Capacitive Touch Hardware Design Guide (Rev. A)

1 DesignGuideSLAA576A May 2013 RevisedNovember2015 CapacitiveTouchHardwareDesignGuideHollyG u, CapTIvate technologyare the mostnoise-immunecapacitive-touchMCUsand first tofeaturean mostconfigurablecombinationofcapacitiveb uttons,sliders,wheels,and proximitysensors,all at the world' learnaboutthesenew solutions,go to sometimesconsideredmoreart multipledesigniterationsbeforethe optimumperformanceis , however,gooddesignpracticesfor circuitlayoutand principlesof materialsthat needto be understoodto keepthe numberof iterationstoa processfor creatingand designingcapacitivetouchsolutions,starti ngwith the schematic,workingthroughthe mechanicals,and finallydesigningthe electrodesfor of and Cross-SectionalViewsof TraceLine in Viewof Connectorand Viewof the ,Dos and Don' a Slideror Wheelfor ,MSP430are trademarksof is a trademarkof a registeredtrademarkof May 2013 RevisedNovember2015 CapacitiveTouchHardwareDesignGuideSubmit DocumentationFeedbackCopyright 2013 2015, collateralfor developingcapacitivetouchsolutions:1.

2 This document,theCapacitiveTouchHardwareDesig nGuide2. CapacitiveTouchSoftwareLibrary(CAPSENSEL IBRARY)3. Tuningguides:CapacitiveTouchSensing,MSP4 30 ButtonGateTimeOptimizationand TuningGuide(SLAA574)CapacitiveTouchSensi ng,MSP430 Sliderand WheelTuningGuide(SLAA575)Goodsensordesig nis the foundationfor a purposeof this designguideisto provideguidancefor the designand layoutof capacitivetouchsensorsso that they can achievingmaximumperformancein the Hardware ,the capacitivetouchsoftwarelibrarycan performthe capacitivetouchmeasurementswith the softwarelibraryis implementedfor the application,the tuningguidescan be usedto tunetheperformanceof the providesan overviewof capacitivetouchdetectionand describesits terms,concepts,andtheoryof and followingare organizedlike a productdesign fromschematictomechanicalsto schematicand mechanicalsare treatedas designrequirementsthat influencethe guidefirst describesrequirementsthat mightbe foundin the schematicandmechanicalrequirementsof the effecton the layoutand mustbeconsideredfirst.

3 Afterthe requirementsof the productare understood,thenthe PCBlayoutcan CapacitiveTouchSensingThis overviewis brokeninto two first part is the theoryof , somebasictermsare definedand discussedto give a basisfor the descriptionof capacitivetouchdetectionprovidedin the secondportionof the overviewin meritusedin usedto evaluatetheperformanceand are referredto frequentlyin the documentas the goal or reasonfor CapacitiveTouchSensingFigure2 showsan equivalentcircuitthat providesa basisfor understandinghow the differentaspectsofthe designcontributeto the equivalentcircuit,somebasictermsare introduced:electrode,traces,and :Electrodes,Traces,CapacitanceAnelectrod eis the physicalconductivestructurethat a structureis typicallythoughtof as the copperon a printedcircuitboard(PCB),but can also be madeof transparentmaterialssuchas indiumtin oxide(ITO)or otherconductivematerialslike shownin Figure1, the electrodeshapecan be very simple(for buttons)or very complex(for inter-digitation).

4 The electrodedoesnot necessarilyneedto be of the benefitsof capacitivetouchdetectionis that it allowsan applicationto use buttons,wheels,or slidersthat conformto the May 2013 RevisedNovember2015 SubmitDocumentationFeedbackCopyright 2013 2015,TexasInstrumentsIncorporatedSimple ElectrodeShape: ButtonComplex ElectrodeShape: SliderContour ElectrodeShape: CapacitiveTouchSensingFigure1. SampleElectrodeShapesAtraceis the conductiveconnectionbetweenthe MSP430 microcontrollerand the electrode,the traceis typicallya coppertraceon a PCB,but it couldalso be madeof materialslikeITO and cablesbetweenthe microcontrollerand electrodealso affectperformanceand are describedalongwith traceroutingin the abilityof the electrodeto storean the contextof capacitivetouchdetection,thereare a two commoncategoriesof capacitance:mutualcapacitanceand self the namesimply,self capacitancerefersto the capacitanceof one electrode,whilemutualcapacitancerefersto the capacitancebetweentwo capacitanceis the topicof this document,and theconceptsdescribedherepertainprimarily to importantconceptwithincapacitivetouchdet ectionis representsthesteady-stateno-interactionc apacitanceseenby the baselinecapacitanceis the sumof the parasiticcapacitances,whichincludethe electrode,trace,and parasiticcapacitancesassociatedwith the MSP430pins,solderpads,and any discretecomponentsassociatedwith the circuit(1).

5 Thebaselinecapacitanceis importantbecausesensitivityis a functionof the relativechangein baselinecapacitanceis too large,thenany changein capacitancecausedby a touchor proximityeventis very smalland mightnot be distinguishablefromthe variouscapacitancesfoundin a capacitivetouchsolution,an equivalentcircuitmodelcan be helpfulin visualizingthe sourceof the differentcapacitancesas well as the effectof is an exampleof an equivalentcircuitfor a singleself-capacitancebutton.(1)For example, May 2013 RevisedNovember2015 CapacitiveTouchHardwareDesignGuideSubmit DocumentationFeedbackCopyright 2013 2015,TexasInstrumentsIncorporatedCtouchC electrodeCtraceCparasiticsCgroundLocalGr oundEarthGroundOverviewof EquivalentCircuitFive differentcapacitancesare shownin Figure2. Cgroundis the capacitancebetweenthe localdeviceundertest (DUT)groundand someapplications,localand earthgroundare connectedwhenthe DUTusesmainspower,but typicallythe localgroundis capacitivecoupledbackto Celectrodeis the capacitancebetweenthe traceand electrodestructuresbackto the capacitanceis mostdirectlyaffectedby surroundingstructures,typicallygroundpou rs,that are eitheron the samelayeror on shownis the capacitancebetweenthe traceand electrodestructuresand not withoutmerit;however,for simplicityand in thecontextof this document,the designguidelinesare givenwith the principleof affectingthe localcapacitance(thatis, separationbetweenlocalgroundand the tracesand electrodes).

6 The capacitanceCparasiticsis a combinationof the internalparasiticcapacitanceof the microcontrollerand any componentswithinthe capacitanceis also referencedto touchcapacitance,Ctouch, is theparallelplatecapacitanceformedbetween the touchinteractionand the the exampleof atouch,as the fingerpressesagainstthe overlay,the flattenedsurfaceof the fingerformsthe upperplateand the electrodeformsthe capacitanceis a functionof the areaof the two plates,thedistancebetweenthem,and the dielectricof the materialthat , Section4, and Section5 describehow the physicalattributesof a designaffectthecapacitancesfoundin the goal is to addressthesecapacitanceswith layoutandmechanicaldesignguidelinesand maximizethe addressesperformanceparametersassociated with signaland NoiseCapacitivetouchdetectionis a type of analog-to-digitalconverter(ADC),specific allya with mostADCs,the termsof interestare resolution,signal-to-noiseratio(SNR)

7 ,andlinearity,in the specificcasesof wheelsand document,the designguidancehelpsto maximizesignal,minimizenoise,and addresswhenthesetwo goalsare at May 2013 RevisedNovember2015 SubmitDocumentationFeedbackCopyright 2013 2015, mentionedin , the basisof capacitivetouchdetectionis the abilityto measurea changein capacitanceis the signalthat the oftenusedto describethe signalstrength a moresensitivesolutionhas a measuredin capacitanceper the contextof capacitivetouchdetection,themagnitudeof changeintroducedby a touchis on the orderof picofaradsor hundredsof isnot uncommonfor a solutionto see a touchintroduce1 pF of changeand be measuredas 300 whilethe sensitivitymightbe fF/count,this shouldnot be considereduntil the noiseis factoredin( ).Sensitivitycan be controlledwithinfirmware,but the goal is to providegoodsensitivitywith the Hardware ,so that the lowestsensitivitysettingscan be usedin firmware,whichprovidesthe the equivalentcircuitin Figure2, Ctouchmustbe maximized,whilethe otherfour capacitancesmustbe capacitancesof the traceand electrodeare approximatedas servesas the basisfor = r 0 A/d(1)The dielectricconstant( r), area(A), and distance(d)

8 Are describedthroughoutthis documentwith theintentof positivelyinfluencingthe capacitancefor a presentedseparately,it is a part of the sensitivityand alreadymentioned,the capacitanceof interestis the relativechangein changein capacitanceisbaseduponthe touchinteraction,but this changeis perceivedrelativeto the parasiticcapacitanceof parasiticcapacitanceis also calledthe steady-statecapacitanceor introducedchangeis 100 fF, thenthe sensitivity,whichis the relativechangein capacitance,can beincreasedby decreasingthe capacitancesCtrace, Celectrode, and Cparasiticsare genericallyreferredto as termresolutioncan be usedin severaldifferentcontextsin the broadestsense,resolutionis sensitivityand definedin termsof measurementsystemmay bedesignedto resolvechangesin capacitancein ,therefore,the resolutionis applications,the termresolutioncan referto distancefor wheels,sliders,and theseapplications,the goal is to resolvechangesin distancealonga slideror abovea inverselyproportionalto the distancebut, in the contextof sliders,wheels,and proximitysensors,resolutionis moreclearlyexpressedin termsof directlylimitedby hardwaredesignmustprovidethe maximumamountofsensitivity,and any additionalsensitivitythat is neededto supportthe requiredresolutionmustbe gainedin firmware(typicallyat the cost of higherpowerconsumption).

9 Termlinearityis oftenusedto describethe performanceof capacitivetouchwheelsand the linearityperformanceof ADCor digitalto analogconverter(DAC),the linearityis how wellthe reportedpositionmatcheswith the wheelor slidercan havepoorlinearitywhichareflat or 'deadspots'or missingcodes,becauseof the designand shapeof the poorexamplesof relatedto increasingsensitivity,the flat or deadspotscan be reducedoreliminated,but this is typicallydonein conjunctionwith shapingof the May 2013 RevisedNovember2015 CapacitiveTouchHardwareDesignGuideSubmit DocumentationFeedbackCopyright 2013 2015, rangeor distanceis directlyproportionalto the hardwaredesign,the greaterthe distanceor rangecan be achievedwith a signalis the changein capacitancethat resultsin a meaningfulchangein ,on the otherhand,is any disturbancethat doesnot changethe capacitancebut doeschangethe the resultof powersupplyswitchingnoise,electrostaticd ischarge(ESD),electricallyfast transients(EFTs)

10 ,radiatednoise,or someothertype of electricalnoisethat couplesintothe generalaggressor-victimapproachis takento ,the capacitivetouchcircuitisthe victim,but whatis not so obviousare the mediathroughwhichthe noisetravelsfromthe aggressortothe the VCCrail, the VSSrail, and the capacitivecouplingbetweentraces(crosstal k).Typically,the aggressoris anythingwithinthe sameoperating(frequency)bandas an aggressorat the samefrequencycannotbe avoided,the mediumbetweenthe aggressorand victimshouldbe is donebymaximizingthe distancebetweenthe aggressorand victim,whichis a commonthemein (SNR)The SNRis a system-levelspecificationand needsto be testedat the adequatebut is part of the overalldesignto achievean mainidea in providinggoodsensitivityis to maximizethe capacitanceassociatedwith a touchand minimizethe noisemustbe mitigatedand reducedwith gooddesignpracticesfor noiseimmunity(from,for example,ESD,RF, or EMC).