mems and sensors whitepaper seriesmaximum base clock of 12.5mhz, provide a raw bitrate of 12.5 mbps...

MEMSandSensorsWhitepaperSeries

IntroductiontotheMIPII3CStandardizedSensorInterface

August2016

Whitepaper Topics: sensors, digital communication (MIPI I3C, I²C, SPI), standardized sensor interface,unifiedsensorlink,high-speeddatarate,sensordatabatching,sensorhubs,mobile,wearablesandIoTAbout Us: MIPIAlliance (MIPI) develops interface specifications for mobile and mobile-influencedindustries.ThereisatleastoneMIPIspecificationineverysmartphonemanufacturedtoday.Foundedin2003,theorganizationhasmorethan270membercompaniesworldwideand14activeworkinggroupsdelivering specifications within the mobileecosystem.Members of the organization include handsetmanufacturers, device OEMs, software providers, semiconductor companies, application processordevelopers,IPtoolproviders,testandtestequipmentcompanies,aswellascamera,tabletandlaptopmanufacturers.Formoreinformation,pleasevisitwww.mipi.org.ContactInformation:MelanieCole([email protected])Manager,IntegratedMarketing&MembershipMIPIAlliance(www.mipi.org)


Copyright2016MIPIAlliance,Inc.Allrightsreserved. 2

Introduction

Thetotalglobalsensormarketisexpectedtoreach$154.4Bby20201.$10.46Bofthiswillbemadeupbyintegratedsensorsthatincludedecisionmaking,logicfunctionsanddigitalcommunication.Sensorsare experiencing an unprecedented growth, from $650M in 2012 and with an expected compoundannualgrowthrateof36.25%through20202.Amajordriverhasbeentheadoptionoflow-cost,smallform factor sensors in smartphones, tablets and a growing number of wearables applications. Theoriginal iPhone was introduced in 2007 with advanced sensing capabilities (i.e. ambient light,accelerometerandproximity)thatprovidedaninnovativeandimproveduserinterface.Sincethen,theadoptionofsensorshasrapidlyescalatedandthelargestsensormanufacturersnowshipbillionsofunitsper year formobile and consumer applications. This trend is continuing. Smartphone providers aretryingtogainacompetitiveadvantagebyadoptingnewsensorsandsensingtechnologiesforimproveduserexperiences.Increasedsensorcontentallowsmorecomplexfeaturesthroughsensorfusion,wheresensor data from multiple sensors is aggregated and analyzed to provide functions that cannot beachieved with a single sensor. Advanced functionality such as dead-reckoning, augmented/virtualreality and others require better performing, faster and sometimes duplicate sets of sensors. Newsensing technology isalsobeingdeveloped. Apossiblenext frontiercouldbe theadditionofgasandchemicalsensorstomonitorairqualityandtoxicitylevelsintheworldaroundus.

Theproliferationofsensors inmobiledevicesrequiresapplicationprocessorsand/orsensorhubswithanincreasednumberoflogicpinsusedforsensorcommunicationandcontrol.Inatypicalapplication,multiple digital communication interfaces are used along with supporting logic lines for dedicatedinterruptandsleepsignals. Top tier smartphones include10ormore sensorsandacriticalpointhasbeenreachedwhere20ormorelogicsignalsarerequired.Thereareothercomplicatingfactorsaswell.The de facto communication standard for sensors in mobile and consumer applications is I²C. I²Crequiresonlytwosignal lines(clockanddata),buthasseveralshortcomings, includingtheinabilityforsensor slaves to initiate communication, an overhead protocol that reduces throughput and pull-upresistors that limit clock speedand increasespowerdissipation. Another commonlyused standard isthe serial peripheral interfaceor SPI. SPI requires four communication lines and is usedwhere largeamounts of data needs to be transferred, such as clearing data batches from first in, first out (FIFO)buffers. To its disadvantage, SPI lacks a clearly defined standard that has resulted inmany differentimplementations.

There is no consistent method for interfacing to sensors, which causes considerable integrationchallenges. Deviceandplatformdesignersare facedwithdigital interface fragmentationandhave todeal with I²C, SPI, UART and others. What if there was a single, scalable, cost-effective and clearlydefinedstandardcommunicationinterfaceforsensors?Andwhatifthisstandardcouldunitethebestof I²C and SPI while adding new functionality that the current standards are lacking? In 2013,MIPIAllianceformedaSensorWorkingGroupwithagoaltodevelopanI²Ccompatibleinterfacewithsensor-focused,differentiated features. Anextensive industrysurveywasperformed inconjunctionwith theMEMS & Sensors Industry Group (MSIG) to collect information about what a new standard shouldinclude.TheculminationofthissurveyalongwiththeongoingeffortsoftheSensorWorkingGroupisthebrandnewMIPI®AllianceSpecificationforI3CSMImprovedInterIntegratedCircuit(MIPII3C)sensorinterfacestandard.1“GlobalMarketforSensorstoReachNearly$154.4Billionin2020;ImageSensorsMovingat11.7%”,BCCResearch,July20142“Smart/IntelligentSensorMarketbyType,Application&byGeography,2013–2020”,MarketsAndMarkets,March2014



MIPII3CScopeandPurpose

The MIPI I3C interface is an evolutionary standard that improves upon the features of I²C, whilemaintaining backward compatibility. This standardoffers a flexiblemulti-drop interface between thehostprocessorandperipheralsensorstosupportthegrowingusageofsensors inembeddedsystems.The main purpose of MIPI I3C is threefold: 1) to standardize sensor communication, 2) reduce thenumberofphysicalpinsused in sensor system integrationand3) support low-power,high-speedandothercriticalfeaturesthatarecurrentlycoveredbyI²CandSPI. DevelopingtheMIPII3Cinterfacehasbeen a communal effort to identify the realmarket needs, define a standard andmerge technologycontributionsfromleadingSoC,sensorhubandsensorvendors.DuringtheinitialphasesoftheMIPII3Cdevelopment,multipleproposalswereconsideredand technicallygraded tocomeupwitha standardthat satisfies a broad range of applications that extends beyond the smartphone. The subsequentdevelopmentworkhasbeenabroadeffortfrommanyMIPIAlliancemembers3.TheMIPII3Cstandardiscurrentlybeingfinalizedandwillbereleasedlaterin2016,exclusivelytoMIPIAlliancemembers.

MIPII3CFactSheetMIPII3CStandardizedSensorInterface

I3CMAINMASTER

I3CSLAVE

I3CSECONDARYMASTER

SDA

SCL

I2CSLAVE

Two-wirecommunicationinterface,clock(SCL)anddata(SDA)

Numberofgates <2,000Bandwidth >33MbpsFeatures In-bandinterrupts

In-bandcommandcodesDynamicaddressingMulti-master/multi-dropHot-joinsupportBackwardcompatiblewithI²C

Table1:MIPII3Cstandardizedsensorinterfaceataglance.

MIPI I3Cwas initially intended formobile applications as a single interface that can be used for anysensor. Modernsmartphones that includeamultitudeofsensorsandaslewofsupporting logic linesarepushingtheboundariesofbothI²CandSPI.MIPII3Cwillaccommodatemanysensorsonthesamecommunication bus, while eliminating additional logic signals needed to support interrupt or sleepmodefunctionality.TheMIPII3Cstandardisusefulforotherapplicationsthansmartphones.Itoffershighspeeddatatransferatverylowpowerlevels,whichishighlydesirableforanyembeddedsystem.Wearablesisagreatexamplewheremultiplesensorsareusedinaverylimitedphysicalspaceandwithstringent power restrictions. Over time, theMIPI I3C could conceivably becomemuchmore than astandardized sensor interface and develop into a de facto bus communication standard for touchsensing,always-onandlowresolutioncameras,acoustics,environmentalsensorsandtransducersthatcurrentlyuseI²C,SPI,UARTandothers.

3AMD,Broadcom,Cadence,Intel,InvenSense,Knowles,LatticeSemiconductor,MediaTek,MentorGraphics,Nvidia,NXP,Qualcomm,QuickLogic,Sony,STMicroelectronics,Synopsys,VLSIPlus,ZMDI(nowIDT)andothers.



MIPII3CFundamentals

TheMIPII3CinterfaceusesanI²C-likeinterfacewithanopendraindataline(SDA)andapush-pullclockline(SCL).Theopendrain(i.e.opencollector)SDAlineallowsforslavestotakecontrolofthedatabusandinitiateinterrupts.Thepush-pullSCLlineisusedbythemastertoclockthecommunicationbusupto12.5MHz. MIPI I3Csupportsmultipleclassesofdevices includingmainmaster, secondarymaster,MIPII3CslaveandI²Cslave.Themastercandynamicallyassign7-bitaddressestoallMIPII3CdeviceswhilesupportingthestaticaddressesoflegacyI²Cdevices.ThisensuresfullcompatibilitybetweenMIPII3CandI²C.TheMIPII3Cinterfacerepresentsashiftinpowerperformancewhileprovidinggreaterthanan order ofmagnitude improvement in speedover I²C. I3C offers four data transfermodes that, onmaximumbaseclockof12.5MHz,providearawbitrateof12.5MbpsinthebaselineSDRdefaultmode,and25,27.5and39.5Mbps,respectivelyintheHDRmodes.Afterexcludingtransactioncontrolbytes,the effective data bitrates achieved in each mode are 11.1, 20, 23.5 and 33.3 Mbps, respectively,protected by I3C's basic error detectionmechanisms. The bar charts in Figure 1 compare the energyconsumption(perbit)ofthevariousMIPII3CmodeswithI²C(left)andthecorrespondingrawbitrates(right). Based on these results, the MIPI I3C is a more power efficient interface even in the I²C-compatiblemode. TheMIPI I3CternaryHDR-TSPmodeisthefastestandmostpowerefficientmode,supportingeffectivedatabitratesover33Mbps.

I3C I3C

0

5

10

15

20

25

30

35

40

45

SDR HDR-DDR HDR-TSL HDR-TSP I2C

Raw BitrateMbps for I3C Data Modes (@12.5MHz)

vs I2C (@400KHz)

0

0.5

1

1.5

2

2.5

3

3.5

4

4.5

SDR HDR-DDR HDR-TSL HDR-TSP I2C

Energy ConsumptionmilliJoules per Megabit for I3C Data Modes (100pF)

vs I2C (100pF, 3.54KOhm)

mJ per Mega-bit, VDD=1.8VmJ per Mega-bit, VDD=3.3V Assumptions: 1) All symbols in each mode have equal probability for use.

2) Energy consumption is the energy delivered by pull-up devices to the bus (which includes drivers and resistors).

Figure1:EnergyconsumptionforMIPII3CmodesincomparisontoI²C(left)andrawbitratesforMIPII3CmodescomparedtoI²C(right).ImagefromMIPIAlliance.

A very useful feature allowsMIPI I3C slaves to initiate in-band interrupts,which currently requires adedicatedsignallineforbothI²CandSPIdevices.Thein-bandinterruptfeatureenablesslavestoissuea“start”whenthebusisavailable(i.e.idle).Themasterprovidesaninterfaceclockfortheslavetodriveitsmaster-assignedaddressonto thebus to initiatean interrupt. If there isaconflictwheremultipleslavesaretryingtoinitiateaninterruptsimultaneously,thelowestassignedaddresswinsbyarbitration.Themastercanacknowledge(ACK)theinterruptandrestartthebusorcontinuetoclockoutdatafromtheslave.Anot-acknowledge(NACK)canbesenttoendthecommunication.Forexample,anambientlightsensoriscommonlyusedtocontrolthebacklightbrightnessofadisplayinmobiledevices.Ifthelightconditionschange,thesensorwillsendaninterrupttothesystemandrequeststhatsensordatais



sent to themaster. When light conditionsare stable, there isnoneed to inquire the sensorandusepowerforunnecessarysensordatatransmissions.

MIPII3Cslavesareallowedtojointhebusafterithasalreadybeenconfigured.Thisiscalledhot-join.Sensors that are connected on the samebus can be powered off until they are needed. Hot-joiningshouldadheretospecificelectricallimitsandnotdisturbtheMIPII3Clines.Asetofcommoncommandcodes (CCCs) has been defined for standard operations like enabling and disabling events,managingMIPI I3C specific features (dynamic addressing, timing control etc.) and others. These CCCs can bebroadcasted (sent to all devices) or directed at a specific device on the bus. Power efficiency andperformance are crucial in sensor applications. Always-on sensors and sensor hubs are constantlyaccumulating sensor data evenwhile themain application processor is idle (i.e. low-powermode ordeepsleep). Accumulatedsensordata iscommonlyorganized inbatches thatneedtobeperiodicallyand quickly transmitted between sensors, sensor hubs and application processor tominimize powerconsumption.TheindustryhasfavoredSPIforhigh-speedtransmissionofbatchedsensordata,butSPIismorecomplexandhashigherpincountthanI²C.

MIPII3CSensorInterfaceStandardization

I²CandSPIhavebecomesynonymouswithdigital communication for sensors. I²Cor inter-integratedcircuitwasoriginallydevelopedbyPhilipsSemiconductor(nowNXPSemiconductors)backin1982asasimple communicationbus forbuilding controlelectronics. The initial versionof I²C ranata100kHzbitrateandfasterbitrateversionswereadderlater.Tothisday,the400kHzversionisthemostwidelyusedandtheprimarycommunicationprotocolforsensors.I²Cusestwobidirectionalopen-drainsignallines,onedataline(SDA)andoneclockline(SCL).TheI²Cbusisdesignedforoneormultiplemastersand one ormore slaves (see Figure 2). Slave devices can be individually addressed by a 7-bit (mostcommon)or10-bitaddressingscheme.Themastergeneratestheclockandinitiatesthecommunicationwiththeslave(s).However,anI²CslavecannotinitiatecommunicationandthisisamajorshortcomingthattheMIPII3Cstandardhasresolvedwithin-bandinterrupts.

I2CMAINMASTER

INERTIALSENSORSLAVE

HUMIDITYSENSORSLAVE

SDA

SCL

ALTIMETERSLAVE

Figure2:I²Csampleschematicwithonemasterandthreeslaves,inertialsensor,humiditysensor,pressuresensorandamicrocontroller(µC)(left)andsampletimingdiagram(right).

S=Start,B1toBnarebitsandP=Stop.Image(right)fromWikipedia.(https://en.wikipedia.org/wiki/I%C2%B2C)

SPI or serial peripheral interface is a synchronous digital communication interface for short distancesandprimarilyusedinembeddedsystems.ThestandardwasinventedbyMotorolaandfirstintroducedasanexternalmicrocontrollerbus in1979. TheSPIbus includes four logicsignals, serialclock (SCLK),master out/slave in (MOSI),master in/slave out (MISO) and slave select (SS). The clock polarity andphase can be programmatically configured to four different SPImodes (i.e. 0, 1, 2 and 3). Figure 3displaysanSPIimplementationwiththreeslaves(left),thefourSPImodes(middle)andacorrespondingsampletimingplotfortherespectivefourSPImodes(right). Alternativenamingconventionsiswidely



usedforthesesignals,butthefunctionalityisthesame.ContrarytotheaddressingschemesfortheI²CandMIPI I3Cstandards, theSPImasteruses thededicatedslaveselect (SS) line toaddresseachslaveindividually. The twoextra lines required forSPIputs it atadisadvantagecompared to the two-wireI²C/MIPI I3C interfaces inapplicationswhere real-estate ispreciousandwhere themasterhas limiteddigitaloutputchannels formultipleSSsignals. TheSPI interface ispush-pull (asopposedto I2Cbeingopendrain)andsupportshigh-speedbitratesupto100MHz.Thereisnolimitationforthenumberofbits per transfer (as opposed to I²C that transmits 8 bits per cycle) and it allows streaming of largeramountsofdata. SPIisconsideredlowerpowerthanI²Csinceitrequireslesssupportingcircuitryandeliminatesthepull-upresistorsusedforI²C.

Figure3:SPIbussinglemasterandthreeslaves(left),SPImodes(middle)andsampletiming/modediagram(right).ImagesfromWikipedia.

(https://en.wikipedia.org/wiki/Serial_Peripheral_Interface_Bus)

BothSPIandI²Carelegacyinterfacesthathavefailedtoevolvealongwiththemarketneedsforlower-power,increasedflexibilityandinterfacestandardizationforsensorapplications.Theyarewidelyusedand accepted acrossmanyapplications inmany industries. While itwouldbe tempting todevelop acompletely new, revolutionary digital communication interface, the evolutionary MIPI I3C interfaceallowsco-existencebetweenI²CandMIPII3Cdevices.ThisenablesagradualmigrationtoMIPII3Canddoesnotrequireanimmediateupgradeofexistingproducts(i.e.sensors)fromI²CtoMIPII3C.TheMIPII3Cinterfacestrivestore-use,builduponandimproveI²Cforeasysystemintegrationwhilemaintainingbackwardcompatibility.

ThemainfocusfortheMIPII3Cstandardhasbeentounifyafragmentedinterfaceindustrybydefiningastandardthatislowcost(i.e.takesupverylittlecircuitreal-estateonthesensorside),high-speedandthatuses as little power aspossible. TheMIPI I3C standardprovides immediatebenefits for existingsensor systems and there are also provisions in anticipation for future mobile and other systemarchitectures. The firstMIPI I3C compliant IP blocks were announced in April 2016 with immediateavailability. These IP blocks can be dropped directly into new IC designs to add theMIPI I3C sensorinterface.ThissolutionsupportsallMIPII3Cdataratesupto33.3Mbps,dynamicaddressallocationandmulti-masteroperations.ItisalsocompliantwiththeMIPICameraControlInterface(CCI)andbackwardcompatiblewithI²C.TheideaismeantfordeveloperstofutureprooftheirdesignsforlegacyI²CdeviceswhileutilizingthenewfunctionalitiesthattheMIPII3Cstandardoffers.Table2comparesandcontrastsMIPII3C,I²CandSPI.



Parameter MIPII3C

(ImprovedInterIntegratedCircuits)

I²C

(InterIntegratedCircuits)

SPI

(SerialPeripheralInterface)Overview

I3CMAINMASTER

I3CSLAVE

I3CSECONDARYMASTER

SDA

SCL

I2CSLAVE

I2CMAINMASTER

I2CSLAVE

I2CSLAVE

SDA

SCL

I2CSLAVE

SLAVE_INT

SLAVE_INT

SPIMASTER

SCLKMOSIMISOSS1SS2

SPISLAVE

SPISLAVE

SLAVE_INT

SLAVE_INT

NumberofLines

2-wire 2-wire(plusseparatewiresforeachrequiredinterruptsignal)

4-wire(plusseparatewiresforeachrequiredinterruptsignal)

Effective DataBitrate

33.3Mbpsmaxat12.5MHz(Typically:10.6Mbpsat12MHz

SDR)

3Mbpsmaxat3.4MHz(Hs)0.8Mbpsmaxat1MHz(Fm+)0.35Mbpsmaxat400KHz(Fm)

Approx.60Mbpsmaxat60MHzforconventionalimplementations(Typically:10Mbpsat10MHz)

Advantages • Onlytwosignallines• LegacyI²Cdevicesco-existonthesamebus(withsomelimitations)

• DynamicaddressingandsupportsstaticaddressingforlegacyI²Cdevices

• I²C-likedataratemessaging(SDR)• Optionalhighdataratemessagingmodes(HDR)

• Multi-dropcapabilityanddynamicaddressingavoidscollisions

• Multi-mastercapability• In-bandInterruptsupport• Hot-joinsupport• Aclearmasterownershipandhandovermechanismisdefined

• In-bandintegratedcommands(CCC)Support

• Onlytwosignallines• Flexibledatatransmissionrates• Eachdeviceonthebusisindependentlyaddressable

• Deviceshaveasimplemaster/slaverelationship

• Simpleimplementation• Widelyadoptedinsensorapplicationsandbeyond

• Supportsmulti-masterandmulti-dropcapabilityfeatures

• Fullduplexcommunication• Push-pulldrivers• Goodsignalintegrityandhighspeedbelow20MHz(higherspeedarechallenging)

• HigherthroughputthanI²CandSMBus

• Notlimitedto8-bitwords• Arbitrarychoiceofmessagesize,contentandpurpose

• Simplehardwareinterfacing• LowerpowerthanI²C• Noarbitrationorassociatedfailuremodes

• Slavesusethemaster'sclock• Slavesdonotneedauniqueaddress• Notlimitedbyastandardtoanymaximumclockspeed(canvarybetweenSPIdevices)

Disadvantages • Only7-bitsareavailablefordeviceaddressing

• SlowerthanSPI(i.e.20Mbps)• Newstandard,adoptionneedstobeproven

• Limitednumberofdevicesonabustoaroundadozendevices

• Only7-bits(or10-bits)areavailableforstaticdeviceaddressing

• Limitedcommunicationspeedratesandmanydevicesdonotsupportthehigherspeeds

• Slavescanhangthebus;willrequiresystemrestart

• Slowerdevicescandelaytheoperationoffasterspeeddevices

• UsesmorepowerthanSPI• Limitednumberofdevicesonabustoaroundadozendevices

• Noclearmasterownershipandhandovermechanism.

• Requiresseparatesupportsignalsforinterrupts

• NeedmorepinsthanI²C/MIPII3C• Needdedicatedpinperslaveforslaveselect(SS)

• Noin-bandaddressing• Noslavehardwareflowcontrol• Nohardwareslaveacknowledgment• Supportsonlyonemasterdevice• Noerror-checkingprotocolisdefined

• Noformalstandard,validatingconformanceisnotpossible

• SPIdoesnotsupporthotswapping• Requiresseparatesupportsignalsforinterrupts

Table2:Comparisonbetweendigitalcommunicationinterfaces:MIPII3C,I²CandSPI.



MIPII3CProtocolandSystemIntegration

TheMIPI I3C interfaceprovides several communicationprotocols includingan I²C like singledata rate(SDR)messagingmoderunningupto12.5MHzandseveralhighdatarate(HDR)messagingmodesthatarenotI²Ccompatible. BoththeSDRandHDRformatsshareatwo-wireinterfacewithabidirectionaldatapin(traditionallySDA)andonepineitherusedasaclockpin(SCLonSDRandHDR-DDRprotocols)or as a bidirectional data pin (on HDR-TSL and HDR-TSP protocols). The I²C compatible SDR formatsupportsamixofvariousmessagetypeslikestandardI²Cmessages,broadcastandCCCmessagesthatallow themaster to communicate to all devices on the bus and slave initiated requests (e.g. in-bandinterruptsorrequeststoassumethemasterrole).TheSDRmodeismoreflexiblethanI²CandallowsasecondarymasterandamixofI²CandMIPII3Cslaves.I²CslavesmustbeaccommodatedbystandardI²C data rate speeds and communication protocol initiated by a start-bit followed by a 7-bit slaveaddress.Inotherwords,theMIPII3CSDRmodecouldbeoperatedexactlylikeanI²Cbus.AlldevicesonthebusmustbeupgradedtoMIPII3CtobeabletotakefulladvantageoftheMIPII3CinterfaceliketheHDRmodes,though.

ThehighspeedMIPII3Cmodessupportspeedsbeyondthe12.5MbpsbaseSDRmode.TherearetwomainHDRmodes,HDR-DDR(doubledatarate)andHDR-TSL/TSP(ternarysymbol). Thesemodesofferbitrates over 33 Mbps at a fraction of the per bit power of I²C (400 kHz, fast mode). Slave-sideimplementationissimpleandthismodecancoexistwithlegacyI²Cdevices.Inpractice,thismeansthatHDR-DDRcanbeusedtocommunicatewithMIPII3CslaveswhileallowingI²Cdevicestobeonthesamebus and communicate with these using legacy I²C interface. The I²C devices are expected to simplyignorethehighspeedMIPI I3CHDRtransmissions. TheHDR-DDRmodeusestheSCLsignalasaclockwithdatabitsbeingclockedonbothSCLedges(I²CandMIPII3CSDRmodeschangeSDAonlywhenSCLis low). HDR-DDRmovesdataby“words”,whichcontain twopreamblebits,16payloadbitsandtwoparity bits. There are four word types defined: commandword, user data word, cyclic-redundancy-check (CRC) word and reserved word. The different word types have specific functions, like thecommandwordthatindicatesthedirectionofdatamovementeitheraswrite(mastertoslave)orread(slavetomaster).TheHDR-TSL/TSPenablesternary(i.e.basethreenumeralscodes)symbolcodingforpureMIPI I3C implementation (TSP)and I²C-legacy inclusive (TSL)systems. HDR-TSL isusingbothSCLandSDAasdatalines,whereatleastonelinemusttransitioneachperiod.Transitionindicesareusedforencodingbinaryintoternarysymbolstransfertoenablehighspeedtransmissionatverylowpower.

Amajorchallengeforsmartphones,wearablesandothersystems is to integratemultiplesensorsthatare using different communication protocols. Similar sensors from various vendors have beennotoriouslyinconsistentinspecifyingsensorperformanceandcommunicationinterface.Backin2013,Intel, Qualcomm and MSIG pioneered a sensor performance standard for mobile devices called the“Standardizedsensorperformanceparameterdefinitions”4.ThislaterbecametheIEEE2700standard.Similarlythere isaneedtostandardizehowmicrocontrollersandapplicationprocessorscommunicatewithsensorsinasystem.Currently,eachsensorhasitsownsetoffunctionsandcommandstructure.Itwouldbehighlybeneficial tohaveaunified“sensor link”thatwoulduseaconsistent implementationforalltypesofsensors. Asanexample,a“ReadPressure”commandforabarometricpressuresensorwould return a 16-bit pressure result independent of its vendor. This sensor link could allow pre-

4“Standardizedsensorperformanceparameterdefinitions”,MEMSIndustryGroup,May2014http://www.memsindustrygroup.org/default.asp?page=SPD



compiledlibrariesthatwouldgreatlysimplifysoftwaredevelopmentforthetransferlayerbetweenthemasterandslavesontheMIPII3Cbus.

CaseStudy|MIPII3Cforsmartphones

The many sensors packed into modern smartphones are enabling advanced features like activityrecognition,pedestriannavigation,healthandfitnesstrackingcapabilitiesandothers.Smartphonesareemployingsensorfusionandalgorithmstorecognizethedifferencebetweenwalkinganddriving,ortomakecleverpowersavingschemeslikeswitchingofftheWi-Fiifthephonehasbeenidleforaperiodoftime. As the trend for adding more sensors continues, the implementation is quickly becomingunmanageable.High-endsmartphonesalreadyhave10sensorsormore,andcanrequireupto20signallines.Additionalsensorswillrequireadditionallogiclinesandincreasetheoverallpowerconsumption.Thentherearethealways-onfeaturesthatenableconstantmonitoringofsensorfunctionsevenwhenadevice is in idlemode. A smartphone located in the user’s pocket or pursewill continue to run thepedometermodeforstepcounting,alongwithotheractivityrecognitionfeatures. Thefactthatsomesensorsarealwaysactiveandthatsensordataisalwaysbeingtransferredbetweendevices,requiresavery low power communication interface. Both I²C and SPI are typically used to support multiplesensors,buttheybothhavedrawbacksforsensorinterconnections.Neitherofthemhaveamethodtonotifythemasteraboutachangeinstateorto initiateasensordatatransfer. Thesenotificationsarecurrentlybeingperformedbyexternalgeneralpurpose inputandoutput (GPIO)signals. MIPI I3Ccanreplace both I²C and SPIwith amore power efficient two-wire interface. TheMIPI I3C standardizedsensorinterfacewilleliminateorreducetheneedforexternalGPIOsandsubstitutethesewithin-bandinterrupts.Theresultisasimplerandmoreflexibleimplementation.Table3listsanumberofsensorsandotherdevicesthatarebeingtargetedbytheMIPII3Cinterface.

Table3:PartiallistofsensorsandotherfunctionstargetedbytheMIPII3Cinterface.

CaseStudy|MIPII3Cforwearables

Wearablesarecharacteristicallysmallandhaveseverepowerlimitations.Itwouldbehighlydesirabletoreplace the commonly used I²C bus with a more power efficient and flexible interface. Similar tosmartphones,theMIPII3Ccouldreplacedigitalcontrolsignalsforsensorswithin-bandinterrupts.Thiswouldsavespaceondenselypopulatedprintedcircuitboards(PCBs).TheimagesinFigure4displaya



simplewearablethatonlycontainsonesensor,anaccelerometer.Thewearable’selectronicmoduleisshowntoscale(middle)alongwiththePCBthatismountedinsidetheelectronicmodule(right).ThereisnotalotofroomonthisPCBforcomponentsandtraces,noristheremuchspaceforabatteryinsidethehousing.Needlesstosay,theabilitytobothsaverealestateandreducepoweraretwoverycriticalaspectsfordevelopingnewandbetterperformingwearables.

Figure4:SimpleactivitytrackerFitbitFlex,product(left),electronicmodule(middle)andPCBassemblywiththeaccelerometermarkedwithayellowsquare(right).ImagesfromUserLib

(http://www.userlib.com/fitbit-flex-manual-tutorial/)andiFixit(https://www.ifixit.com/Teardown/Fitbit+Flex+Teardown/16050).

CaseStudy|MIPII3CforIoTdevices

InternetofThings(IoT)devicesaremakingeverydayitemslikehomesandcarssmartertoimproveourdailylives.Thiswouldnotbepossiblewithouttheuseofsensorstogatherandanalyzedatafromtheworldaroundus.Figure5showsatypicalsystemwithtwotypesofsensorhubsthatinterfacedirectlytoarangeofsensors.Thesensorhubsactasthemasterandcontrolthecommunicationatalltimes,forboth I2C and SPI. In this system,MIPI I3C can replace all other communication interfaces with twowires.Itwillallowslavedevicestoinitiatecommunicationwithsimplein-bandinterruptrequests.

Figure5:Twocommontypesofsensorhubarchitecturesformobile,wearablesandIoTdevices;applicationprocessorwithsensorhub(left)andexternalsensorhub(right).



Summary

TheMIPII3Cstandardizedsensorinterfaceisagamechangerforintegratedsensorsystems.Ithasbuilta superset of features on top of the existing I²C (two-wire) interface with additional high data ratemodes that can satisfy sensor use cases that currently require an SPI bus (four-wire). MIPI I3C is anevolutionarystandardthatcombinestheadvantageofI²CandSPIwhileaddingnewfeaturessuchasin-band interrupts, dynamic addressing and advanced power management. It is defined to maintainbackwardcompatibilitywithI²Candoffersdrasticallylowercost,lowerpowerandbetterscalabilitythanI²C,SPI,UARTandotherdigitalinterfaces.Sensorusageinmodernsmartphonesthatcurrentlyrequiresup to 20 signal lines including interrupts, can now be replaced by two. This is a major shift instreamliningsensorintegration,whichweexpecttodrivecostefficienciesandstandardizeafragmentedindustry. MIPI I3Chasclearadvantages inmobileandconsumerelectronics likesmartphones, tabletsand wearables. It is also practical for other use cases that employ sensors, like IoT devices andapplicationsformedical, industrial,automotiveandothers. Withawidespreadadoption,MIPII3Chasgreatpotentialandcouldextendtoothernon-sensordevicessuchastouchsensing,always-on,andlow-resolution cameras (higher resolution cameras and displays are covered by other MIPI Allianceinterfaces).Ifthishappens,MIPII3Ccouldtrulybeastandardforthefuture.

ContactInformation:MelanieCole([email protected])

Manager,IntegratedMarketing&MembershipMIPIAlliance(www.mipi.org)

MIPIisaregisteredservicemarkofMIPIAlliance.AllotherMIPIspecificationnamesareservicemarksofMIPIAlliance.Thirdpartymarksarethepropertyoftheirrespectiveowners.

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