR =Z(VV )I-BATOUCH(7)R =ZVD21(ZZ ) / Q- VZD1/ Q R= RX / QXXAZZ21-1(8)P =Ra-b Z(9)TouchScreenPanelPressureTheresistive touchpanelpressureisproportionaltotheres istancebetweenpointsAandBinFigure4,denot edbyRZ, : VA=VD Z1/Q VB=VD Z2/QZ1andZ2arethemeasuredZ1-PositionandZ 2-Position,respectively; ,Q=256iftheTSCresolutionis8-bit,Q=1024if itis10-bit,orQ= (VDD)isaddedbetweenY+andX ,asshowninFigure4b,ITOUCH=VA/RXA=(VD Z1/Q)/RXA,withtheresistanceatpointAbeing RXA=RX X/Q,whereXisthemeasuredX-positionwhenthe XdriverisON, ,bybringingtheexpressionofITOUCH hereintoEquation7,wegetEquation8:Equatio n8isthefirstdatasheetequationformanyTITS C devices; , , ,itbecomesconductive,withresistancefroms everalhundredohmsto1k ,inverselyproportionaltothepressureonthe touchpanel(denotedbyP).

6 Thatis:thepressureontouchpanel,P,canbeex pressedasafunctionofRZanditisgivenbyEqua tion9:Where and July2007 , **//unsignedintP;//thetouchpressureunsig nedintThreshold//thetouchpressurethresho ld//SampleX,Y,Z1andZ2 (P>Threshold){//thesetoftouchcoordinates (X,Y)isvalid}//** 'sTSCEVM Systems ,suchasthatintheTSC2003 EVM-PDKortheTSC2005 EVM-PDK, , ; , July2007 , , ,W.(2007.). , ,W.(2007.). :I2C ,12-Bit,Nanopower,4-WireTouchScreenContr ollerwithI2C ,12-Bit,Nanopower,4-WireTouchScreenContr ollerwithSPI :Nanopower,4-WireTouchScreenControllerwi thSPI ,12-Bit,Nanopower,4-WireMicroTouchScreen ControllerwithI2C ,W.(2007.). July2007 RevisedSeptember2007 ReducingAnalogInputNoiseinTouchScreenSys tems11 SubmitDocumentationFeedbackIMPORTANTNOTI CET exasInstrumentsIncorporatedanditssubsidi aries(TI)reservetherighttomakecorrection s,modifications,enhancements,improvement s, , , ,eitherexpressorimplied,isgrantedunderan yTIpatentright,copyright,maskworkright,o rotherTIintellectualpropertyrightrelatin gtoanycombination,machine, , ,conditions,limitations, (suchaslifesupport)whereafailureoftheTIp roductwouldreasonablybeexpectedtocausese verepersonalinjuryordeath, ,andacknowledgeandagreethattheyaresolely responsibleforalllegal,regulatoryandsafe ty-relatedrequirementsconcerningtheirpro ductsandanyuseofTIproductsinsuchsafety-c riticalapplications, , "enhancedplastic.

7 " 'srisk, ,iftheyuseanynon-designatedproductsinaut omotiveapplications, :TexasInstruments,PostOfficeBox655303,Da llas,Texas75265 Copyright 2007,TexasInstrumentsIncorporat