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SystemonChipInterfacesfor LowPowerDesign SystemonChipInterfacesfor LowPowerDesign SanjeebMishra NeerajKumarSingh VijayakrishnanRousseau
AcquiringEditor: ToddGreen
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ProjectManager: PunithavathyGovindaradjane
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—NeerajKumarSingh Acknowledgments Wewouldliketoexpressgratitudetothepeoplewhohelpedusthroughthisbook;someofthem directlyandmanyothersindirectly.It’simpossibletonotriskmissingsomeone,butwewill attemptanyway.
Firstandforemost,wewouldliketoacknowledgeBalamuraliGouthamanforwritingthesensor,security,andinput/outputinterfacechapters;andKiranMathforhishelponthestorage sectionofthebook.
WewouldliketothankStuartDouglasandDavidClarkfortheirhelpinreviewingtheconcept, structure,andcontentofthebookandarrangingforpublishingwithElsevier—David,your meticulousreviewshelpedthebooksignificantly.
ThankyousomuchToddGreen,LindsayLawrence,PunithaGovindaradjaneandallthe Elsevierpublishingteamfortheoutstandingwork,help,guidance,andsupport;youhavegone theextramiletomakethebookwhatitis.
WewouldliketothankIntelmanagement,inparticularPramodMaliandSiddanagoudaS.,for thesupportandencouragement.
Aboveall,wethankourfamilyandfriendsfortheirunderstanding,support,andforbeingcontinuoussourcesofencouragement.
Chapter 1 SoCDesignFundamentalsandEvolution Thischapterdiscussesvarioussystemdesignintegrationmethodologies alongwiththeiradvantagesanddisadvantages.Thechapteralsoexplains themotivationforcurrentsystemdesignstomovefrom“systemonboard” designstoward“systemonchip”(SoC)designs.IndiscussingthemotivationforthemovetowardSoCdesign,thechapteralsodiscussesthetypical chipdesignflowtradeoffsaswellashowtheyinfluencethedesignchoices.
INTRODUCTION Asystemissomethingthatachievesameaningfulpurpose.Likeeverything else,itdependsonthecontext.Acomputersystemwillhavehardwarecomponents(theactualmachinery)andsoftwarecomponents,whichactually drivethehardwaretoachievethepurpose.Forexample,talkingaboutapersonalcomputer(alsocommonlyknownasaPC),alltheelectronicsarehardware,andtheoperatingsystemplusadditionalapplicationsthatyouuseare software.
However,inthecontextofthisbook,bya system wemeanthehardwarepart ofthesystemalone. Figure1.1 showsaroughblockdiagramofasystem. Thesysteminthediagramconsistsofaprocessingunitalongwiththeinput/ outputdevices,memory,andstoragecomponents.
Typicalsystemcomponents Roughlyspeaking,atypicalsystemwouldhaveaprocessortodothereal processing,amemorycomponenttostorethedataandcode,somekind ofinputsystemtoreceiveinput,andaunitforoutput.Inaddition,weshould haveaninterconnectionnetworktoconnectthevariouscomponents togethersothattheyworkinacoherentmanner.Itshouldbenotedthat basedontheusagemodelandapplicabilityofthesystem,thevariouscomponentsinthesystemmaycomeindifferingformats.Forexample,inaPC environment,keyboard,andmousemayformtheinputsubsystem,whereas inatabletsystemtheymaybereplacedbyatouchscreen,andinadigital
healthmonitoringsystemtheinputsystemmaybeformedbyagroupofsensors.Inadditiontothebareessentials,theremaybeothersubsystemslike imaging,audio,andcommunication.In Chapter3 we’lltalkaboutvarious subsystemsingeneral,involving:
1. Processor
2. Memory
3. Inputandoutput
4. Interconnects
5. Domain-specificsubsystems(camera,audio,communication,and soon)
Categorizationofcomputersystems Computersystemsarebroadlycategorizedas:general-purposecomputer systemslikepersonalcomputers,embeddedsystemsliketemperaturecontrolsystems,andreal-timesystemslikebrakingcontrolsystems.Generalpurposecomputingsystemsaredesignedtobemoreflexiblesothatthey canbeusedfordifferenttypesoffunctions,whereasembeddedsystems aredesignedtoaddressaspecificfunctionandarenotmeanttobegeneric. Theyareusuallyembeddedaspartofalargerdevice,andtheuserseldom directlyinteractswithsuchasystem.Real-timesystemsareembedded systemswithstringentresponsetimerequirements.Allthesecomputing systemsarebuiltusingthesamebasicbuildingblocksasshownin Figure1.1.Theflavorofthebuildingblocksmayvaryfromsystemtosystembecausethedesignparametersanddesignrequirementsaredifferent. Forexample,sinceembeddedsystemshaveafixedknownusage,thecomponentscanoptimallybechosentomeetthatfunctionalrequirement.The
Main memory
Secondary memory/storage
Input devices
Processor
Output devices
n FIGURE1.1 Asystemwithmemory,processor,input/output,andinterconnects.
general-purposesystem,ontheotherhand,mighthavetosupportarangeof functionalityandworkloads,andthereforecomponentsneedtobechosen keepinginmindthecostanduserexperiencefortherangeofapplications. Similarlythecomponentsforreal-timesystemsneedtobechosensuchthat theycanmeettheresponsetimerequirement.
SYSTEMAPPROACHTODESIGN Duetothetighterbudgetoncost,power,andperformancediscussedinthe previoussection,thewholesystemisbeingthoughtaboutanddesignedas completeandnotasanassemblyofdiscretepieces.Thephilosophyofsystemdesigntherebybringstheopportunitytooptimizethesystemforaparticularusage.Thereisnorealchangeinthesystemfunctionality;it’sjust thatitisadifferentwayofthinkingaboutthesystemdesign.Wealready talkedaboutthetypicalsystemcomponents;nextwewilldiscussthehardwaresoftwareco-design,followedbyvarioussystemdesignmethodologies.
Hardwaresoftwareco-design Asdiscussedearlier,asystemingeneralhassomehardwareaccompanied bysomesoftwaretoachievethepurpose.Generally,thesystem’sfunctionalityasawholeisspecifiedbythebaredefinitionofthesystem.However, whatpartofthesystemshouldbededicatedhardwareandwhatshouldbe softwareisadecisionmadebythesystemarchitectanddesigner.Theprocessoflookingatthesystemasawholeandmakingdecisionsastowhat becomeshardwareandwhatbecomesasoftwarecomponentiscalled hardwaresoftwareco-design.Typicallytherearethreefactorsthatinfluencethe decision:
n Input,output,memory,andinterconnectsneedtohavehardware (electronics)todothefundamentalpartexpectedfromthem.However, eachoftheseblockstypicallyrequiressomeprocessing;forexample, touchdatareceivedfromtheinputblockneedstobeprocessedtodetect gestures,ortheoutputdataneedstobeformattedspecificallytosuitthe display.Theseprocessingparts,generallyspeaking,arepartofthe debateastowhetheradedicatedhardwarepieceshoulddothe processingorwhetherthegeneral-purposeprocessorshouldbeableto takecareoftheprocessinginpartorfull.
n Thesecondfactorthatcontributestothedecisionistheexperience thatwewanttodelivertotheuser.Whatthismeansisthat,dependingon theamountofdatathatneedstobeprocessed,thequalityofthe outputthatisexpected,theresponsetimetotheinput,andsoon,wehave todecidethequalityofthededicatedhardwarethatshouldbeused,
andalsothishelpsmakethedecisionastowhichprocessingshouldbe donebydedicatedhardwareandwhichbysoftwarerunningontheCPU. Theassumptionhereisthathardwarededicatedtodoingspecific processingwillbefasterandmoreefficient,sowhereverweneedfaster processing,wededicatehardwaretodotheprocessing,suchas,for example,graphicsprocessingbeingprocessedbyagraphics processingunit.
n Thethirdfactoristheoptimality.Therearecertaintypesofprocessing thattakealotmoretimeandenergywhendonebygeneral-purpose processingunitsasopposedtoaspecializedcustomprocessor,suchas digitalsignalprocessingandfloatingpointcomputations,whichhave dedicatedhardware(DSPunitandfloatingpointunit,respectively) becausetheyareoptimallydoneinhardware.
Systemdesignmethodologies Earlyon,thescaleofintegrationwaslow,andthereforetocreateasystemit wasnecessarytoputmultiplechips,orintegratedcircuits(ICs),together. Today,withvery-large-scaleintegration(VLSI),designingasystemona singlechipispossible.So,justlikeanyotherstream,systemdesignhas evolvedbasedonthetechnologicalpossibilitiesofthegeneration.Despite thefactthatsystemonasinglechipispossible,however,thereisnoone designthatfitsall.Incertaincasesthedesignissocomplexthatitmay notfitonasinglechip.Why?Basedonthetransistorsize(whichislimited bytheprocesstechnology)andsizeofthedie(againlimitedbytheprocess technology)thereisalimitednumberoftransistorsthatcanbeplacedona chip.Ifthefunctionalityiscomplexandcannotbeimplementedinthatlimitednumberoftransistors,thedesignhastobebrokenoutintomultiple chips.Also,thereareotherscalabilityandmodularityreasonsfornot designingthewholesysteminonesinglechip.Inthefollowingsectionwe’ll discussthethreemajorsystemdesignapproaches:systemonboard(SoB), systemonchip(SoC),andsysteminapackage(SiP)oronapackage(SoP).
Systemonboard SoBstandsforsystemonboard.Thisistheearliestevolutionofsystem design.Backinthe1970sand1980swhenasinglechipcoulddoonlyso much,thesystemwasdividedintomultiplechipsandallthesechipswere connectedviaexternalinterconnectinterfacesoveraprintedcircuitboard. SoBdesignsarestillapplicabletodayforlargesystemdesignsandsystem designsinwhichdisparatecomponentsneedtobeputtogethertoworkasa system.
AdvantagesofSoB Despitethefactthatthisistheearliestapproachtosystemdesignandbackin theearlydaysitwastheonlyapproachfeasibletobeabletodoanything meaningful,theSoBdesignapproachisprevalenteventodayandhasa lotofadvantagesoverotherdesignapproaches:
n Itisquickandeasytododesignspaceexplorationwithdifferent components.
n Proven(prevalidatedandused)componentscanbeputtogethereasily.
n Designcomplexityforindividualchipsisdivided,sotheriskofabug isless.
n Thedebuggingofissuesbetweentwocomponentsiseasierbecausethe externalinterfacescanbeprobedeasily.
n Individualcomponentscanbedesigned,manufactured,anddebugged separately.
DisadvantagesofSoB SincethereisamovetowardSiP/SoPandSoC,theremustbesomedisadvantagestotheclassicalSoBdesignapproach;thesecanbesummarizedas follows:
n Becauseoflongconnectivity/interconnects,thesystemconsumesmore powerandprovideslessperformancewhencomparedtoSoC/SiP/SoP designs.
n Overallsystemcostisgreaterbecauseoflargersize,morematerials requiredinmanufacturing,higherintegrationcost,andsoon.
n Sinceindividualcomponentsaremadeandvalidatedseparately,they cannotbecustomizedoroptimizedtoaparticularsystemrequirementor design.
Systemonchip BydefinitionSoCmeansacompletesystemonasinglechipwithnoauxiliarycomponentsoutsideit.ThecurrenttrendtodayisthatallthesemiconductorcompaniesaremovingtowardSoCdesignsbyintegratingmoreand morecomponentsofasystemasSoC.However,thereisnotasingleexampleofapureSoCdesign.
AdvantagesofSoC
SomeoftheadvantagesofSoCdesignare
n lowersystemcost, n compactsystemsize, n reducedsystempowerconsumption,
n increasedsystemperformance,and n intellectualpropertyblocks(IPs)usedinthedesigncanbecustomized andoptimized.
DisadvantagesofSoC EventhoughitlooksasthoughSoCdesignisveryappealing,therearelimitations,challenges,andreasonsthatnoteverythinghasmovedtoSoC. Someofthereasonsareoutlinedbelow:
n Forbigdesigns,fittingthewholelogiconasinglechipmaynotbe possible.
n Siliconmanufacturingyieldmaynotbeasgoodbecauseofthebigdie sizerequired.
n TherecanbeIPlibrary/resourceproviderandlegalissues.
n Chipintegration:Componentsdesignedwithdifferentmanufacturer processesneedtobeintegratedandmanufacturedononeprocess technology.
n Chipdesignverificationisachallengebecauseofthehugemonolithic design.
n Chipvalidationisachallenge,alsobecauseofthemonolithicdesign.
Systeminapackage SiPorSoPdesignisapracticalalternativetocounterthechallengesposed bytheSoCapproach.Inthisapproach,variouschipsaremanufacturedseparately;however,theyarepackagedinsuchawaythattheyareplacedvery closely.Thisisalsocalleda multichipmodule (MCM)or multichippackage (MCP).ThisisakindofmiddlegroundbetweenSoBandSoCdesign methodologies.
AdvantagesofSiP Inthisapproachthechipsareplacedcloseenoughtogivecompactsize, reducedsystempowerconsumption,andincreasedsystemperformance. Inaddition:
n IPsbasedondifferentmanufacturingtechnologiescanbemanufactured ontheirowntechnologiesandpackagedasasystem.
n Becauseofsmallersizesofindividualchips,themanufacturingyield isbetter.
n Developmentcomplexityislessbecauseofdivisionofdesigninto multipleparts.
n Bigdesignsthatcannotbemanufacturedasasinglechipcanbemadeas aSiP/SoP.
DisadvantagesofSiP Despitethefactthatthedifferentchipsareplacedverycloselytominimize thetransmissionlatency,theSiPdesignislessthanoptimalintermsof powerandperformanceefficiencywhencomparedtoSoCdesigns.Inaddition,thepackagingtechnologyfortheMCM/MCPsystemismorecomplex andmorecostly.
Inmostoftheliterature, SiP and SoP areusedinterchangeably;however, sometimestheyhavedifferentmeanings.SiPreferstoverticalstackingof multiplechipsinapackageandSoPreferstoplanarplacementofmorethan onechipinapackage.Forexample,aSiPorSoPcancontainmultiplecomponentslikeprocessor,mainmemory,flashmemoryalongwiththeinterconnects,andauxiliarycomponentslikeresistor/capacitoronthesame substratetomakeitaSiPorSoP.
Application-specificintegratedcircuit Application-specificintegratedcircuit(ASIC)isafunctionalblockthatdoes aspecificjobandisnotsupposedtobeageneral-purposeprocessingunit. ASICdesignsarecustomizedforaspecificpurposeorfunctionalityand thereforeyieldsmuchbetterperformancewhencomparedtogeneralpurposeprocessingunits.ASICdesignsarenotacompetingdesignmethodologytoSoC,butrathercomplementary.So,whendesigninganSoC, thedesignermakesadecisionastowhatIPsorfunctionalblockstointegrate.AndthatdecisioncomesbasedonwhethertheSoCismeanttobe generalpurpose,cateringtovariousdifferentapplicationneeds(likeatablet SoCthatcanbeusedwithdifferentoperatingsystemsandthencustomized toserveasrouterordigitalTVorGPSsystem),orforaspecificpurposethat issupposedtocatertoonlyaspecificapplication(e.g.,aGPSnavigator).
AdvantagesofASIC So,onemightthinkthatitisalwaysbettertomakeageneral-purposeSoC, whichcancatertomorethanjustoneapplication.However,therearesignificantreasonstochoosetomakeanASICoverageneral-purposeSoC:
n Cost:Whenwemakeageneral-purposeSoCanditiscustomizedfora specificpurpose,agoodpieceoflogiciswastedbecauseitisnotusedfor thatspecificapplication.IncaseofASIC,thesystemorSoCismadeto suit;thereisnoredundantfunctionality.Andthereforethedieareaofthe systemissmaller.
n Validation:ValidationofanASICismucheasierthanthegeneralpurposeSoC.Why?Becausewhenavendorcreatesageneral-purpose SoCandmarketsitassuch,thereareaninfinitenumberofpossibilities
forwhichthatSoCcanbeused,andthereforethevendorneedsto validatetoitsspecificationtoperfection.Ontheotherhand,whenone createsanASIC,thatpieceissupposedtobeusedforthatspecific purpose.Therefore,thevendorcanlivewithvalidationoftheASICfor thattargetedapplication.
n Optimization:Sinceit’sknownthattheASICwillbeusedforthe specificapplication,thedesignchoicescanbemademoreintelligently andoptimally;forexample,howmuchmemory,howmuchshouldbe memorythroughput,howmuchofprocessingpowerisneeded,and soon.
DisadvantagesofASIC Therearealwaystradeoffs.Ofcourse,therearesomedisadvantagestothe ASICdesignapproach:
n Weallknowthatthehardwaredesignandthemanufacturingcycleare longandintensive(effortandcost).So,makinganASICforevery possibleapplicationisnotgoingtobecosteffective,unlesswecan guaranteethatthevolumeofeachsuchASICwillbehuge.
n Customerswantonesystemtobeabletodomultiplethings,ratherthan carryingonedeviceforGPS,oneforphonecalls,anotherforInternet browsing,anotheroneforentertainment(mediaplayback),andyet anotheroneforimaging.Also,sincetherearecommonfunctionblocks ineachofthesesystems,itismuchcheapertomakeonesystemtodoit all,whencomparedwithamortizedcostofallthedifferentsystems,each dedicatedforonefunctionality.
Systemonprogrammablechip Becauseofaneedforfastdesignspaceexploration,anewtrendisfastgaininginpopularity:thesystemonaprogrammablechip,orSoPC.InanSoPC solutionthereisanembeddedprocessorwithon-chipperipheralsandmemoryalongwithlotsofgatesinafield-programmablegatearray(FPGA).The FPGAcanbeprogrammedwiththedesignlogictoemulate,andthesystem behavior,orfunctionality,canbeverified.
AdvantageofSoPC SoPCdesignsarereconfigurableandthereforecanbeusedforprototyping andvalidatingthesystem.BugfixesaremucheasiertomakeinthisenvironmentthaninanSoCdesign,whereinoneneedstochurninanotherversionofsilicontofixandverifyabug,whichhasasignificantcost.
DisadvantageofSoPC TheSoPCdesignmodelsthefunctionalityinanFPGA,whichisnotasfast asrealsiliconwouldbe.Itisthereforebestfitforthesystemprototypingand validation,andnotreallyforthefinalproduct.
Systemdesigntrends Asweseefromtheprecedingdiscussion,therearemanyapproachestoa systemdesign:onemoresuitableforonescenariothanother.Itshould,however,benotedthattheSoCapproach,whereverpossible,bringsmany advantagestothedesign.Andtherefore,notsurprisingly,theSoCapproach isthetrend.However,forvariousreasonsapureSoCinidealtermsisnot possibleforarealsystem.Infact,initiallyitwasonlypossibletodesign smallerembeddeddevicesasSoCduetothelimitednumberoftransistors onachip.Itisnowpossibletointegrateevenageneral-purposecomputing deviceontoasinglechipbecauseMoore’slawhasallowedmoretransistors onasinglechip.SoCsforgeneral-purposecomputingdevicesliketablets, netbooks,ultrabooks,andsmartphonesarepossiblethesedays.Giventhe advantagesofSoCdesign,thelevelofintegrationinachipisgoingtodecide thefateofonecorporationversusanother.
HARDWAREICDESIGNFUNDAMENTALS Intheprevioussectionwetalkedaboutvarioussystemdesignapproaches andtheconceptofhardwaresoftwareco-design.Irrespectiveofthesystem designmethodology,thecomputersystemismadeofICs.Weallknowthat theintegratedchipdesignisacomplexpipelineofprocessculminatinginan ICchipthatcomesoutofmanufacturing.Inthissectionwetalkalittlebit aboutthepipelineofprocessesinanICdesign.
ThebasicbuildingblockofanyICisatransistor,andmultipletransistorsare putandconnectedtogetherinaspecificwaytoimplementthebehaviorthat wewantfromthesystem.Sincetheadventoftransistorsjustafewdecades back,thesizeoftransistorshasgonedownexponentially,andthereforethe numberoftransistorsintegratedinachiphasgrownsimilarly.Justtobring insomeperspective,thenumberoftransistorsonachipin1966wasabout 10ascomparedtobillionsoftransistorsonthelatestonein2014.
Theminimumwidthofthetransistorisdefinedbythemanufacturingprocesstechnology.Foracademicpurposes,thelevelofintegrationhasbeen classifiedbasedonitsevolution:
1. SSI ¼ small-scaleintegration(upto10gates)
2. MSI ¼ medium-scaleintegration(upto1000gates)
3. LSI ¼ large-scaleintegration(upto10,000gates)
4. VLSI ¼ very-large-scaleintegration(over10,000gates)
Giventhecomplexityofthedesignstoday,theICdesignfollowsavery detailedandestablishedprocessfromspecificationtomanufacturingthe IC. Figure1.2 illustratestheprocess.
CHIPDESIGNTRADEOFF Tradeoffisthewaytolife.Tradeoffbetweencostandperformanceisfundamentaltoanysystemdesign.Costofthesiliconisadirectfunctionofthe areaofthediebeingused,discountingtheotherone-timeexpensesin designingtheIC.Butchangingsetoftheusagemodelandexpectationfrom Physical verification System specification Architectural design
n FIGURE1.2 High-levelflowofchipdesign.
thecomputersystemhasbroughtintwoothermajordesigntradeoffs:thatof powerandthatofconfigurabilityandmodularity.
Poweruntilafewyearsagowasaconcernonlyformobiledevices.Itisand wasimportantformobiledevicesbecause,withthesmallbatterysizes requiredduetoportabilityandothersimilarreasons,itisimperativethat thepowerconsumptionforthefunctionalityisoptimal.However,asthe hardwaredesignsgotmorecomplexandnumberofsystemsinuseinenterprisesgrewexponentiallytohandletheexponentialgrowthintheworkload, enterprisesrealizedthattheelectricitybill(that’stherunningcostofthe computersystems)wasequally(ormaybemore)importantthanone-time systemcost.Sothechipvendorsstartedtoquotepowerefficiencyinterms ofperformanceperwatt.Andthebuyerswillpaypremiumforpowerefficientchips.
Theotherparametersconfigurabilityandmodularityaregainingorrather havegainedimportancebecauseoftheincessantpursuittoshortentime tomarket(TTM).Theamountoftimeittakestodesign(andvalidate)a functionalblockinthechipfrombaseisquitesignificant.However,if welookatthemarket,newproducts(orsystems)arelaunchedrather quickly.So,thereisaneedforthechipvendortodesignabaseproduct andbeabletoconfigurethesameproducttocatertovariousdifferentmarket segmentswithvaryingconstraints.Theotherfactorthatisbecoming(and againinfacthasbecome)importantismodularityofthedesign.Whyis modularityimportant?ThereasonagainisthattheTTMfromconception tolaunchofaproductissmall,andthedesignofthefunctionalblocksof systemisreallycomplexandtimeconsuming.So,thesystemdevelopment companiesaretakinganapproachtousethefunctionalblocksfromother designers(vendors)asIPandintegratethatintheirproduct.ThefundamentalrequirementforsuchastitchingtogetheristhatthesystemdesignandIP designbeingsourcedmustbothbemodularsotheycanworkwitheachother seamlessly.TheapproachhelpsboththesystemdesignerandIPdesigner: thesystemdesignerbyreducingtheirTTMandtheIPdesignerbyallowing themtospecializeandselltheirIPtoasmanysystemsvendorsaspossible.
Chapter 2 UnderstandingPowerConsumption Fundamentals Thischapterstartsbyexplainingwhypoweroptimizationisimportant,then triestohelpthereaderunderstandthesourcesofpowerconsumption, measurementorhowtomonitorpowerconsumption,anddiscussesthestrategiesappliedtoreducepowerconsumptionattheindividualICandsystem level.However,beforewestarttodelveintothedetails,afewthingsabout whyit’simportant.
WHYPOWEROPTIMIZATIONISIMPORTANT Savingenergyisbeneficialfortheenvironmentandalsofortheuser.There isalotofliteraturethatdiscussesthebenefitsindetail,buttogivejustafew obviousexamples,thebenefitsincludelowerelectricbillsforconsumers, longeruptimeofthedeviceswhenrunningonbatterypower,andsleeker mobilesystemdesignmadepossiblebysmallerbatteriesduetoenergy efficiency.
Knowingthatpowerconservationisimportant,nextweshoulddiscussand understandthefundamentalsofpowerconsumption,itscauses,andtypes. Onceweunderstandthem,wecanbetterinvestigatewaystoconservepower.
Sincetheuseofelectronicdevicesisprevalentacrosseveryaspectofour lives,reducingpowerconsumptionmuststartatthesemiconductorlevel. Thepower-savingtechniquesthataredesignedinatthechiplevelhavea far-reachingimpact.
Inthefollowingsectionwewillcategorizepowerconsumptionintwoways: powerconsumptionattheIClevelandpowerconsumptionatthesystemlevel.
PowerconsumptioninIC Digitallogicismadeupofflip-flopsandlogicgates,whichinturnaremade upoftransistors.Thecurrentdrawnbythesetransistorsresultsinthepower beingconsumed.
n FIGURE2.1 Diagramofatransistordepictingvoltageandcurrentflow.
Figure2.1 showsatransistor,voltage,andthecurrentcomponents involvedwhilethetransistorisfunctioning.Sofromthediagram,the energyrequiredforthetransitionstatewillbe CL *Vdd 2 .Andthepower (energy*frequency)consumptioncanbeexpressedas CL *Vdd 2 *f.Goingfurther,thepowerconsumedbythedigitallogichastwomajorcomponents: staticanddynamic.
Staticpower Staticpoweristhepartofpowerconsumptionthatisindependentofactivity. Itconstitutesleakagepowerandstandbypower.Leakagepoweristhepower consumedbythetransistorinoffstateduetoreversebiascurrent.Theother partofstaticpower,standbypower,isduetotheconstantcurrentfrom Vdd to ground.Inthefollowingsectionwediscussleakagepoweranddynamic power.
Leakagepower Whenthetransistorsareintheoffstatetheyareideallynotsupposedtodraw anycurrent.Thisisactuallynotthecase:Thereissomeamountofcurrent drawnevenintheoffstateduetoreversebiascurrentinthesourceanddrain diffusions,aswellasthesubthresholdcurrentduetotheinversioncharge thatexistsatgatevoltagesunderthresholdvoltage.Allofthisiscollectively referredtoas leakagecurrent.Thiscurrentisverysmallforasingletransistor;however,withinanICtherearemillionstobillionsoftransistors,so thiscurrentbecomessignificantattheIClevel.Thepowerdissipateddueto thiscurrentiscalledleakagepower.Itisduetoleakagecurrentanddepends primarilyonthemanufacturingprocessandtechnologywithwhichthe transistorsaremade.Itdoesnotdependonthefrequencyofoperationof theflip-flops.
Standbypower Standbypowerconsumptionisduetostandbycurrent,whichisDCcurrent drawncontinuouslyfrompositivesupplyvoltage(Vdd)toground.
Dynamicpower Dynamicpower,duetodynamiccurrent,dependsonthefrequencythetransistorisoperatingon.Dynamicpowerisalsothedominantpartoftotal powerconsumption.Dynamiccurrentagainhastwocontributors:
1. Shortcircuitcurrent,whichisduetotheDCpathbetweenthesupplies duringoutputtransition.
2. Thecapacitancecurrent,whichflowstocharge/dischargecapacitive loadsduringlogicchanges.
Thedominantsourceofpowerdissipationincomplementarymetal-oxide semiconductor(CMOS)circuitsisthecharginganddischarging.Therate atwhichthecapacitiveloadischargedanddischargedduring“logiclevel transitions”determinesthedynamicpower.Asperthefollowingequation, withtheincreaseinfrequencyofoperationthedynamicpowerincreases, unliketheleakagepower.
ForCMOSlogicgate,thedynamicpowerconsumptioncanbeexpressedas:
where CL istheloadcapacitance(thecapacitanceduetoload), C istheinternalcapacitanceoftheIC, f isthefrequencyofoperation,and N isthenumber ofbitsthatareswitching.
So,fundamentally,asperformanceincreases(meaningthespeedandfrequencyoftheICincreases)theamountofdynamicpoweralsoincreases. Itcanalsobenotedthatdynamicpowerisdatadependentandiscloselytied tothenumberoftransistorsthatchangestates.
Asisevidentfromtheequationforpowerconsumption,attheIClevel,we haveafewfactorstotweakin:voltage,frequency,capacitance,andthe numberoftransitions,tocontrolorreducethepowerconsumption.
PoweroptimizationinIC So,thetaskofpowerminimizationormanagementcanbedefinedas:minimizingpowerconsumptioninallmodesofoperation(bothdynamicwhen activeandstaticwhenidle/standby)withoutcompromisingontheperformancewhenneeded.Asdiscussedpreviouslythereareafewfactorsthat weneedtotweakintooptimize/minimizethepowerconsumption:
n Voltage.Asisevidentfromtheequation,loweringthesupplyvoltage quicklybringsdownthetotalpowerconsumption.So,whydon’twe bringthevoltagedownbeyondapoint?It’sbecausewepayaspeed penaltyforsupplyvoltagereduction,withdelaysdrastically increasingas Vdd approachesthethresholdvoltage(Vt)ofthedevices. Thistendstolimittheusefulrangeof Vdd toaminimumoftwoto threetimes Vt.Thelimitofhowlowthe Vt cangoissetbythe requirementtosetadequatenoisemarginsandcontroltheincreasein subthresholdleakagecurrents.Theoptimum Vt mustbedetermined basedonthecurrentgainoftheCMOSgatesatlowsupplyvoltage regimeandcontroloftheleakagecurrents.
n Capacitance.Letusnowconsiderhowtoreducephysicalcapacitance. Werecognizethatcapacitancescanbekeptataminimumbyusingless logic,smallerdevices,andfewerandshorterwires.Someofthe techniquesforreducingtheactiveareaincluderesourcesharing,logic minimization,andgatesizing.Aswithvoltage,however,wearenot freetooptimizecapacitanceindependently.Forexample,reducing devicesizesreducesphysicalcapacitance,butitalsoreducesthe currentdriveofthetransistors,makingthecircuitoperatemore slowly.Thislossinperformancemightpreventusfromlowering Vdd asmuchaswemightotherwisebeabletodo.
n Switchingactivity.Ifthereisnoswitchinginacircuit,thennodynamic powerwillbeconsumed.However,thiscouldalsomeanthatno computationoccurs.Sincetheswitchingactivityissodependenton theinputpattern,forageneral-purposeprocessor,containing switchingactivitymaynotberealistic.So,wedonotfocusonthis.It shoulddefinitely,however,bekeptinmindwhiledefiningand designingprotocolssothatweminimizetheswitchingactivitytoas littleaspossibleforaveragecasescenarios.
Applyingthefundamentalsdiscussedintheprevioussection,toaddressthe challengeofreducingpower,thesemiconductorindustryhasadoptedamultifacetedapproach,attackingtheproblemonthreefronts:
n Reducingcapacitance.Thiscanbeachievedthroughprocess developmentsuchassilicononinsulatorwithpartiallyorfully depletedwells,CMOSscalingtosubmicrondevicesizes,and advancedinterconnectsubstratessuchasmultichipmodules.Since thisisdependentontheprocesstechnologybeingused,thereisa limitationdependingonthecurrentprocesstechnology,andany advancementhasitsownpaceofdevelopment.
n Scalingthesupplyvoltagecapacitance.Thisisagainaprocessdependentfactor,anditalsorequireschangeintheauxiliarycircuit
andcomponentsinuse.However,moreimportantly,thesignal-to-noise ratioshouldbepropersothatthecommunicationisnotbrokenbecause ofnoisesignalsofcomparablestrength.
n Usingpowermanagementstrategies.Thisisoneareawherethe hardwaredesignercanmakeahugedifferencebyeffectively managingthestaticanddynamicpowerconsumption.Itshould howeverbenotedthattheactualsavingsdependalotontheusage scenarioortheapplicationofthesystem.Someexamplesofpower managementtechniquesaredynamicvoltageandfrequencyscaling (DVFS),clockgating,andsoforth,whichwillbediscussedinalittle detailinsubsequentsections.
Inthenextsectionwediscussthesestrategiesinsomedepth.Thevarious parametersinterferewitheachother,andthereforetheycannotbechosen independentofotherparametersorvectors.Forexample,CMOSdevice scaling,supplyvoltagescaling,andchoiceofcircuitarchitecturemustbe donecarefullytogetherinordertofindanoptimumpowerandperformance. Itmayseemthatprocessscalingcanhelpsolveallthepowerconsumption problems;however,weshouldnotethatleakagepoweroftheprocessof smallersizeismorethanleakagepoweroftheprocessofhighersize.Especiallywith45nmandbelow,theleakagepowerismorebecauseof increasedelectricfield.Tocounterthisproblem,newmaterialswerediscoveredandemployed.Silicondioxidehasbeenusedasagateoxidematerial fordecades.Thetablein Figure2.2 comparesvariousparametersofsilicon onthreedifferentprocesstechnologies.Inthetable90nmistakenasreferenceorbaseline.Pleasenotethatthetablecomparestheparametersin termsofmultiplierandthevaluesarenotabsolute.Itshouldalsobenoted thatthevaluesarearoughestimatebecausetheseparameterswillbeinfluencedbyotherfactorsaswell.Thekeytakeawayorthepointtonotefrom
Parameter/technology 90 nm65 nm45 nm
Node length (nm)1x0.7x0.5x
Frequency (GHz)1x1.43x2x
power (W)1x0.7x0.5x
Dynamic power density (W/cm2) 1x 1.4x2x
Leakage power density (W/cm2) 1x 2.5x6.5x
Power density(W/cm2) 1x 2x 4x
n FIGURE2.2 Comparisonofvariousparametersdrivenbyprocessin90,65,and45nm.
thetableisthattheleakagepowerisgrowingfasterthandevicelength goingdown.
So,anumberofpower-savingmechanismsareappliedacrossdesign,architecture,andprocesstosavedynamicpower:
n MultipleVdd:
– Staticvoltagescaling.InanSoC,differentblockscanworkon differentvoltages,andthelowerthevoltageofablock,theless poweritislikelytoconsume;therefore,itisimperativetocreate multiplevoltagedomains.Tosupportthis,typicallyvoltage regulatorsareusedtocreatedifferentsuppliesfromonesource supply.IPsoperatingononeparticularvoltagewillbeputinthe respectivevoltageisland.So,in Figure2.3,forexample,theIPs operatingon1.2VwillsitonVoltageIsland-1,whiletheIPs operatingon1.8VwillsitonVoltageIsland-3,andsoon.In Figure2.3,theCPUisshowntobeplacedina1.2-Visland, graphicsandtouchina1.8-Visland,audioina1.5V,andeMMC ina1.9-Visland.Thesevoltagelevelsandtheseparationarejust forthesakeofillustration.Theexactnumberofrails/islands dependsonthedesign,IPsbeingused,andtheintendedusage modelofthesystem.
– DVFS.DVFSisatechniqueusedtooptimizepowerconsumptionin differingworkloadscenarios.Inotherwords,theIPisdesignedin suchawaythatitdoesnotconsumefixedpowerallthetime; instead,thepowerconsumptiondependsontheperformancelevel theIPisoperatingon.So,inheavyworkloadscenarios,theIP willbeoperatingonahigherperformancemodeandthereby consuminghigherpower,whileinlighterworkloadscenarios,the IPwillbeoperatingonlowerperformancemodeandthereby consuminglowerpower.Toimplementthis,intheIPdesign,
n FIGURE2.3 Systemwithmultiple Vdd
variousperformancemodesarecreated.Eachoftheperformance modeshasanassociatedoperatingfrequencyandeachofthe operatingfrequencieshasanassociatedvoltagerequirement.So, dependingontheworkload(andtherebytheperformance requirement),thesystemsoftwarecanchoosetheoperatingmode. WhatthisfundamentallymeansisthataparticularIPiscapableof runningonmultiplefrequenciesonrespectivevoltages,andthe softwarewillchoosetherightfrequencyandvoltage,atthepoint. Ageneraldesignmaylooklike Figure2.4.Asshowninthe figure,thesoftwarewillchoosetherightmodeviamodecontrol, whichwilltranslatetorespectivevoltageandfrequencysetting.
n Adaptivevoltagescaling(AVS).AfurtherextensiontoDVFSisAVS, whereinthemodecontrollermonitorsthestate/performance requirementoftheblockandtunesthevoltage/frequencyofthe block.Inthisdesigntheneedforsoftwarecontrolgoesawayand thereforeafinercontrolonDVFSispossible.Agenericblock diagrammaylooklike Figure2.5
Itmustbenotedthatthemodemonitorandcontrollerunitcontinuously keepmonitoringtheIPsindifferentvoltageislandsandregulatethe voltagebasedontheminimumrequired.
n Clockgating.Sincetheclocktreeconsumessignificantpower (approximately50%ofdynamicpower)itisimportanttoreducethe powertakenbytheclocktree.Fundamentallyclockgatingmeans stoppingtheclocktoalogicblockwhentheoperationsofthatblock arenotneeded.Thisclockgatingsavesthepowerconsumedbylogic operatingoneachclockedge,powerconsumedbyflops,andthe clocktreeitself.Manyvariationsweredevisedthathavebuiltonthis basicconcept.
Voltage Island-1
n FIGURE2.4 Systemwithmultiple Vdd andDVFS.
n Frequency/voltagethrottling.Thisisoneofthevariationsofclock gating,whereintheclockisnotcompletelyshutoff,butrather, dependingontheperformancerequirementoftheclockfrequency,is adjustedtoalowervaluesuchthattheperformancerequirementis metwithminimumpowerconsumption.Sincethesupplyvoltage requirementdependsontheclockfrequency,thevoltageisalso adjustedappropriatelytoalowvalue,therebyreducingthepower consumptionevenfurther.
n Powergating.Alogicalextensiontoclockgatingispowergating,in whichthecircuitblocksnotinusearetemporarilyturnedoff.By powergatingwebringthevoltagetozerofordevicesnotinuse.For example,ifthemediaIPisonaseparatevoltagerail,thenitcanbe completelyturnedoffwhenthereisnomediaplayback.Thisismade possiblebythemultiplevoltagerailsanddomainsinthedesign. Powergatingsavestheleakagepowerinadditiontodynamicpower. Gettingthisrightrequiressignificanteffort.Therearetworeasonsfor this:
–Sincethetimeittakestobringupthedevicefrompowerofftopower onissignificantandnoticeable,collateralsneedtoaccommodatefor powerdownstateanddefinetheiroperationflowaccordingly.
–Thedevicemaynotbeabletorespondwhenpowereddown,and worse,maycauseundesirableeffectswhenaccessedinpower downstate;theblocksaccessingthepowered-downunitsshould includeamechanismtocheckwhethertheblockcanbeaccessed.
n Processimprovement.Astransistorshavedecreasedinsize,the thicknessofthesilicondioxidegatedielectrichassteadilydecreased toincreasethegatecapacitanceandtherebydrivecurrentandraise deviceperformance.Asthethicknessscalesbelow2nm,leakage
Voltage Island-1 Voltage Island-2
Voltage Island-3 Mode monitor and controller Voltage regulator
n FIGURE2.5 Systemwithadaptivevoltagescaling.
currentsduetotunnelingincreasedrastically,leadingtohighpower consumptionandreduceddevicereliability.Replacingthesilicon dioxidegatedielectricwithahigh-κ materialallowsincreasedgate capacitancewithouttheassociatedleakageeffects.
So,tosummarizetheabovediscussion:Therearevariouswaysormechanismsemployedtosavepower,andthesemechanismsdonotworkinisolation,butratherhaveinterdependencies.Thereforevariousmechanismsare tweakedandputtogethertominimizeoroptimizethepowerconsumption.
POWERCONSUMPTIONOFASYSTEM Roughlyspeaking,systemshavetwomodeswhenpoweredon,thefirst beingtheactivemodewhenthesystemisactivelybeingused,thesecond modeofoperationbeingstandbymodewhereinthesystemisonbutis onstandbyandwaitingforinputfromtheuser.Inthestandbymode,tosave power,mostofthesystemcomponentswillbeturnedoffsincetheyareidle. Toeffectivelymanagethepowerandstatetransition,theAdvancedConfigurationandPowerInterface(ACPI)standarddefinesvarioussystemstates anddevicestatesindetail.Generallyspeaking,thedevice/IPisnonfunctionalinlowpowerstates.Inordertousethedevice/IPagain,oneneeds tobringthedevice/IPbacktoafunctionalstatefromthelowpowernonfunctionalstate.Thetimetakenintheprocessiscalledwake-uplatency.Again,a generalruleofthumbis,thelowerthepowerstate,thelongerittakesto bringthedevice/IPtofullyfunctionalstate(themorethewake-uplatency).
So,speakingofthepowerconsumedbyasystem,asshownin Figure2.6,the totalpowerconsumedisasummationofactivemodepowerconsumption, standby(sleep)modepowerconsumption,andthewake-uppower.Inthe figurethe x-axisrepresentstime,whilethe y-axisrepresentsthepowerconsumedattime x.Wake-uppowerrepresentsthepowerwastedduringwakeup.Inanutshell,therearethreecategoriesofpowerconsumption,andseparatestrategiesareappliedtooptimizeeachoftheminasystem:
1. Powerconsumptioninactivemode
2. Powerconsumptioninstandbymode
3. Powerwastageduringsystemwake
Poweroptimizationatthesystemlevel Whilediscussingpoweroptimizationatthesystemlevel,wewilldiscussthe optimizationonthethreefronts:activepowermanagement(APM),idle powermanagement,andconnectedstandbypowermanagement.
