Iphone 破解 耳机口通信

Ye-ShengKuo,SonalVerma,ThomasSchmid,andPrabalDutta

ElectricalEngineeringandComputerScienceDepartment

UniversityofMichiganAnnArbor,Michigan48109

{samkuo,sonalv,thschmid,prabal}@eecs.umich.edu

ABSTRACT

Weendowthemobilephonewithalow-cost,openinterfacethatcanparasiticallypowerexternalperipherals,andtransferdatatoandfromthem,usinganalog,digital,andserialsignaling,usingonlytheexistingheadsetaudioport.Thisinterface,calledHiJack,allowsthemobilephonetoeasilyintegratewitharangeofexternalsensors,openingthedoortonewphone-centricsensingapplica-tions.Inthispaper,wecharacterizethesignalingandpowerde-liverycapabilityoftheaudiojack,designcircuitsandsoftwaretotransferdataandharvestenergy,andevaluatetheperformanceofourdesigns.Wealsousethemobilephone’saudiochanneltocre-atealayeredcommunicationsstackthatsupportslow-level,analogsignalingandhigh-level,multiplexeddatacommunicationswithexternaldevices.Ourdesignsupportsasingle,bi-directionalcom-municationschannelatadatarateof8.82kbpsoveraManchester-encodedserialstream,usingjustafewdiscretecomponentsandthehardwareperipheralsfoundinalmostanymicrocontroller.Ourharvesterdelivers7.4mWtoaloadwith47%efficiencyusingcom-ponentsthatcost$2.34in10Kvolume.Integratingthepieces,wepresentacombinedsystemfordeliveringdataandpoweroverau-dio,anddemonstrateitsusebyturninganiPhoneintoaninexpen-siveoscilloscope,EKGmonitor,andsoilmoisturesensor,allatpricepointsaccessibletomostconsumersindevelopingregions.

CategoriesandSubjectDescriptors

B.4.2[HARDWARE]:Input/OutputandDataCommunications—Input/OutputDevices;C.3[COMPUTER-COMMUNICATIONNETWORKS]:Special-PurposeandApplication-BasedSystems

GeneralTerms

Design,Experimentation,Measurement,Performance

Keywords

Mobilephones,Energyharvesting,Phoneperipherals,Audiocom-munications,Participatorysensing

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ACMDEV’10,December17–18,2010,London,UnitedKingdom.Copyright2010ACM978-1-4503-0473-3-10/12...$10.00.

1234Figure1:TheiPhoneheadsetpluganditspinout.iPhonesusea3.5mmphonojack/plugtooutputaudiotoheadphonesandreceiveinputfromamicrophone.Theheadphoneconnectionsare:(1)leftearphone(tip),(2)rightearphone(ring),(3)com-mon/ground(ring),and(4)microphone(sleeve).ThemeasuredimpedanceoftheiPhoneheadsetbetweentheleft(orright)ear-phoneandcommonis33Ω.Themeasuredimpedancebetweenthemicrophoneandcommonisapproximately640Ω.

1.INTRODUCTION

Themobilephoneisthemostpervasivepersonalcommunica-tionsandcomputingplatformevercreatedandyet,amongitsvar-iousanaloginterfaces,onlyoneistrulyopen:theheadsetport,asshowninFigure1.Inthispaper,wetakeacloserlookatthiscommoninterfaceandassessitsutilityforaugmentingthemobilephonewitharangeofphone-poweredperipherals.Weshowthatthemobilephoneheadsetportcanbeusedtoefficientlypowerex-ternalperipheralsandcommunicatewiththem,enablingmanynewphone-centricapplications.Butwhy,beyondopenness,shouldweusetheheadsetport?Therearemanyreasons:itissimple,inex-pensive,ubiquitous,anddocumented.

Butperhapsevenmoreimportantisthefactthattheheadsetin-terfaceisbackward-compatiblewithmostphonesinusetoday,andmanyrecycledonestoo,soexistingphonescouldformthebasisformanyhealthandcommunicationsapplicationsindevelopingre-gions.Thispaperexploresthefeasibilityofthisinterfaceforpowerdeliveryandthegeneralityoftheinterfacefordatatransfer.Ourgoalistounderstandthedesignspace,enablephone-poweredpe-ripherals,andmakethephonethecenterofanewinstrumentationecosystemfordevelopingregions.

Anaffordableandubiquitousplug-and-playinterfaceforattach-ingsensorscouldhavemanyapplications.Itcouldturnthemobilephoneintoascientificdatacollectioninstrument,amedicalde-vice,oranagriculturaltoolbyconnectinglow-powersensorstoasourceofconnectivityandpower.Themobilephone,withitseverimprovingcomputing,communications,andgraphicscapabil-itiesisanobviouschoice.Moregenerally,byleveragingastandardandubiquitousinterfaceliketheheadsetjack,thephonecouldalsoserveasamultimeteroroscilloscope–usefultoolsthatotherwisemaybeoutofthereachofmostcitizensindevelopingregions,butwellwithintheirgraspasa$5phoneaccessory.

ParameterValueHardwareiPhone3GS[1]SoftwareSignalScopePro[2]FunctionSignalGeneratorOutputHeadphonesTypeToneFrequency20Hzto24kHz(5kHznom)Amplitude0.00dBPan0.000%VolumeMaximumTable1:ExperimentparametersfordeterminingtheavailablepowerfromtheiPhone3GSheadsetport.

Usingtheheadsetporttopowerandcommunicatewithexternalperipheralsposesseveralengineeringandresearchchallenges.Theheadsetoutputisalowvoltagesignal,oftenevenlowerthantypicaltransistorthresholdvoltages.Tobeuseful,itmustbeconvertedtoahighervoltageusingenergyharvestingandvoltageboostingcir-cuitsthatcanoperatewithinputACvoltagesinthe200mVrange.Duetothelimitedvoltageheadroom,simplerectificationisdiffi-cultwithoutsubstantialpowerlosses,andmaximumpowerpointtrackingmayberequiredinsomecases.Matchingtheharvestingcircuit’scost,complexity,andconversionefficiencywiththeidealaudiowaveformalsopresentsaniterativeco-designproblem.Us-ingtheaudiooutputtodeliverpoweranddatafunctionalityrequiresexploringthesedesigntradeoffs.

Inthispaper,wecharacterizethepoweravailablefromtheau-diojack,designacircuittoharvestthispower,andevaluatetheefficiencyoftheconversion.Wefindthattheheadsetcandeliver15.8mWperchannelfromtheiPhone’sheadsetport.Wepresentacircuitthatcanharvestenergyfromasinglechannelandanau-diosignalthatwhenplayedonthephonecanmaximizetheoutputpowerfromtheharvestingcircuit.Ourenergyharvestingcircuitdeliversupto7.4mWtoaload–a47%powertransferefficiencycomparedwiththeoutputpowercapabilityoftheheadsetport–usingjust$2.34inelectroniccomponents.

Wealsodemonstratethatapairof(coded)audiosignalscanbegeneratedbythephoneprocessorandtransmittedtoboththeen-ergyharvestingcircuit(forpowertransfer)andamicrocontroller(fordatatransfer).Conversely,weshowthatthemicrocontrollercangenerateacodedsignalthatcanbesampledbythemobilephone’smicrophoneinputanddecodedbythephonetopresentastreamofdigitaldata,establishingan8.82kbpsbi-directionaldatastreamusingthehardwareresourcesfoundinamicrocontroller.Integratingthepieces,wepresentanoscilloscope,EKGmonitor,andsoilmoisturesensorthatrunspartlyonthephoneandpartlyonanexternalmicrocontroller(MCU)poweredusingthemobilephone’srightaudiochannel.Thetwoprocessorscommunicateus-ingtheleftaudiochannel(phonetoMCU)andmicrophonechannel(MCUtophone).Wealsopresentasinglecircuitboard,measuringjust1.0”x1.0”,onwhichapplication-specificsensorsareattached.

2.ENERGYHARVESTING

Ourfirstdesigngoalistoharvestenergyfromtheheadsetjackofamobilephone,convertitintoamoreusableform,andachievehighconversionefficiencyintheprocess.Webeginbycharacteriz-ingtheACwaveformsthatareavailable,theoutputimpedanceoftheheadsetport,andtherangeofvariablesavailableformanipula-tion.Wethendesignandevaluateanenergyharvestingcircuittoconverttheavailablepowerintoamoreusableform.

17517.515015)sm12512.5)r ,WAmm( (r 10010tenweorrpu tc757.5u tputputuOO505252.500100Output voltage (mV, rms)2003004005000Figure2:AvailablepowerfromtheiPhoneheadsetjack.TheIVcurvedatashowthatitispossibletodraw15.8mWfromaniPhone3GSwithanideally-matched3.6Ωload.TheiPhonecansupplyenoughtopowertooperatemanylow-powerelec-tronics.Tobeuseful,however,thecurrentmustberectified,thevoltagemustbeboosted,andtheoutputmustbefiltered.

2.1DeterminingAvailablePower

Wenowexplorethequestionofhowmuchpowercanbehar-vestedfromiPhone’sheadsetport.Todoso,weusetheFaberAcousticaliPhoneSignalScopeProsoftware[2]togeneratearangeofaudiofrequencies,from20Hzto24kHz,andoutputthemovertheiPhone’saudioport.Wefindthattheoutputpowerisindepen-dentoffrequency,soweusea5kHzACtoneinoursubsequentexperiments.Table1showsthesettingstogeneratetheoutput.Aloadisconnectedbetweentherightaudiochannelandcom-monlineontheheadsetandvariedfrom0Ωto15kΩ,andtheoutputvoltageandloadcurrentaremeasuredatseveralpoints.Alinearfitofthedatayieldsthe(essentiallylinear)IVcurveshowninFigure2.Fromthesedata,wegeneratethepowertransfercurve,whichshowsthatmaximumpowertransferoccursat240mVrmsandwhendelivering66.0mArms,fora3.6Ωloadimpedance.

2.2ExploringtheDesignSpace

Wenextexplorethequestionofhowtoefficientlyharvesttheenergyproducedfromtheheadsetoutput.ThetwoengineeringchallengesaretoincreasethesignalamplitudeandconverttheACsignalintoaDCone.Figure2showsthattheopencircuitvolt-age,Voc,islessthan500mVandthatthemaximumpowerpointvoltage,Vmpp,occursat240mV.Thesevoltagesarefarbelowtheturnonvoltagesofswitchingregulators(typicallyintherangeof800mVto900mV).Theyarealsobelowtherequiredstartupvolt-age,afterrectification,ofultra-lowvoltagestep-upDC-DCcon-verters,liketheSeikoS-882Z[3],whichrequire300mVtostart.Rectificationlossescanbesignificantinbothhigh-powerandlow-voltagesystems.Inourcase,forexample,toachievemaxi-mumpowertransfer,anRMScurrentof66mAisrequired.Whenrectifiedusingevenalow-VfSchottkydiodeliketheDFLS120L,a200mVforwardvoltagedropoccurs(SeeFig1inreference[4]),meaningthat80%ofthepowerislostduringrectification,andonly20%canbedeliveredtotheload.1

1

wouldThisassumesthatonlyasinglerectifierdiodeisonarewouldontheofcoursereducetheavailablepowerby50%.theIfpath,twodiodeswhichbesubstantiallypath,aswouldhigher.

bethecaseforabridgerectifier,thelossesBCDFigure3:Theaudiooutputwaveformobservedexternallywhena20HzsquarewaveEistransmittedovertheaudioout-putchannel.Thesharptransitionsfollowedbycharacteristicexponential-decaycurvesshowtheoutputisAC-coupled,elim-inatingthesimpleoptionofdrivingtheaudiooutputwithaconstantDCvoltage.

Synchronousrectificationissometimesusedtoreducelosses,whereaFETswitchisusedinsteadofadiode[28].Inlow-voltageapplicationslikeours,theproblemisgeneratingasufficientlyhighgatedrivevoltagetoturnontheFETswitch.Giventhelowvolt-agesinvolved,thiswouldrequireseveralstagesofinefficientvolt-agemultiplication,perhapsusingaladdercircuit.

Weendthissectionondesignalternativesbyeliminatingtwosimple,butultimatelyunworkable,options:harvestingDCdirectlyfromtheaudiooutputandharvestingDCfromthemicrophonebiasvoltage.Figure3showsthewaveformthatisobservedontheaudiooutputwhena20Hzsquarewavesignalisgeneratedonthephone.ThecharacteristicexponentialdecaycurvessuggestthattheoutputisAC-coupled,andisthereforehigh-passfiltered,blockingDC.ThiseliminatesthepossibilityofsimplygeneratingaDCoutputvoltagetopowertheexternaldevices.Usingthemicrophonebiasvoltageisalsoproblematicbecauseweplantouseitasthedatainputchanneltothephone,whichwillbemodulatedexternallybyamicrocontroller.

2.3HarvestingEnergyEfficiently

Tosidestepthetwobasicengineeringchallenges–low-supplyvoltageandneedforrectification–weuseastep-upmicrotrans-former,followedbyFET-basedrectification,followedbyablock-ingSchottkydiode,followedbyfiltercapacitors,asshowninFig-ure4.Onekeyelementofthedesign,themicrotransformer,lever-agesarecentlyintroduceddeviceforflybackandstep-upforen-ergyharvestingapplications.Thesenewtransformersaresmall(6mmx6mmx3.5mm),havehighcouplingcoefficients(>0.95),andareavailableinarangeofturnsratios[5].Weusea1:20ratioinourenergyharvesterdesign.

Thestepped-voltageispassedthroughaFETbridgeforrectifi-cation.Sincethestepped-upvoltageissubstantiallyhigherthantheFETthresholdvoltage,theFETsareinconductionandoffermarginalloss.Anotherbenefittostepping-upthevoltageisare-ductionincurrentflowthroughtheblockingdiode,andthereforeareductioninforwardvoltagedrop.However,sincethediodeisanexponentialdevice,thisunfortunatelydoesnotresultinasubstan-tialdecreaseintheforwardvoltagedrop,butitdoeseliminatethe

T1Q1Q3J1141D12L1L2ACR11J223Q42C1C21J32+C3Q2D2Figure4:Theenergyharvestingcircuit.A1:20microtrans-formerbooststheinputvoltage.AFETbridgeefficientlyrecti-fiestheACcurrenttoDC.Alow-VfSchottkydiodeprovideslow-lossblockingtopreventtheoutputfiltercapacitorfromdischargingthroughtheFETbridge.An(optional)LEDwithcurrent-limitingresistorprovidesavisualpowerindicator.Figure5:Energyharvestingcircuit.Thetransformer,Schottkydiode,LED,andresistorarevisibleonthetopside.TheFETrectifierandfiltercapacitorsarevisibleonbottomside.voltagedropfromaseconddiodeintherectifier.And,sincethediodeforwardvoltagedropisonlyasmallfractionoftheboostedvoltage,thisdesignincursasmallinefficiencycomparedtodirectrectificationoftheoriginallow-voltagesignal.

Matchingtheloadandsourceimpedancesiscriticaltoachievinghighpowertransferefficiencyfromapowersupplytoitsload.Inthiscase,theimpedanceofferedbythemicrotransformer’sprimarywindingshouldbematchedtotheiPhone’saudiooutputport’simpedanceof3.6Ω.Thetransformer’sdatasheetstatesthatthepri-maryDCresistanceis200mΩandprimaryinductanceis25µH,whichwealsoverifyempirically.Sincethetransformer’sDCresis-tanceissmallcomparedtothepowersupply’soutputimpedance,wefocusonthetransformer’simpedance.Theimpedance,XL,offeredbyaninductoris

XL=jωL=j2πfL.

Rearrangingtosolveforf,thedesiredexcitationfrequency,givesf=

XL

2πL.Substitutingourmeasuredimpedanceandinductancevaluesgives

f=

3.6Ω

2π×25µH

=22.9kHz.

NotethatthetargetexcitationfrequencysitsjustattheedgeofwhattheiPhoneiscapableofproducing.Fortunately,however,wehavecompletecontrolovertheexcitationfrequencywithintheaudioband,sowecangeneratea22kHzwaveformwhichwillachievenearoptimalpowertransfertotheenergyharvestercircuit.

2.4EvaluatingPerformanceandCost

Toevaluatetheperformanceandcostofourdesign,weimple-menttheenergyharvestingcircuit,asshowninFigure5.Thecir-cuitonlyrequiresafootprintof1.0”x0.35”(althoughthecurrentboardincludesa0.15”unusedareaformanufacturingreasons)andseveralheaderexportlines.Thecircuit’ssmallsizemakesitsuit-ableforembeddinginsideasmallperipheraldeviceorevenahead-setplug,liketheoneshowninFigure1.

BCDEFigure6:Energyharvestingcircuitoperation.Channel1(or-ange)showsthefilteredaudioexcitationsignal.Channel2(cyan)showsthesignalafterrectification.Channel3(magenta)showstheoutputafterpassingthroughablockingdiodeandgettingfiltered.Channel4(green)showsthevoltageacrosstheLED.Overall,alowamplitudeACsignalisefficientlycon-vertedintomoreusableform.

2.4.1Performance

Figure6showsatraceofthecircuitinoperation.TheiPhonegeneratesa22kHz,500mVpeak-to-peaksquarewavethatisband-passfiltered(substantiallylowerexcitationfrequenciesresultinpoorpowertransfer).TheRMSvalueofthesignalis207mV,meaningthatapproximately15mWisdeliveredbythephone,orabout90%ofpeakpower.Channel1(orange)showsthisfilteredaudiooutputsignal.Channel2(cyan)showsapeak4.24Vsig-nalafterrectificationusingtheFETbridge.Channel3(magenta)showstheoutputafterpassingtherectifiedsignalthrough(asingle)blockingdiode,whichdrops230mVatpeakcurrent,providingatworsta94.5%efficiency.Channel4(green)showsthevoltageacrosstheLEDafterthesignalpassesthrougha699Ωresistor.Figure7showstheenergyharvesterpowerdeliverycapability.Aloadresistanceisconnectedacrosstheharvester’soutputterminalsandisvariedfrom0Ωto15kΩ.Theoutputvoltageandloadcur-rentaremeasuredatseveralpoints.Fromthesedata,wegeneratethepowertransfercurve,whichshowsthatmaximumpowertrans-feroccursat3.5Vwhendelivering2.11mA,fora1.7kΩloadimpedance.Theopencircuitvoltageis9.95Vandshortcircuitcurrentis3.54mA.

Thepowersupplyrippleislessthan±10mVaslongastheaudiosignalispresentandtheloadisstatic.Ourdesigncurrentlydoesnotincludevoltageregulationforseveralreasons:(i)itmaynotbeneededinsomeapplications;(ii)itisnotneededforourexampleapplication;and(iii)theoutputvoltageandsupplyfilteringisoftenspecifictotheparticularapplication.Theseresultsshowwecanharvestenergyfromthephone’sheadsetport,andconvertitintoamoreusefulform,usingasimpleandinexpensivecircuitconsistingofahalf-dozencomponents.

2.4.2ImpactonPhonePower

Toestimatetheimpactofenergyharvestingonthephone,wemodifyinganiPodTouchtomeasurebatterycurrentandvoltage.Weplayanaudiofilethatgeneratestherequired22kHztoneandfindthattheiPoddraws91mAat4.04Vwhentheharvesterisattachedand37mAat4.04Vwhentheharvesterisnotattached.

483.5736))AmW(m 2.55(tn reerrwu24ocp ttuuptu1.53ptuOO120.51002468100Output voltage (V)Figure7:Powerdeliverycapabilityoftheharvester.Thehar-vesterdelivers7.4mW,at3.5Vand2.11mA,foranideally-matchedloadof1.7kΩ.Thepowertransferefficiency,ηeff,is47%.Althoughahigherconversionefficiencymaybedesir-able,theoutputpower,voltage,andcurrentisenoughtodrivemanylow-powerelectronics.

Thesefiguresillustratethesubstantialinefficiencyofthisapproachcomparedwithdirectlysupplyingpowertoaperipheral.Wenote,however,thatthepoweroutputofaniPodishigherthananiPhone,makingthenumbersappearworsethantheyareontheiPhone.

2.4.3MarginalCost

Table2showthecostbreakdownoftheenergyharvestercompo-nents.Thetotalmarginalcostis$2.34,assuming10Kvolumesandlistprices.Thesefiguresincludetheaudioheadsetplugandboardcosttobuildanoperationalenergyharvester.Thecostsreflectlistpricesandexcludeoptionalparts(J2,J3,R1,C3,andD2).RefDescMfgPartCost(ea)J1171-7435-EX$0.92Q1,T1XFMRPlugKobiconnQ3,Q2Q4N-FETCoilcraftP-FETZetexZXM61N03LPR6235$0.40$0.14C1,D1C2DiodeDiodesZetexZXM61P03$0.14$0.16Fab

PCB

Cap4pcb.com

TDKY5V1A225ZDFLS120LHIJACK_A

$0.02$0.26

Table2:Energyharvestercostbreakdownincludingcircuitboardandheadsetplug.Totalcostatlistprices,excludingop-tionalcomponents,is$2.34at10Kunits.

3.PERIPHERALINTERFACING

Ourseconddesigngoalistoenablebi-directionalinterfacingbe-tweenthemobilephoneandexternalperipherals.Thetwocon-straintsfortheinterfacingchannelare:(i)itmustoperateintheaudiofrequencyrange,and(ii)itmustbelow-costandenergy-efficient.Thefirstconstraintreflectsthebandwidthlimitationsofthephoneaudiochannelwhilethesecondreflectsthecostandpowerconstraintsofthespace.Withintheseconstraints,wecon-siderthreeapproachestodatatransfer:directencodinginana-logvoltages,encodingusingmodulatedwaveforms,anddigitallytransmittingserialdataasa(Manchester-encoded)UARTstream.

Figure8:Diodedetectorcircuit.Theoutputvoltage,vo(t)fol-lowstheenvelopeoftheinputvoltage,vi(t).

3.1AnalogSignaling

Sincetheaudiointerfacealreadybi-directionallytransfersana-logsignals,itwouldseemstraightforwardtoextendthisinterfacetotransferinformationencodedasananalogvoltage.Indeed,forsensorsthatrequireasmallbiasvoltageandofferatransient(AC)response,thesolutionissimple.Suchsensorscanbeconnecteddirectlybetweenthemicrophoneandcommonlines,orperhapsthesensorsmightrequiresomeminorimpedancematching,buffering,filtering,oramplification.Sensorsthatfallintothiscategoryaremicrophones,piezoaccelerometers,andmagneticcardreaders.Conversely,asimpletechniqueforobtainingacontrolledoutputvoltagefromtheAC-coupledoutputistouseanenvelopedetectorcircuitwithafastattack,slowdecayresponse.Suchadetectorcanbeconstructedfromadiode,capacitor,andresistor(andoptionallyanopamp),asshowninFigure8.Adetectortracksthepositiveenvelopeofitsdrivingwaveformandrespondsquicklytoincreasesintheinputvoltage.Trackingadecreaseintheinputvoltageisalittlemorecomplicatedsincethediodeisreversebiased.Insuchcases,theresistordischargesthecapacitoruntilthecapacitorvolt-agefallsbelowtheinputvoltage,atwhichtimethediodeisagainforwardbiasedandchargesthecapacitor.

Inourapplication,thechoiceofresistorhastobalancethreefac-tors:outputripple(whentheresistoristoosmallgiventhedrivingfrequency),trackingdelay(whentheresistoristoolargefortheenvelopedecay),andtrackingerror(whentheresistoristoosmallandresultsinanon-trivialvoltagedropacrossthediode).How-ever,theseareallparticulartotheapplication,sowedonotdiscussthemfurther.

3.2ModulatedDigitalCommunications

Analogsignalingallowsjustoneinputchannelandoneoutputchannelforperipheralinterfacing.Inthissection,weexploremod-ulateddigitalcommunicationsovertheaudioport,usingamicro-controller(MCU)thatispoweredbytheenergyharvestingcircuitasoneendofthecommunicationslink,andthephone’sprocessorastheotherendofthelink,withthetwoendsconnectedviatheheadsetport.Usingamicrocontrolleraffordstwobenefits.First,itallowsustomultiplexthechannel.Second,itenablesustocon-nectany(suitablylow-power)sensorbyusingthemicrocontroller’sADC,GPIO,I2C,SPI,UART,orotherinterface.

Onesourceofinspirationformodulateddigitalcommunicationsovertheheadsetportcomesfromacousticdial-upcomputermodemsusedtoconnectcomputerstogetherovertheacoustictelephonenet-work.Unfortunately,manyhigh-performancedigitalsignalpro-cessingtechniquesemployedinconventionalmodemsarenotfea-sibleinoursystemduetothelimitedpowerandcomputationre-sources.Forexample,atypicalintegratedmodulator/demodulatorcircuitisoftennotsufficientlylowpower,drawingtensofmilli-watts[6],andsoaloweroverheadapproachisneeded.

From PhoneAC Coupling Comparator Timer CompareTo Phone MicLeft Channel at Vcc/2at Vcc/2Signal Gen.LP FilterEdge Time Tone MeasurementEncoderTone 0/1Decoder0/1UART RXUART TXFrom PhoneRight Channel Power VccCommonHarvesterGNDMSP430 MCUFigure9:Digitalcommunicationssystemarchitecture.Thephone’sheadsetportexportsfourwires:rightchannel,leftchannel,microphone,andcommon.Therightchannelpro-videspowertotheenergyharvestingcircuit.Theleftchan-nelprovidesdataoutputfromthephonetothemicrocontroller.Themicrophoneprovidesdatainputfromthemicrocontrollertothephone.Boththephoneandthemicrocontrollerimple-mentanFSKmodulator/demodulator,althoughusingverydif-ferentimplementationtechniques.

TheoriginalBell202modemsignalingofferssuchasimplescheme,employingfrequencyshiftkeying(FSK)usingtwotones.Adigitalzero,or“space,”isrepresentedbya1200Hztone,whileadigitalone,or“mark,”isrepresentedbya2200Hztone.However,unliketheBell202standardwhichspecifies1200baudcommuni-cations,weusealowerdatarateof300baudinordertofacilitateanimplementationonalow-powermicrocontroller.Atthedigitallevel,weuseRS-232signalingtocreateavirtualUARTabstractionovertheaudioserialbitstream.Sincethisprotocoladdsastartandstopbittoeverybyte,theeffectivedatarateislimitedto30byte/s.InordertoefficientlyimplementFSKencodinganddecodingonamicrocontroller,itisusefultomakemaximumuseoftheavailablemicrocontrollerhardwareperipherals.Oneexample,forinstance,usesseveralhardwarefeaturesincludingtimersandUSARTstoefficientlymodulateanddemodulateFSKsignals[24].Figure9presentsanoverviewofourimplementationofthisunconventional,combinedhardwareandsoftware,approach.Therearetwokeybenefitsofusingthisapproach.First,thehardwareacceleratorsmakeitfeasibletoactuallyimplementthemodulationschemesonalow-endmicrocontroller.Second,theacceleratorsenablelowerpoweroperationthanwouldbepossibleiftheoperationswereex-ecutedinsoftware.

Usingthisscheme,themaincommunicationinterfaceforanap-plicationrunningonthemicrocontrolleristheUARTreceiverandtransmitterperipherals,whicharefamiliartomanyprogrammers.ThepinsofboththereceiveandtransmitunitoftheUARTareconnectedtoothermicrocontrollerperipheralswhichprovidead-ditionallow-levelsymbolanddataprocessingthatoccurtranspar-entlytotheapplicationcode.

Ontheencoderside,theUARTtransmittergeneratesthedatabitsat300baud(Channel2,cyan,topofFigure10).Thesebitsarefedbackintoamicrocontrollerinterruptline.AtimercompareunitgeneratesthecorrectfrequencyaccordingtotheincomingbitfromtheUARToutput.Theoutputofthetimercompareisasquarewaveateither1200Hzor2200Hz(Channel1,orange,bottomofFigure10).Thissignalislow-passfiltered(notshown)beforeitisfedintothemicrophoneportofthephone.ThisfilteringreducesthehighfrequencycomponentsofthesquarewavethatcanbeclearlyseeninFigure10.

(a)Noisespectrum(b)Communicationsspectrum

Figure11:Audiospectrumutilization.(a)showsthenoisespectrumofthesystem(andthe22kHzpowertone).Thesoundpressurelevelislessthan40dB.(b)showstheaudiospectrumwithdatacommunicationsandpowertransfer.Twopeaksareclearlyvisibleat1200Hzand2200Hz.The22kHztonethatprovidespowerisshowncouplingbackthrougha-30dBpath.WeusetheFFTfunctionintheSignalScopeProsoftwareontheiPhonetocapturethisdata.

ImplementingtheFSKencoderanddecoderonthephoneiseas-ieraswehaveamorepowerfulprocessoravailable,allowingforamorerobustalgorithm–anon-coherentFSKdemodulator–onthephone[25].Innon-coherentFSKdemodulation,theinputiscor-relatedwithfourdifferentsignals:apairofsine/cosinesignalsat1200Hz,andapairofsine/cosinesignalsat2200Hz.Thecorrela-tionvalueofthetwopairsissampledandaddedtogether.Then,thesumsofthetwopairsarecomparedtoeachothertodecidewhichfrequencyisbeingreceived.AstatemachineprocessesthestreamofonesandzerostodetecttheUARTstartbitbeforedecodingthebytevaluetransmittedintheFSKsignal.

TheFSKencoderonthephonecreatesacontinuous-phasesignalthatswitchesbetweenthetwotonesaccordingtothetransmittedbit.Figure11showstheaudiospectrumusedbythephone.Twopeaksareclearlyvisibleat1200Hzand2200Hz,plusthe22kHztoneusedtopowertheenergyharvester.Atestofthecommunica-tionchannelfrommicrocontrollertophonefoundabiterrorrateof3.0×10−4overarunlengthof27,872bytes.

Figure10:Thedatastreamanditsmodulatedtransmissionfromthemicrocontrollertothephone.Thetoptraceshowsa300bpsdatastream,whereazero-bitisshownfrom2.0msto5.3ms,followedbyaone-bitfrom5.3msto8.6ms.ThebottomtraceshowstheFSK-encodedtransmissionofthisdata,wherethezero-bitisencodedasa1200Hztoneandtheone-bitisencodedasa2200Hztone.Althoughthemicrocontroller-generatedsignalsaresquarewaves,theyarelow-passfiltered.

3.3DirectDigitalCommunication

ThefinaloptionweexploreisdirectlytransmittingUARTdataovertheheadsetport,withoutfirstFSKmodulatingit.However,sincetheaudiochannelisAC-coupled,alongsequenceofzerosoronessaturatesthechannelandisdifficulttodecode.Tosidestepthisproblem,weManchesterencodetheUARTdatastream.Manch-esterencodingisabalanced1:2codethatreducestheoveralldataratebyafactoroftwobuttransmitsanequalnumberofzerosandones.Theencoderworksbyreplacingevery1intheinputstreamwitha01,andevery0with10.Thisencodingisself-synchronizingsinceitguaranteesatransitionafteratmosttwohalf-bits.

Figure12showstheUARTdatastreamanditscorrespondingManchesterencodingasseenontheoutputoftheheadsetportofaphone.Wenotetwothings.First,thewaveformisnotsquareandisthusband-limited.Second,theedgedirectionaftereveryotherbitintheManchesterencodeddataindicatestheUARTbit.Thesecondobservationiskeytothesynchronizationofourdecoder.AstheUARTlineishighifthereisnocommunication,theManchesterencodingoftheidleUARTisasequenceofbinary01.However,thereceivercannotrobustlydistinguishalongrunof10sfrom01s,leadingtopotentialsynchronizationloss.

Theleftchannelheadphoneoutputisaground-referencedACsignalsowecannotdirectlyprocessit.ThusweACcoupleittoavoltagedividerwhosemidpointissettothemicrocontroller’sVcc/2.ThemainideabehindtheFSKdecoderistomeasurethezero-crossingtimeofthesignal,andestimatetheinstantaneous,cycle-levelfrequency.WeachievethisbycomparingthesignaltoVcc/2inacomparatorthatisinternallyconnectedtoatimercap-tureunit.Insoftware,wecalculatethetimedifferencebetweenrisingandfallingedgesusingatimercaptureinterrupt.TheresultofthisdecisionisthenoutputonadigitalIOlinethatisexternallyconnectedbackintothereceiveportoftheUARTperipheral.TheUARTreceiveristhenresponsiblefordecodingthesignalat300baud,asifitreceivedanormalUARTframe.

UART Manchester Encoding

UART Data

Manchester Encoding

Figure12:UARTdataanditsequivalentManchesterencodedandlow-passfilteredrepresentation.LongrunsofzerosandonesareindistinguishablebutthekeytosynchronizationisthataUARTstartbit,encodedasahigh-to-lowtransition,createsadouble-baudlengthpositivesymbol.Thestartbit’shigh-to-lowtransitionintheUARTdataiscoincidentwiththefallingedgeofthisdouble-baudpositivesymbol.Thissymbolsynchronizesthereceiverandtransmitterafterthelinkhasbeenidle.

Ourdesignaddressestheresynchronizationfromidleasfollows.OncethetransmittersendsaUARTstartbit,thereceiverdetectsadouble-baud-lengthpositivesymbol,asseeninthefirsthigh-to-lowtransitionoftheUARTdatainFigure12.Thissymbolsynchro-nizesthereceivertothebitstreamasitindicateswiththefallingzero-crossingthatthecurrentUARTbitisa0,andhencethepriorsequenceofbitswereasequenceof1s.

Onceagain,weusemicrocontrollerperipheralstoenableanef-ficient,low-powerimplementation.Wemeasurethezero-crossingtimesoftheManchesterencodedsignalusingacomparatoratVcc/2drivingtheinputlineofatimercaptureunitsensitiveonbothrisingandfallingedges.Theedge-to-edgetimeiscapturedandcomparedtothebaudrate,andthenastatemachinedecodesandoutputsthereceivedUARTbit.

Asimilarapproachcanbeusedforthedecoderrunningonthephone.However,insteadofusinghardwareperipherals,weimple-mentthecomparatorandtimercaptureinsoftware.Thecomparatorsimplyimplementsalessthantestwhichisusedtoslicetheincom-ingaudiosamplesintoonesandzeros.Toemulateatimerunit,weexploitthefactthattheaudiosamplesaretakenbythephoneatpre-cisely44.1kHz.Therefore,thetimebetweensuccessivesamplesis22.68µs.Bycountingthenumberofsamplesbetweenapairofzerocrossings,weestimatethesymbolwidthandthuscandecodetheManchesterencodedUARTsignalusingthesameapproachasonthemicrocontroller(butinsoftware).

Twofactorslimittheachievablebaudrateusingthistechnique.Thefirstisthatthephone’saudiosamplingrateat44.1kHz,result-inginamaximumUARTbaudrateof22.05kbaud.Thesecondistheclocksourceusedonthemicrocontroller.Commonlyused32kHzcrystalsareneitherfastenough,noroffercleanbaudratedivisors,togeneratereliableManchesterencodinganddecodingat22.05kbaud.Andfaster,digitally-controlledRCoscillatorsarenotstableenoughoverlongerperiodsoftime.Inaddition,ahigherfrequencyRCoscillatorcanpreventthemicrocontrollerfromen-teringsleepmodes,thusincreasingaveragepower.Therefore,itisimportanttobalanceclockspeedanddatarate.

Voltage DividerEpic DevKit withCommunication

with PotentiometerTI MSP430 MCUFilters

EnergyHeadsetSimple OscilloscopeHarvester

Audio PortApplication on iPhone

Figure13:Prototypephone-centricoscilloscopeapplicationinoperation.Thesystemconsistsoffourdistinctsubsystemswhichareshownworkingtogether:(i)iPhone;(ii)communi-cationfilters;(iii)energyharvester;(iv)microcontrollerwithpotentiometersimulatingaresistivesensor.TheblueLED,lo-catedatthebottomoftheenergyharvester,isturnedonandclearlyvisibleinthisimage.Asthepotentiometeristurned,theiPhone-basedoscilloscopedisplaysthechangingvoltages.WetestseveralbaudratecombinationstodeterminethelimitsofourcurrentcombinationofiPhone3GandtheTexasInstrumentsMSP430F1611.Wefindthatat7,350baud,therearenotransmis-sionerrorsaftertransmittingmorethan30,000bytes.At8,820baud,wefind2byteerrorsafter65,000transmittedbytes,indicat-ingabyteerrorrateoflessthan3.1×10−5.Finally,at11,025baud,wefindmorethan30droppedbytesoutof250transmittedbytes–anunacceptableerrorrate.Basedontheseresults,weconcludethatourcurrentimplementationcansustainbi-directionalcommu-nicationsreliablyat8,820baud,orapproximately1kbyte/s.

4.APPLICATIONS

Ourfinaldesigngoalistoillustratethatthevariousdifferentsub-systems–energyharvesting,datainput,anddataoutput–canallbecombinedintoanintegratedandfunctionalapplication.Toexplorethedesignspace,wecreatefourapplications.Thefirst,asimpleoscilloscope,demonstratestheproofofconceptandallowsustomicrobenchmarkpower,code,andmemorymicrobenchmarks.WethenintegratethevariousHiJacksubsystemsintoabasicplatformthatexposeskeypower,communications,analog,anddigitalinter-faces.Finally,webuildtwoapplications–anEKGmonitorandasoilmoisturesensor–usingthisbasicplatform.Ourresultsdemon-stratethegeneralityandextensibilityoftheHiJackinsupportingnovelapplications.

4.1Oscilloscope

Figure13showsaprototypehandheldoscilloscopebuiltaroundtheenergyharvestingandperipheralinterfacingprimitivesdescribedintheprecedingsections.Thissystemillustratesacanonicalhand-heldinstrumentthatusesthephone’sdisplayforvisualization,andthemicrocontroller’sADCtomeasureanexternalsignal.Thesys-temincludesaniPhone,abreakoutboardpopulatedwithtransmitandreceivecommunicationfilters,theprototypeenergyharvesterboard,theEpicMote[21](whichincludesaTIMSP430F1611[7])inabreakoutboard,andabreadboardwithapotentiometer.Thepotentiometerapproximatesanexternalanalogsensor,butadigitalorserialsensorcouldhaveeasilybeenused.

StateLCDAudioHiJackPoweriPodStandbyoffoffdiscon.<3mWiPodHomeScrn.onoffdiscon.280mWOscilloscopeoffondiscon.280mWOscilloscopeoffonattached480mWTable3:Powerdrawbreakdownoftheoscilloscopeapplica-tionrunningonaniPodTouch2G.Thebaselinestandbypowerislessthan3mW.Theactivebaselinepower(screenon,noappsrunning)is280mW.RunningtheOscilloscopeapplicationwithoutHiJackconnected(screenoff)draws200mW.Attach-ingHiJackaddsanadditional200mW.

4.1.1PhoneSoftware

AttheheartofthissystemliesanapplicationrunningonthephonethatisbasedontheaurioTouchsampleapp[8].Ourversiongeneratesa22kHztoneontherightaudiochanneltopowertheen-ergyharvestingcircuit.TheleftaudiochannelsendsaManchester-encodeddatastreamtothemicrocontrollerasdescribedinSec-tion3.3.Thephone’smicrophoneinputlinereceivesasimilar,Manchester-encoded,datastreamgeneratedbythemicrocontroller.Thephoneapplicationgeneratestheraw,low-leveldatastreamandiscapableofdecodingastreamencodedinasimilarmanner.

TheiPhoneapplication’suserinterface,alsoshowninFigure13,providesvisualfeedbackofthevoltagemeasuredbythemicrocon-trollerADCinput.Thegraphdisplaysascrollinghistoricalviewofmeasureddatawhilethetextboxshowsthelastreceivedmeasure-ment.Thedataaretransmittedfromthemicrocontrolleratregularsampleintervals.Aslidercontrolontheapplicationallowstheusertochangethesamplingratefromabout1/10Hzto1kHz,withtheupperlimitgivenbythe8.82kbpsencodingofthedatastreamoftheaudiochannel.Thisslidersettingissentovertheleftchanneltothemicrocontroller,whichdecodesthesignalandchangesitssam-plerateaccordingly.WehaveverifiedthatourapplicationrunsontheiPhone3G,3GS,4G,iPodTouch2G,andiPad.Wehypothesizethatitcouldbeportedtootherphoneswithrudimentaryprogram-mingsupportaswell,includingAndroid,Windows,andNokia.

4.1.2ImpactonPhonePower

WemodifyaniPodTouch2Gtomeasurethevoltageandcurrentofthebatterywhilerunningouroscilloscopeapplication.Table3summarizestheenergybreakdown.Wefindthatgeneratingthe22kHztoneandManchesterencodinganddecodingonthephonedrawaboutasmuchpowerastheLCDscreen.ThepowerdrawoftheharvesterisagaincomparabletotheenergyusedbytheLCDscreen.Thus,giventhetypicalbatterysizeof1200to1400mAhfoundonseveralgenerationsofiPhones,andatanominal3.7V,weestimateabatterylifeof10to11hoursofcontinuousopera-tionifthescreenisturnedoff.However,wedonotenvisionthisasthetypicalusagemodel.Rather,becauseofitssmallsize,weenvisionauserwillcarryaHiJacksensorintheirpocket.Whentheuserthenwishestotakeameasurement,shetakesitout,plugsthesensorintoherphone,andrunsthemeasurementapplication.Thisisthetypicalusagemodelemployedbyparticipatorysensingapplications.

4.1.3MicrocontrollerSoftware

Themicrocontrollerdrawsamere0.7mAat2.8Vwhilerun-ningthefullapplicationincludingcommunicationsencoderanddecoder,UART,andADCtosamplethesensor.Thisislessthan2mW,leavingover5mWformoresophisticatedsensingandsig-nalconditioningelectronics.

Figure14:TheHiJackbaseplatform,witha1”x1”footprint,offerspower(>5mW),analog(2x12-bit),digital(1xGPIO),andserial(1xI2Cand1xUART)interfaces,exportedviacon-nectors,andallmultiplexedovertheheadsetport.Thisboardprovidesthefunctionalityneededtobuildavarietyofexternalsensorinterfacesforthemobilephone.

ModuleSize(bytes)

TotalRAM109Timers,Alarms1,280HiJackApplication808Others(UART,Arbiters,Init,etc)2,378TotalROM4,466Table4:CodeandmemoryusageoftheMSP430application.WithasmallTimeroptimization,thetotalROMusagecanbebroughtdownto3316bytes.

4.2PhoneAccessoryPlatform

Oneofourmotivationsforharvestingenergy,ratherthandirectlypoweringaperipheralwithanexternalbattery,istoreducethepe-ripheral’sformfactor.Whiletheprototypeoscilloscopeapplica-tionislargeduetotheuseofmultipleprototypeboardsandbreak-outboards,theactivecomponentsofthesystemcanbeintegratedintoamuchsmallerfootprintcircuitboard.Forexample,thetwolargestcomponents–atransformerandaTIMSP430F1611mi-crocontroller–measureamere6mmx6mmand9mmx9mm,respectively,allowingustoeasilyintegratethecomponentsontoadouble-sidedboardmeasuringjust1.0”x1.0”,asFigure14shows.Table4showsthatweuse109bytesofRAM,andwithsomesmallcodeoptimization,lessthan4kBofROM.Thus,inthenextversionofHiJack,weplantouseanewerTIMSP4302allowingustoincorporateallthecomponentsonacircuitboardthatrequiresonlyabouta1.0\"x0.20\"footprint,whichissmallenoughtoplaceinapocket,allowinganyonetocarryarangeofphone-poweredperipheraldeviceswiththem.

Byprovidingthemostcommonanalog,digital,andserialinter-faces,thisbaseboardmakesiteasytointegratemanylow-powersensorsdirectlyintonearlyanymobilephonewithaheadsetcon-nectorandrudimentaryprogrammingfacilities.

Onecomponentthatismissingfromthisplatformisavoltageregulator.Unsureaboutthespecificpowerrequirementsofpe-ripheraldevices,thisdesignallowsthesensorstoregulatevolt-ageasneeded.Inhindsight,thisdesignchoicecomplicatestheinterfacebetweenthebaseplatformandperipheralssincethebaseboardpowerisunregulated.Subsequentdesigns(describedbelow)correctthisoversightbyintegratingtheregulatordirectlyintotheHiJackbaseboard.

2

F2121orF2122,dependingontheapplicationrequirements,16mm2or25mm2,$1.30or$1.80in10kunits,respectively.

(a)Phone-poweredEKGInterface(b)SoilMoistureSensor

Figure15:Application-specificHiJacksensorboards.(a)A3-lead,phone-poweredEKGsignalconditioningboardthatinterfacestoHiJack,drawslessthan400µA,andprovidesamplification,filtering,andbaselinedriftcorrection.(b)Anultralow-cost,passivesoilmoisturesensorthatinterfacestoHiJackusingaresistivevoltagedivider.ThesesensorboardsillustratethebreadthofdifferentsensingapplicationsthatcanbebuiltusingHiJack,showingHiJack’sversatilityinmeetingarangeofsensingtasks.

4.3Phone-PoweredEKGMonitor5.RELATEDWORK

Hospitals,clinics,andurgentcarefacilitiesindevelopedregionshaveaccesstoabatteryofmedicalinstrumentsunavailabletohealthcareprofessionalsindevelopingregions.OneexampleofsuchaninstrumentistheEKGmonitor.EKGsignalsareusedtodiagnoseawiderangeofmedicalconditionsbuttheyareoftenunavailableinallbutthemostadvancedhospitalsindevelopingregions.Wear-gue,inthissection,thatthisneednotbethecase.Rather,anyhos-pital,villageclinic,ordoctorcanhaveaccesstoadvancedhealthcareinstrumentsforlittlemorethanthecostofamobilephone.Weillustrateourclaimbydesigningalow-cost,low-powerEKGmonitorthatusesHiJackforpowerandcommunications,andthemobilephoneforvisualizingtheEKGwaveform,asshowninFig-ure15(a).TheEKGmonitorattachestothesquare-inchform-factorHiJackbasenode,andextendsthePCBtoprovidespaceforconnectorstoattachEKGleadsina3-leadconfiguration.TheEKGboarddrawslessthan400µA,amplifiestheinputsignal,passesitthroughanotchfilter,andcorrectsforbaselinedrift.ThemobilephonevisualizestheEKGandcanoptionallytransmitthedatatothecloudforstorageorreal-timeremotedisplay(includingonan-othermobilephoneorevenaFacebookapplication).TheEKGboardledtoaniterationintheHiJackbaseboardwherebythebaseboardnowprovidesregulatedpower(andawiderelectricalinter-facewhichisvisibleasperimeterpinsinFigure15(a)).

4.4SoilMoistureSensor

Aninexpensivesoilmoisturesensorcouldenablefarmersinde-velopingregionstobetterunderstandconditionsonandundertheground,andthismayleadtobettercropmanagementandhighercropyields[20].Thekeychallengeisindevisingasensorsystemthatcanbedeployedatmodestscaleindevelopingregions.Thisre-quiresinexpensivesensing,signalconditioning,andvisualization.Toaddressthisneed,wedesignalow-costsoilmoisturesen-sorasourfinalexampleofaHiJackphoneperipheral.Thesensor,showninFigure15(b),usestheHiJackphoneaccessoryplatformpresentedinSection4.2augmentedwithasimplebreakoutboardthatholdsthetophalfofavoltagedivider.Thesoilmoisturesen-soritselfisbuiltfromtwonails,gypsum,andapairofwires.Thenailsaresuspendedinthegypsumwhichisthencured.Onewireisattachedtoeachtonail.Theseriescircuitformedbywire,nail,gypsum,nail,andwireformthebottomhalfofavoltagedivider.Thesensorisburiedunderground.Whenthesurroundingearthisdamporwet,waterseepsintothegypsumandchangesitsconduc-tivity[9].ThesechangesandcanbemeasuredbytheHiJackboardandvisualizedusingthemobilephone.

Energyharvestinghasreceivedconsiderableresearchinterestinrecentyears.Harvesterdesignssuitedtoradiofrequency[27],piezovibration[26],outdoorsolar[23]andindoorlighting[22]havebeenexplored.Manyoftheproposeddesignsinvolveeitherdiode-basedvoltagedoublerswhichhavelowefficiencyatsmallinputvoltagesorintegratedcircuitswhichhavehighupfrontde-signandmaskcosts,makingbothoftheseapproachesill-suitedtothetargetdomain.

Recently,industryhasbegunshippingintegratedcircuitsdesignedforenergyharvestingapplications.LinearTechnologyofferstheLTC3108[10]butthedevicerequiresaDCinputvoltage.Inthispaper,weshowedthatdirectrectificationleadstohighlosses,andsothischipisnotidealforthetargetdomain.Inaddition,thechiprequiresseveralexternalcomponentswhoseaggregatecostishigherthanthecostpointweareabletoachievewithourdiscreteimplementation.Anotherchip,theLTC3588[11],canoperatedi-rectlyfromanACinput,butitrequiresatleast2.7V,aswellasfourcapacitorsandaninductor,againincreasingcost.

OthershaveproposedusingtheiPhone’saudiojackasacom-municationinterface.TheSquareCardReader[12]isacreditcardreaderthatattachestoaphone’saudiojack.BasedonourownteardownanddiscussionswithSquareengineers,wefindthatthereaderisapassivedevice,andthatthevoltageinducedbyacardswipeonthemagneticheadisdirectlyappliedtothemicrophoneinput.Thus,thedevicedoesnotactivelyharvestenergyortransmitdigitally-codedinformation.Rather,itisrepresentativeofthesim-plestkindofanalogsensorthatcanbeattachedtoamobilephone.Anotherdevice,iData[13],plugsintoaniPhoneheadsetportandclaimstocommunicateusingFSKmodulationat1200baud,equiv-alenttotheBell202standard.Nopublicdetailsareavailableabouthowthedeviceispowered.Prologicalprovidesadevelopmentkitcalledhomemadeforintegration(H4I)[14]towhichlicensedde-velopersgetaccesstoanSDK,andplansforthenecessaryexternalhardwarecomponents.H4Iclaimsspeedsofupto19.2kbaud,butexternalperipheralsstillneedtheirownsourceofpower.SerialoutputontheAndroidplatformhasalsobeendemonstrated[15].Ourworkisdistinctasitprovidesbothcommunicationsandpowertoadevicedirectlyfromtheaudiojackatthesametime.

Therearemanywaystointerfaceexternalperipheralswithaphone.BluetoothandWiFiarethemostcommonwaysofcon-nectingwirelessperipherals,likeheadsets,keyboards,carstereos,heartratemonitors,orevencars[16],toaphone.Whilethesestan-dardsarecommononmodernphones,theybothrequireanexternalpowersourcefortheperipherals.

AdditionalinterfacesonsomephonesincludemicroSD[17]andUSBOn-The-Go(OTG)[18].AlthoughthemicroSDcardslotisdesignedspecificallyforsmallperipherals,anditincludessupportforSDIOmodewithSPIsupport,theportissmallandoftennoteasilyaccessible.Thus,connectingperipheralsthroughtheSDIOinterfacecanbedifficult.And,somedevices,liketheiPhonedonotofferamicroSDcardslot.However,microSDcanbeaviablein-terface,asrecentlyannouncedmicroSDWiFicardsillustrate[19].WhileUSBhostcapabilitiesseemliketheidealinterface,onlyafewphonesprovideOTGcapability.And,evenwhenphonesofferOTG,thestandardonlyrequires5Vat8mA(40mW),lessthanthreetimesthepowerthattheheadsetcandeliveroneachchan-nel.OTGalsorequiresanexternalUSBslave.Ontheotherhand,USBOTGcansupplypowerveryefficientlyandinregulatedform,whichaccruestothelifetimeofthemobilephoneandsimplicityofaperipheral’spowersupplydesign.Therefore,asUSBOTGgainsmorewidespreadadoptiononmobilehandsets,itmaywellemergeasthepreferredmethodforinterfacingperipherals.

6.CONCLUSION

Weshowthatitispossibletoadapttheubiquitousmobilephoneheadsetporttoprovidepoweranddata,allowingthephonetointer-facewitharangeofexternalperipheralseasilyandeconomically.Withthebasicsofpoweringexternaldevicesandcommunicatingwiththemnowinplace,itwillsoonbepossibletoplugacubic-inchsensorintoalmostanymobilephone,powerthedevice,readitsoutputback,processthedata,andpresentortransmitaresult.Byleveragingthewidelyusedheadsetinterfaceforpoweranddata,HiJackenablesplug-and-playperipheralswhosechargingcyclesconvergewiththoseofthemobilephone.Aparasitically-poweredexpansioninterfaceforthemobilephonewillenablemanynewapplicationsfordevelopingregionsthatareinfeasiblewithcurrentapproaches.Themobilephonehasalreadybecomethepersonalcomputer.Tomorrowitcouldbecometheoscilloscope,portableEKGmonitor,andsoilmoisturesensoraswell.Thisworktakesasmallsteptowardthatfuturebyenablinganewclassofuniversal,low-costinterfaceoptionsforthemobilephone.

7.ACKNOWLEDGMENTS

SpecialthankstoEricBrewerfororiginallysuggestingthemo-bilephone’saudiointerfacecouldbeusedtopowerexternalsen-sors,engineersatSquare,Inc.fordetailsonhowtheircardreaderworks,andtheanonymousreviewersfortheirhelpfulfeedback.ThismaterialissupportedinpartbyNationalScienceFounda-tionAward#0964120(“CNS-NeTS”).AdditionalNSFsupportwasprovidedunderGrant#1019343totheComputingResearchAsso-ciationfortheCIFellowsProject.

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