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Event-Based Neuromorphic Systems

Tobi Delbruck

Giacomo Indiveri

Adrian Whatley

Rodney Douglas

EVENT-BASED NEUROMORPHIC SYSTEMS

EVENT-BASED NEUROMORPHIC SYSTEMS

TobiDelbruck

GiacomoIndiveri

AdrianWhatley

RodneyDouglas

UniversityofZurichandETHZurich

Switzerland

Thiseditionfirstpublished2015 ©2015JohnWiley&Sons,Ltd

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ISBN:9780470018491

Setin10/12ptTimesbyAptaraInc.,NewDelhi,India 12015

ThisbookisdedicatedtothememoriesofMishaMahowald, JorgKramer,andPaulMueller.

ListofContributorsxv Forewordxvii Acknowledgmentsxix

ListofAbbreviationsandAcronymsxxi

1Introduction1

1.1OriginsandHistoricalContext3

1.2BuildingUsefulNeuromorphicSystems5 References5

PartIUNDERSTANDINGNEUROMORPHICSYSTEMS7

2Communication9

2.1Introduction9

2.2Address-EventRepresentation12

2.2.1AEREncoders13

2.2.2ArbitrationMechanisms13

2.2.3EncodingMechanisms17

2.2.4MultipleAEREndpoints19

2.2.5AddressMapping19

2.2.6Routing19

2.3ConsiderationsforAERLinkDesign20

2.3.1Trade-off:DynamicorStaticAllocation21

2.3.2Trade-off:ArbiteredAccessorCollisions?23

2.3.3Trade-off:QueueingversusDroppingSpikes24

2.3.4PredictingThroughputRequirements25

2.3.5DesignTrade-offs27

2.4TheEvolutionofAERLinks28

2.4.1SingleSender,SingleReceiver28

2.4.2MultipleSenders,MultipleReceivers30

2.4.3ParallelSignalProtocol31

2.4.4Word-SerialAddressing32 2.4.5SerialDifferentialSignaling33 2.5Discussion34 References35 3SiliconRetinas37 3.1Introduction37

3.2BiologicalRetinas38

3.3SiliconRetinaswithSerialAnalogOutput39

3.4AsynchronousEvent-BasedPixelOutputVersusSynchronousFrames40 3.5AERRetinas40

3.5.1DynamicVisionSensor41

3.5.2AsynchronousTime-BasedImageSensor46

3.5.3AsynchronousParvo–MagnoRetinaModel46

3.5.4Event-BasedIntensity-CodingImagers(OctopusandTTFS)48

3.5.5SpatialContrastandOrientationVisionSensor(VISe)50 3.6SiliconRetinaPixels54 3.6.1DVSPixel54 3.6.2ATISPixel56 3.6.3VISePixel58 3.6.4OctopusPixel59

3.7NewSpecificationsforSiliconRetinas60

3.7.1DVSResponseUniformity60

3.7.2DVSBackgroundActivity62

3.7.3DVSDynamicRange62

3.7.4DVSLatencyandJitter63 3.8Discussion64 References67 4SiliconCochleas71 4.1Introduction72 4.2CochleaArchitectures75 4.2.1Cascaded1D76

4.2.2Basic1DSiliconCochlea77

4.2.32DArchitecture78

4.2.4TheResistive(Conductive)Network79

4.2.5TheBMResonators80

4.2.6The2DSiliconCochleaModel80

4.2.7AddingtheActiveNonlinearBehavioroftheOHCs82

4.3Spike-BasedCochleas83

4.3.1Q-controlofAEREAR2Filters85

4.3.2Applications:Spike-BasedAuditoryProcessing86 4.4TreeDiagram87 4.5Discussion87 References89

5LocomotionMotorControl91

5.1Introduction92

5.1.1DeterminingFunctionalBiologicalElements92

5.1.2RhythmicMotorPatterns93

5.2ModelingNeuralCircuitsinLocomotorControl95

5.2.1DescribingLocomotorBehavior96

5.2.2FictiveAnalysis97

5.2.3ConnectionModels99

5.2.4BasicCPGConstruction100

5.2.5NeuromorphicArchitectures102

5.3NeuromorphicCPGsatWork108

5.3.1ANeuroprosthesis:ControlofLocomotion inVivo 109

5.3.2WalkingRobots111

5.3.3ModelingIntersegmentalCoordination112

5.4Discussion113 References115

6LearninginNeuromorphicSystems119

6.1Introduction:SynapticConnections,Memory,andLearning120

6.2RetainingMemoriesinNeuromorphicHardware121

6.2.1TheProblemofMemoryMaintenance:Intuition121

6.2.2TheProblemofMemoryMaintenance:QuantitativeAnalysis122

6.2.3SolvingtheProblemofMemoryMaintenance124

6.3StoringMemoriesinNeuromorphicHardware128

6.3.1SynapticModelsforLearning128

6.3.2ImplementingaSynapticModelinNeuromorphicHardware132

6.4TowardAssociativeMemoriesinNeuromorphicHardware136

6.4.1MemoryRetrievalinAttractorNeuralNetworks137

6.4.2Issues142

6.5AttractorStatesinaNeuromorphicChip143

6.5.1MemoryRetrieval143

6.5.2LearningVisualStimuliinRealTime145

6.6Discussion148 References149

PartIIBUILDINGNEUROMORPHICSYSTEMS153

7SiliconNeurons155

7.1Introduction156

7.2SiliconNeuronCircuitBlocks158

7.2.1ConductanceDynamics158

7.2.2Spike-EventGeneration159

7.2.3SpikingThresholdsandRefractoryPeriods161

7.2.4Spike-FrequencyAdaptationandAdaptiveThresholds162

Contents

7.2.5AxonsandDendriticTrees164

7.2.6AdditionalUsefulBuildingBlocks165

7.3SiliconNeuronImplementations166

7.3.1SubthresholdBiophysicallyRealisticModels166

7.3.2CompactI&FCircuitsforEvent-BasedSystems169

7.3.3GeneralizedI&FNeuronCircuits170

7.3.4AboveThreshold,Accelerated-Time,andSwitched-Capacitor Designs174

7.4Discussion176 References180

8SiliconSynapses185 8.1Introduction186

8.2SiliconSynapseImplementations188

8.2.1NonConductance-BasedCircuits188

8.2.2Conductance-BasedCircuits198

8.2.3NMDASynapse200

8.3DynamicPlasticSynapses201

8.3.1Short-TermPlasticity201

8.3.2Long-TermPlasticity203

8.4Discussion213 References215

9SiliconCochleaBuildingBlocks219

9.1Introduction219

9.2Voltage-DomainSecond-OrderFilter220

9.2.1TransconductanceAmplifier220

9.2.2Second-OrderLow-PassFilter222

9.2.3StabilityoftheFilter223

9.2.4StabilisedSecond-OrderLow-PassFilter225

9.2.5Differentiation225

9.3Current-DomainSecond-OrderFilter227

9.3.1TheTranslinearLoop227

9.3.2Second-OrderTauCellLog-DomainFilter229

9.4ExponentialBiasGeneration230

9.5TheInnerHairCellModel233 9.6Discussion234 References234

10ProgrammableandConfigurableAnalogNeuromorphicICs237 10.1Introduction238

10.2Floating-GateCircuitBasics238

10.3Floating-GateCircuitsEnablingCapacitiveCircuits238

10.4ModifyingFloating-GateCharge242

10.4.1ElectronTunneling242

10.4.2pFETHot-ElectronInjection242

10.5AccurateProgrammingofProgrammableAnalogDevices244

10.6ScalingofProgrammableAnalogApproaches246

10.7Low-PowerAnalogSignalProcessing247

10.8Low-PowerComparisonstoDigitalApproaches:AnalogComputingin Memory249

10.9AnalogProgrammingatDigitalComplexity:Large-ScaleField ProgrammableAnalogArrays251

10.10ApplicationsofComplexAnalogSignalProcessing253

10.10.1AnalogTransformImagers253

10.10.2AdaptiveFiltersandClassifiers253

10.11Discussion256 References257

11BiasGeneratorCircuits261 11.1Introduction261

11.2BiasGeneratorCircuits263

11.2.1BootstrappedCurrentMirrorMasterBiasCurrentReference263

11.2.2MasterBiasPowerSupplyRejectionRatio(PSRR)265

11.2.3StabilityoftheMasterBias265

11.2.4MasterBiasStartupandPowerControl266

11.2.5CurrentSplitters:ObtainingaDigitallyControlledFractionofthe MasterCurrent267

11.2.6AchievingFineMonotonicResolutionofBiasCurrents271

11.2.7UsingCoarse–FineRangeSelection273

11.2.8Shifted-SourceBiasingforSmallCurrents274

11.2.9BufferingandBypassDecouplingofIndividualBiases275

11.2.10AGeneralPurposeBiasBufferCircuit278

11.2.11ProtectingBiasSplitterCurrentsfromParasiticPhotocurrents279

11.3OverallBiasGeneratorArchitectureIncludingExternalController279

11.4TypicalCharacteristics280

11.5DesignKits281 11.6Discussion282 References282

12On-ChipAERCommunicationCircuits285 12.1Introduction286

12.1.1CommunicationCycle286

12.1.2SpeedupinCommunication287

12.2AERTransmitterBlocks289

12.2.1AERCircuitswithinaPixel289

12.2.2Arbiter290

12.2.3OtherAERBlocks295

12.2.4CombinedOperation297

12.3AERReceiverBlocks298

12.3.1Chip-LevelHandshakingBlock298

12.3.2Decoder299

12.3.3HandshakingCircuitsinReceiverPixel300 12.3.4PulseExtenderCircuits301 12.3.5ReceiverArrayPeripheralHandshakingCircuits301 12.4Discussion302 References303 13HardwareInfrastructure305 13.1Introduction306

13.1.1MonitoringAEREvents307 13.1.2SequencingAEREvents311

13.1.3MappingAEREvents313

13.2HardwareInfrastructureBoardsforSmallSystems316 13.2.1SiliconCortex316 13.2.2CentralizedCommunication317 13.2.3ComposableArchitectureSolution318 13.2.4Daisy-ChainArchitecture324 13.2.5InterfacingBoardsusingSerialAER324 13.2.6ReconfigurableMesh-GridArchitecture328 13.3Medium-ScaleMultichipSystems329 13.3.1OctopusRetina+IFAT329

13.3.2MultichipOrientationSystem332 13.3.3CAVIAR335

14.1.1ImportanceofCross-CommunityCommonality350 14.2ChipandSystemDescriptionSoftware350 14.2.1ExtensibleMarkupLanguage351 14.2.2NeuroML351 14.3ConfigurationSoftware352 14.4AddressEventStreamHandlingSoftware352

14.4.1Field-ProgrammableGateArrays353

14.4.2StructureofAEStreamHandlingSoftware353 14.4.3BandwidthandLatency353 14.4.4Optimization354 14.4.5ApplicationProgrammingInterface355 14.4.6NetworkTransportofAEStreams355 14.5MappingSoftware356 14.6SoftwareExamples357

14.6.1ChipDatabase–ASystemforTuningNeuromorphicaVLSIChips357 14.6.2SpikeToolbox359 14.6.3jAER359 14.6.4PythonandPyNN360

14.7Discussion363 References363

15AlgorithmicProcessingofEventStreams365 15.1Introduction365

15.2RequirementsforSoftwareInfrastructure367

15.2.1ProcessingLatency369 15.3EmbeddedImplementations369 15.4ExamplesofAlgorithms370

15.4.1NoiseReductionFilters370

15.4.2Time-StampMapsandSubsamplingbyBit-ShiftingAddresses372

15.4.3EventLabelersasLow-LevelFeatureDetectors372

15.4.4VisualTrackers374

15.4.5Event-BasedAudioProcessing378 15.5Discussion379 References379

16TowardsLarge-ScaleNeuromorphicSystems381 16.1Introduction381 16.2Large-ScaleSystemExamples382

16.2.1SpikingNeuralNetworkArchitecture382

16.2.2HierarchicalAER384 16.2.3Neurogrid386

16.2.4HighInputCountAnalogNeuralNetworkSystem388 16.3Discussion390 References391

17TheBrainasPotentialTechnology393 17.1Introduction393 17.2TheNatureofNeuronalComputation:PrinciplesofBrainTechnology395 17.3ApproachestoUnderstandingBrains396 17.4SomePrinciplesofBrainConstructionandFunction398 17.5AnExampleModelofNeuralCircuitProcessing400 17.6TowardNeuromorphicCognition402 References404

407

ListofContributors

Editors: Shih-ChiiLiu

TobiDelbruck

GiacomoIndiveri

AdrianWhatley

RodneyDouglas InstituteofNeuroinformatics UniversityofZ ¨ urichandETHZ ¨ urich Z ¨ urich,Switzerland

Contributors: CoreyAshby JohnsHopkinsUniversity Baltimore,MD,USA

RalphEtienne-Cummings JohnsHopkinsUniversity Baltimore,MD,USA

PaoloDelGiudice DepartmentofTechnologiesandHealth IstitutoSuperiorediSanit ` a Rome,Italy

StefanoFusi CenterforTheoreticalNeuroscience ColumbiaUniversity NewYork,NY,USA

TaraHamilton TheMARCSInstitute UniversityofWesternSydney Sydney,Australia

JenniferHasler

GeorgiaTech Atlanta,GA,USA

AlejandroLinares-Barranco UniversidaddeSevilla Sevilla,Spain

BernabeLinares-Barranco NationalMicroelectronicsCenter (IMSE-CNM-CSIC) Sevilla,Spain

RajitManohar CornellTech NewYork,NY,USA

KevanMartin InstituteofNeuroinformatics UniversityofZ ¨ urichandETHZ ¨ urich Z ¨ urich,Switzerland

Andr ´ evanSchaik TheMARCSInstitute UniversityofWesternSydney Sydney,Australia

JacobVogelstein

JohnsHopkinsUniversity Baltimore,MD,USA

ContributorsbyChapter:

Chapter1:TobiDelbruck

Chapter2:RajitManohar,AdrianWhatley,Shih-ChiiLiu

Chapter3:TobiDelbruck,BernabeLinares-Barranco

Chapter4:Andr ´ evanSchaik,TaraHamilton,Shih-ChiiLiu

Chapter5:CoreyAshby,RalphEtienne-Cummings,JacobVogelstein Chapter6:StefanoFusi,PaolodelGiudice

Chapter7:GiacomoIndiveri

Chapter8:Shih-ChiiLiu,GiacomoIndiveri

Chapter9:Andr ´ evanSchaik,TaraHamilton

Chapter10:JenniferHasler

Chapter11:TobiDelbruck,BernabeLinares-Barranco Chapter12:Shih-ChiiLiu

Chapter13:AdrianWhatley,AlejandroLinares-Barranco,Shih-ChiiLiu

Chapter14:AdrianWhatley

Chapter15:TobiDelbruck

Chapter16:RajitManohar

Chapter17:RodneyDouglas,KevanMartin

Foreword

Themotivationforbuildingneuromorphicsystemshasrootsinengineeringandneuroscience. Ontheengineeringside,inspirationfromhowthebrainsolvescomplexproblemshasledto newcomputingalgorithms;however,thegoalofreverseengineeringthebrainisadifficult onebecausethebrainisbasedonabiologicaltechnologythatwasevolvedandnotdesigned byhumanengineers.Ontheneuroscienceside,thegoalistounderstandbrainfunction, whichisstillatanearlystageowingtheextremelyheterogeneousandcompactnatureof neuralcircuits.Neuromorphicsystemsareabridgebetweenthesetwoambitiousenterprises. Thelessonslearnedfrombuildingdevicesbasedonneuralarchitecturesareprovidingnew engineeringcapabilitiesandnewbiologicalinsights.

BuildingneuromorphicVLSIchipsandperfectingasynchronousevent-basedcommunicationbetweenthemhasrequiredagenerationoftalentedengineeringscientists.Thesepeople wereinspiredbyCarverMeadandhis1989landmarkbookon AnalogVLSIandNeural Systems.IwasaWiersmaVisitingProfessorofNeurobiologyattheCaliforniaInstituteof Technologyin1987andattended‘Carverland’groupmeetings.Neuromorphicengineering wasstillinitsinfancy,butthestrengthsandweaknessesofthetechnologywerealready apparent.Thepromiseofanewmassivelyparallel,low-power,andinexpensivecomputing architecturewasbalancedbythetechnicalchallengesofworkingwiththetransistormismatch andnoisethatplaguedanalogVLSIchips.Thebrainwasanexistenceproofthattheseproblemscouldbeovercome,butittookmuchlongertimethanexpectedtofindthepractical solutionswhicharediscussedindetailin Event-BasedNeuromorphicSystems.

Ataboutthesametimethatneuromorphicsystemswereintroduced,theneuralnetwork revolutionwasgettingunderwaybasedonsimulationsofsimplifiedmodelsofneurons.The two-volume1986bookon ParallelDistributedProcessing1 hadchaptersontwonewlearning algorithmsformultilayernetworkmodels,thebackpropagationoferrorsandtheBoltzmann machine.Thesenetworkswerelearnedfromexamples,incontrasttoengineeredsystemsthat werehandcrafted.Theincreaseinoverallcomputingpowerbyafactorofamillionoverthe last25yearsandthelargesizesofdatasetsnowavailableontheInternethavemadedeep learninginhierarchiesofsimplemodelneuronsbothpowerfulandpractical,atleastwhen powerisunlimited.TheNeuralInformationProcessingSystems(NIPS)meetingin2013 had2000attendeesandtheapplicationsofmachinelearningrangedfromvisionsystemsto advertisementrecommendersystems.

1 RumelhartDEandMcClellandJL.1986. ParallelDistributedProcessing. MITPress,Cambridge,MA.

Systemsneurosciencehasmadeprogressoneneuronatatimesincesinglecorticalneurons werefirstrecordedin1959.Inthelast10years,newopticaltechniqueshavemadeitpossibleto recordsimultaneouslyfromhundredsofneuronsandallowedresearcherstobothselectively stimulateandsuppressthespikesinsubtypesofneurons.Analytictoolsarebeingdevelopedtoexplorethestatisticalstructureofbrainactivityinlargepopulationsofneurons,and reconstructionoftheconnectivityofthebrainfromelectronmicrographs,aidedbymachine learning,isproducingintricatewiringdiagrams.Thegoal,whichisfarfromyetrealized,isto usethesenewtoolstounderstandhowactivityinneuralcircuitsgeneratesbehavior.

Thenext25yearscouldbeagoldenperiodasthesethreeinteractingareasofresearchin neuromorphicelectronicsystems,artificialneuralnetworks,andsystemsneurosciencereach maturityandfulfilltheirpotential.Eachhasanimportantroletoplayinachievingtheultimate goal,tounderstandhowthepropertiesofneuronsandcommunicationssystemsinbrainsgive risetoourabilitytosee,hear,plan,decide,andtakeaction.Reachingthisgoalwouldgiveusa betterunderstandingofwhoweareandcreateanewneurotechnologysectoroftheeconomy withfar-reachingimpactonoureverydaylives. Event-BasedNeuromorphicSystems isan essentialresourceforneuromorphicelectricalengineerspursuingthisgoal.

TerrenceSejnowski

LaJolla,California

December15,2013

Acknowledgments

ThisbookwouldnevertakenformwithouttheconsistentsupportoftheUSNationalScience Foundation(NSF)infundingtheTellurideNeuromorphicCognitionEngineeringWorkshop andthehelpoftheEUFET(FutureandEmergingTechnologies)programinsupporting theEuropeanCapoCacciaCognitiveNeuromorphicEngineeringWorkshop.Theseunique, hands-onworkshopsweretheplaceswheremanyoftheideasdevelopedinthisbookwere firstdiscussedandprototyped.Formanyneuromorphicengineers,theseworkshopsarea highlightoftheyearandakindofworkingholidaytimethatothervenues,suchasIEEE conferences,cannotreplace.

Theeditorsandcontributorsto Event-BasedNeuromorphicSystems acknowledgethefollowingpeopleforreadingandcommentingonvariouschaptersinthebook:LucaLonginotti, BjornBeyer,MichaelPfeiffer,JosepMariaMargaritTaul ´ e,DiederikMoeys,BradleyMinch, SimBamford,Min-HaoYang,andChristophPosch.TheyacknowledgeSrinjoyMitraforhis contributiontotheon-chipAERcircuitchapter,PhilippHafligerforhiscontributiontothe synapsechapter,RaphaelBernerforhiscontributiontotheretinachapter,andAntonCivitfor hiscontributiontothehardwareinfrastructurechapter.TheyacknowledgeKwabenaBoahen foruseofthematerialinthecommunicationschapter;DianaKasabovforproofreadingand corrections;andDanielFasnachtforsettinguptheoriginalDokuWikiatthestartofthebook project.Theeditorsalsoacknowledgethestudentsofthe NeuromorphicEngineeringI course attheInstituteofNeuroinformatics,UniversityofZ ¨ urichandETHZ ¨ urichwhogavefeedback onChapters7and8.

TheyfurtheracknowledgetheInstituteofNeuroinformatics,UniversityofZurichand ETHZurich;theNSFTellurideNeuromorphicCognitionEngineeringWorkshop;andthe CapoCacciaCognitiveNeuromorphicEngineeringWorkshop.

ThefigureattheheadoftheSiliconCochleaschapterandreproducedonthecoveris courtesyofEricFragni ` ere.ThefigureattheheadoftheLearninginNeuromorphicSystems chapteriscourtesyofValentinN ¨ agerlandKevanMartin.ThefiguresattheheadoftheSilicon NeuronsandSiliconSynapseschaptersarecourtesyofNunoMiguelMac¸aricoAmorimda Costa,JohnAnderson,andKevanMartin.

ListofAbbreviations andAcronyms

1D onedimensional

2D twodimensional

3D threedimensional

ACA analogcomputingarrays

ACK acknowledge

A/D analog–digital(converter)

ADC analog–digitalconverter

AdEx Adaptiveexponentialintegrate-and-firemodel

AE addressevent

AEB address-eventbus

AER address-eventrepresentation

AEX AERextensionboard

AFGA autozeroingfloating-gateamplifier

AGC automaticgaincontrol

ALOHA Notactuallyanabbreviation,ALOHAreferstoanetworkmediaaccess protocoloriginallydevelopedattheUniversityofHawaii

ANN artificialneuralnetwork

ANNCORE analogneuralnetworkcore

API ApplicationProgrammingInterface

APS activepixelsensor

AQC automaticQ(qualityfactor)control

ARM AcornRISCMachine

ASIC application-specificintegratedcircuit

ASIMO AdvancedStepinInnovativeMObility(robot)

ASP analogsignalprocessor/processing

ATA ATAttachment(alsoPATA:ParallelATA);aninterfacestandardforconnectingmassstoragedevices(e.g.,harddisks)incomputers

ATIS asynchronoustime-basedimagesensor

ATLUM AutomaticTape-collectingLatheUltra-Microtome

aVLSI Analogverylargescaleintegration

BB biasbuffer

BGA ballgridarray

BJT bipolarjunctiontransistor

BM basilarmembrane

BPF band-passfilter

bps bitspersecond

Bps bytespersecond

BSI back-sideillumination

C4 capacitivelycoupledcurrentconveyor

CAB computationalanalogblock

CADSP cooperativeanalog–digitalsignalprocessing

CAVIAR ConvolutionAERVisionArchitectureforReal-time

CCD charge-coupleddevice

CCN cooperativeandcompetitivenetwork

CCW counterclockwise

CDS correlateddoublesampling

CIS CMOSimagesensor

CLBT compatiblelateralbipolartransistor

CMI current-mirrorintegrator

CMOS complementarymetaloxidesemiconductor

CoP centerofpressure

CPG centralpatterngenerator

CPLD complexprogrammablelogicdevice

CPU centralprocessingunit

CSMA carriersensemultipleaccess

CV coefficientofvariation

CW clockwise

DAC digital-to-analogconverter

DAEB domainaddress-eventbus

DAVIS DynamicandActive-PixelVisionSensor

DC directcurrent

DCT discretecosinetransform

DDS differentialdoublesampling

DFA deterministicfiniteautomaton

DIY doityourself

DMA directmemoryaccess

DNC digitalnetworkchip

DOF degree(s)offreedom

DPE dynamicparameterestimation

DPI differentialpairintegrator

DPRAM dual-portedRAM

DRAM dynamicrandomaccessmemory

DSP digitalsignalprocessor/processing

DVS dynamicvisionsensor

EEPROM electricallyerasableprogrammablereadonlymemory

EPSC excitatorypost-synapticcurrent

EPSP excitatorypost-synapticpotential

ESD electrostaticdischarge

ETH EidgenossischeTechnischeHochschule

EU EuropeanUnion

FACETS FastAnalogComputingwithEmergentTransientStates

FE frameevents

FET fieldeffecttransistor

FET also FutureandEmergingTechnologies

FG floatinggate

FIFO First-InFirst-Out(memory)

fMRI functionalmagneticresonanceimaging

FPAA field-programmableanalogarray

FPGA field-programmablegatearray

FPN fixedpatternnoise

FPS framespersecond

FSI frontsideillumination

FSM finitestatemachine

FX2LP AhighlyintegratedUSB2.0microcontrollerfromCypressSemiconductor Corporation

GALS globallyasynchronous,locallysynchronous

GB gigabyte,230 bytes

Gbps gigabitspersecond

Geps gigaeventspersecond

GPL generalpubliclicense

GPS globalpositioningsystem

GPU graphicsprocessingunit

GUI graphicaluserinterface

HCO half-centeroscillator

HDL HardwareDescriptionLanguage

HEI hotelectroninjection

HH Hodgkin–Huxley

HiAER hierarchicalAER

HICANN highinputcountanalogneuralnetwork

HMAX HierarchicalModelandX

HMM HiddenMarkovModel

HTML Hyper-TextMarkupLanguage

HW hardware

HWR half-waverectifier

hWTA hardwinner-take-all

I&F integrate-and-fire

IC integratedcircuit

IDC insulationdisplacementconnector

IEEE InstituteofElectricalandElectronicsEngineers

IFAT integrate-and-firearraytransceiver

IHC innerhaircell

IMS intramuscularstimulation

IMU inertialorintensitymeasurementunit

INCF InternationalNeuroinformaticsCoordinatingFacility

INE InstituteofNeuromorphicEngineering

I/O input/output

IP intellectualproperty

IPSC inhibitorypost-synapticcurrent

ISI inter-spikeinterval

ISMS intraspinalmicrostimulation

ITD interauraltimedifference

JPEG JointPhotographicExpertsGroup

KB kilobyte,210 bytes

keps kiloeventspersecond

LAEB localaddress-eventbus

LFSR linearfeedbackshiftregister

LIF leakyintegrate-and-fire

LLN log-domainLPFneuron

LMS leastmeansquares

LPF low-passfilter

LSM liquid-statemachine

LTD long-termdepression

LTI lineartime-invariant

LTN linearthresholdneuron

LTP long-termpotentiation

LTU linearthresholdunit

LUT look-uptable

LVDS lowvoltagedifferentialsignaling

MACs multiplyandaccumulateoperations

MB megabyte,220 bytes

MEMs microelectromechanicalsystems

Meps megaeventspersecond

MIM metalinsulatormetal(capacitor)

MIPS microprocessorwithoutinterlockedpipelinestages(amicroprocessorarchitecture)

MIPS also millionsofinstructionspersecond

MLR mesencephaliclocomotorregion

MMAC millionsofmultiplyaccumulateoperations

MMC/SD Multimediacard/securedigital

MNC multi-neuronchip

MOSFET metaloxidesemiconductorfieldeffecttransistor

MUX multiplex;multiplexer

NE neuromorphicengineering

NEF neuralengineeringframework

nFET n-channelFET

NMDA N-Methyl-d-Aspartate

NoC NetworkonChip

NSF NationalScienceFoundation

OHC outerhaircell

OR OctopusRetina

ORISYS orientationsystem

OS operatingsystem

OTA operationaltransconductanceamplifier

PC personalcomputer

PCB printedcircuitboard

PCI PeripheralComponentInterconnect

PCIe PeripheralComponentInterconnectExpress

PDR phasedependentresponse

pFET p-channelFET

PFM pulsefrequencymodulation

PLD programmablelogicdevice

PRNG pseudo-randomnumbergenerator

PSC post-synapticcurrent

PSRR powersupplyrejectionratio

PSTH peri-stimulustimehistogram

PTAT proportionaltoabsolutetemperature

PVT process,voltage,temperature

PWM pulsewidthmodulation

PyNN PythonforNeuralNetworks

Q qualityfactoroffilter

QE quantumefficiency

QIF quadraticintegrate-and-fire

QVGA QuarterVideoGraphicsArray;320 ×240pixelarray

RAM randomaccessmemory

REQ request

RF radiofrequency

RF also receptivefield

RFC RequestforComments(apublicationoftheInternetEngineeringTaskForce andtheInternetSociety

RISC reducedinstructionsetcomputing

RMS rootmeansquare

RNN recurrentneuralnetwork

ROI regionofinterest

SAC selectiveattentionchip

SAER serialAER

SAM spatialacuitymodulation

SATA serialATA(aninterfacestandardforconnectingmassstoragedevices(e.g., harddisks)tocomputers,designedtoreplaceATA)

SATD sumofabsolutetimestampdifferences

SC spatialcontrast

S-C switched-capacitor

SCX SiliconCortex

SDRAM synchronousdynamicrandomaccessmemory

SerDes serializer/deserializer

SFA spike-frequencyadaptation

SiCPG siliconCPG

SIE serialinterfaceengine

SiN siliconneuron

SNR signal-to-noiseratio

SOS second-ordersection

SpiNNaker spikingneuralnetworkarchitecture

SRAM staticrandomaccessmemory

SS shiftedsource

SSI stackedsiliconinterconnect

STD short-termdepression

STDP spiketiming-dependentplasticity

STRF spatiotemporalreceptivefield

SW software

sWTA softwinner-take-all

TC temporalcontrast

TCAM ternarycontent-addressablememory

TCDS timecorrelateddoublesampling

TCP TransportControlProtocol

TN TrueNorth

TTFS timetofirstspike

UCSD UniversityofCaliforniaatSanDiego

UDP UserDatagramProtocol

USB universalserialbus

USO unitsegmentaloscillators

V1 primaryvisualcortex

VGA VideoGraphicsArray;640×480pixelarray

VHDL VerilogHardwareDescriptionLanguage

VISe VIsionSensor

VLSI verylargescaleintegration

VME VERSAbusEurocardbusstandard

VMM vector-matrixmultiplication/multiplier

WABIAN-2R WAsedaBIpedalhumANoidNo.2Refined(robot)

WKB Wentzel–Kramers–Brillouin

WR_OTA wide-linearrangeOTA

WTA winner-take-all

XML ExtensibleMarkupLanguage

ZMP zeromomentpoint

1 Introduction

Theeffortlessabilityofanimalbrainstoengagewiththeirworldprovidesaconstantchallenge fortechnology.Despitevastprogressindigitalcomputerhardware,software,andsystem concepts,itremainstruethatbrainsfaroutperformtechnologicalcomputersacrossawide spectrumoftasks,particularlywhentheseareconsideredinthelightofpowerconsumption. Forexample,thehoneybeedemonstratesremarkabletask,navigational,andsocialintelligence whileforagingfornectar,andachievesthisperformanceusinglessthanamillionneurons, burninglessthanamilliwatt,usingionicdevicephysicswithabulkmobilitythatisabout 10milliontimeslowerthanthatofelectronics.Thisperformanceismanyordersofmagnitude moretask-competentandpower-efficientthancurrentneuronalsimulationsorautonomous robots.Forexample,a2009‘cat-scale’neuralsimulationonasupercomputersimulated 1013 synapticconnectionsat700timesslowerthanrealtime,whileburningabout2MW (Ananthanarayananetal.2009);andtheDARPAGrandChallengeroboticcarsdrovealonga denselyGPS-definedpath,carryingoverakilowattofsensingandcomputingpower(Thrun etal.2007).

Althoughwedonotyetgraspcompletelynature’sprinciplesforgeneratingintelligent behavioratsuchlowcost,neurosciencehasmadesubstantialprogresstowarddescribingthe components,connectionarchitectures,andcomputationalprocessesofbrains.Allofthese areremarkablydifferentfromcurrenttechnology.Processingisdistributedacrossbillionsof elementaryunits,theneurons.Eachneuroniswiredtothousandsofothers,receivinginput throughspecializedmodifiableconnections,thesynapses.Theneuroncollectsandtransforms thisinputviaitstree-likedendrites,anddistributesitsoutputviatree-likeaxons.Memory instantiatedthroughthesynapticconnectionsbetweenneuronsisco-localizedwithprocessing throughtheirspatialarrangementsandanaloginteractionsontheneurons’inputdendritictrees. Synapticplasticityiswonderfullycomplex,yetallowsanimalstoretainimportantmemories overalifetimewhilelearningonthetimescaleofmilliseconds.Theoutputaxonsconvey asynchronousspikeeventstotheirmanytargetsviacomplexarborizations.Intheneocortex themajorityofthetargetsareclosetothesourceneuron,indicatingthatnetworkprocessing isstronglylocalized,withrelativelysmallerbandwidthdevotedtolong-rangeintegration. Thevariousperceptual,cognitive,andbehavioralfunctionsofthebrainaresystematically organizedacrossthespaceofthebrain.Neverthelessatleastsomeaspectsofthesevarious

Event-BasedNeuromorphicSystems,FirstEdition. EditedbyShih-ChiiLiu,TobiDelbruck,GiacomoIndiveri,AdrianWhatley,andRodneyDouglas. ©2015JohnWiley&Sons,Ltd.Published2015byJohnWiley&Sons,Ltd.

processescanbediscernedwithineachspecializedarea,andtheirorganizationsuggestsa coalitionofrichlyintercommunicatingspecialists.Overallthen,thebrainischaracterizedby vastnumbersofprocessors,withasynchronousmessagepassingonavastpoint-to-pointwired communicationinfrastructure.Constraintsontheconstructionandmaintenanceofthiswiring enforceastrategyoflocalcollectivespecialization,withlongerrangecoordination.

Forthepasttwodecadesneuromorphicengineershavegrappledwiththeimplementation oftheseprinciplesinintegratedcircuitsandsystems.Theopportunityofthischallengeis therealizationofatechnologyforcomputingthatcombinestheorganizingprinciplesofthe nervoussystemwiththesuperiorchargecarriermobilityofelectronics.Thisbookprovides someinsightsandmanypracticaldetailsintotheongoingworktowardthisgoal.Theseresults becomeevermoreimportantformoremainstreamcomputing,aslimitsoncomponentdensity forceevermoredistributedprocessingmodels.

Theoriginofthisneuromorphicapproachdatesfromthe1980s,whenCarverMead’s groupatCaltechcametounderstandthattheywouldhavetoemulatethebrain’sstyleof communicationiftheyweretoemulateitsstyleofcomputation.Theseearlydevelopments continuedinahandfuloflaboratoriesaroundtheworld,butmorerecentlytherehasbeen anincreaseofdevelopmentbothinacademicandindustriallabsacrossNorthAmerica, Europe,andAsia.Therelevanceoftheneuromorphicapproachtothebroaderchallenges ofcomputationisnowclearlyrecognized(Hof2014).Progressinneuromorphicmethods hasbeenfacilitatedbythestronglycooperativecommunityofneuroscientistsandengineers interestedinthisfield.Thatcooperationhasbeenpromotedbypracticalworkshopssuchas theTellurideNeuromorphicCognitionEngineeringWorkshopintheUnitedStates,andthe CapoCacciaCognitiveNeuromorphicEngineeringWorkshopinEurope.

Event-BasedNeuromorphicSystems arosefromthiscommunity’swishtodisseminatestateof-the-arttechniquesforbuildingneuromorphicelectronicsystemsthatsense,communicate, compute,andlearnusingasynchronousevent-basedcommunication.Thisbookcomplements theintroductorytextbook(Liuetal.2002)thatexplainedthebasiccircuitbuildingblocksfor neuromorphicengineeringsystems. Event-BasedNeuromorphicSystems nowshowshowthose buildingblockscanbeusedtoconstructcompletesystems,withaprimaryfocusonthehotfield ofevent-basedneuromorphicsystems.Thesystemsdescribedinthisbookincludesensorsand neuronalprocessingcircuitsthatimplementmodelsofthenervoussystems.Communication betweenthemodulesisbasedonthecrucialasynchronousevent-drivenprotocolcalledthe address-eventrepresentation(AER),whichtransposesthecommunicationofspikeeventson slowpoint-to-pointaxons,intodigitalcommunicationofsmalldatapacketsonfastbuses(see, forexample,Chapter2).Thebookasawholedescribesthestateoftheartinthefieldof neuromorphicengineering,includingthebuildingblocksnecessaryforconstructingcomplete neuromorphicchipsandforsolvingthetechnologicalchallengesnecessarytomakemulti-chip scalablesystems.Aglanceattheindexshowsthewidebreadthoftopics,forexample,nextto ‘Moore’slaw’is‘motionartifact’andnextto‘bistablesynapse’is‘bootstrappedmirror.’

Thebookisorganizedintotwoparts:PartI(Chapters2–6)isaccessibletoreadersfroma widerrangeofbackgrounds.ItdescribestherangeofAERcommunicationarchitectures,AER sensors,andelectronicneuralmodelsthatarebeingconstructedwithoutdelvingexhaustively intotheunderlyingtechnologicaldetails.Severalofthesechaptersalsoincludeahistorical treethathelpsrelatethearchitecturesandcircuitstoeachother,andthatguidesreaderstothe extensiveliterature.ItalsoincludesthelargelytheoreticalChapter6onlearninginevent-based systems.

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Europe which would soon make every kernel of that seed-wheat to spring up over a renovated hemisphere, and to shake like Lebanon. His bonds never restrained his heroic zeal, but continued preaching the Gospel, and converted many of every rank, even some who were "saints of Cæsar's household." When set at liberty, he sailed to Syria, rapidly passed through Asia Minor, and returned through Macedonia and Corinth to Rome. Britain may have witnessed his devotion, and Spain caught the inspiration of his heavenly zeal. But his chief anxiety was centred in that great fountain of influence, Rome, where he had founded a church containing a "vast multitude," according to the expression of Tacitus, A.D. 65, and where, according to his own presentiment, he was martyred the same year.

The confessors who followed the apostles, like them won the approving testimony of conscience, and the profound esteem of all good men. Their blood was considered the seed of the church, which said concerning them: "To each victor is promised now the tree of life and exemption from the second death, now the hidden manna with the white stone, and an unknown name: now to be clothed in white, not to be blotted out of the book of life, and to be made a pillar in the temple of God, inscribed with the name of his God and Lord of the heavenly Jerusalem: and now to sit down with the Lord on his throne, once refused to the sons of Zebedee." About the beginning of the third century arose a discussion which throws light upon the spirit manifested by the martyr-victims of those days. Celsus, on the part of the heathen, reproached his opponents with the fortitude of Anaxarchus, who, when pounded in a mortar, exclaimed, "Pound the shell of Anaxarchus, himself you touch not." "What," he asks, "did your Deity say in his sufferings comparable to this?" Origen returned the appropriate answer, that a pious submission to God's will, or even a prayer, such as "if it be possible, let this cup pass from me," is more truly magnanimous than the affectation of insensibility, so lauded by stoical paganism. The martyr's surrender of his body to the executioner was esteemed an act of faith, a baptism unto Christ, and came to be regarded as a sacrament of certain efficacy, seeing that no subsequent fall could

annul its power. "Be thou faithful unto death," was evermore whispered in the ear of the confessor, "and I will give thee a crown of life." Thus pacific and defenceless, the primitive church conquered the proud array of pagan and imperial power; and the doubting world, forced to admit a divine interposition in behalf of this new religion, beheld a testimony from heaven to its truth. Perhaps the strongest confidence in the resurrection, and the most energetic subscription to the declaration, "If our earthly house of this tabernacle be dissolved, we have a building of God," was expressed by Ignatius, who, knowing the danger often incurred in obtaining the remains of the martyrs, expressed a wish to be so entirely devoured by beasts, that no fragment of his body should be found.

The emperor Julian was ambitious of establishing the old polytheism on the ruins of Christianity; and, without doubt, Diocletian was resolved at all hazards to extirpate the new creed. But the cause of truth was strong, and its strength received imperial protection in the triumph of Constantine. Under his auspices, a new metropolis arose on the site of antique Byzantium, and soon left eclipsed the ancient capital of the world. Thus the old pagan traditions were annihilated, and its prestige, so vivid and powerful in the imagination of all nations, was no more. The empire underwent a new division, and Constantine commenced a modification of the superseded institutions, which, under the law of continuous change, have lasted until our time. Fatal heresies arose during the fourth and fifth centuries, which caused much Christian activity to be wasted on purely theological subjects; still the church exercised the most preeminent influence, presenting the spectacle of a boundless and universal activity in intellectual labors, and in the progressive development, and advancement of civilization. Many, doubtless, like Celsus, were bold to say, "He must be void of understanding who can believe that Greeks and barbarians, in Asia, Europe, and Lybia, all nations to the ends of the earth, can unite in the reception of one and the same religious doctrine." But such happily was proved to be the fact. Such was the design of Jehovah, in that faith given to change all existing polities, Jewish as well as Gentile, into nations

and states, governed by a law founded upon justice and charity; and taking its highest inspirations from the love of God, as the common Father of mankind, declared, in the words of its great Founder, that "the field is the world."

The Roman bore little noblenesss of soul in life, and found corresponding gloom at its end. Brutus, whose patriotism was darkened by despair, and who died a suicide, exclaimed, "O, virtue! thou art but a name." In reviewing the moral condition of the ancients, we find something to admire, but much to condemn. All things that illustrate their religious views and customs, go not only to exemplify the apostolic declaration, "the world by wisdom knew not God," but equally attest the same writer's description of the vices common to the heathen world. Frivolity and mirth generally prevailed, but true happiness was unknown. A tone of sadness dwelt deepest in the popular heart, as appears not only in the choral odes of tragedy, but even in their comic writings; a sadness inseparable from the condition of gifted minds, conscious of present evils, ignorant of future bliss, and having no other resource than that insane philosophy, "Let us eat and drink, for to-morrow we die." Gleams of divine Providence lay amid the gloomy abodes of polytheism; the great truth of future retribution was suggested in the poetic follies of Tartarus and Elysium. A few torn wreaths from the wreck of Paradise seem to have floated to the Italian shores, elegant to suggest, but impotent to save.

Many of the classic legends indicate a remote and universal consciousness of the natural and perpetual course of all civilizing powers. When Ulysses set sail from the isle of Circe, with tears he launched his dark vessel upon the sea, and, after sailing all day with a favorable wind, he arrived at sunset at the boundaries of the "deep flowing Oceanus," and the city of the Cimmerians, whose darkness is never dispelled. He there evokes the dead; then sails from outer ocean back into the sea, and when he returns to the Circean isle, whose site had been so clearly fixed in the West, he finds the gates of morning and of Aurora. In Læstrygonia, beyond the western horizon, were placed the herds of the sun, and the gardens of the

Hesperides adjoined Eurythia, ruddy with the setting ray. There lived the aged Cronus, the three-bodied giant of the West, guarding his oxen, or the years sunk beneath the wave. But Hercules, in the character of Greek devotion, warring against Phœnician superstition, slays the dog Orthos, and the gloomy herdsman Eurythion, and brings back the lost kine to Argos. Under the guidance of Minerva, or divine wisdom presiding over nature, he is enabled to wield his arms of light against the prince of darkness; but these labors have ever to be repeated, that the apples and the dog may be carefully restored by Minerva to their original and rightful places. These mythological fables are interesting, so far as they indicate the glimmerings of great events, but they also remind us of dark and desperate national characteristics. The Romans, especially, like the favorite deity, Bacchus, were terrible in war, but voluptuous and cruel in peace. Their demi-god, Hercules, who turned rivers from their courses, withdrew the dead from the world of shades, and struck terror into the powers of Orcus, was yet the slave of his appetites, and the dupe of his mistress. Mental imbecility was in him, as in his worshipers, the concomitant of extreme physical force. It was from no love of humanity that Cæsar led his warriors into Britain; and yet the circumstance of that conquest at exactly that time, affected the whole civilization of what is now earth's leading race. It is thus that every successive improvement rises, phœnix-like, from the ashes of the past.

In all ages, the most thoughtful have regarded religion as the unique foundation of duties, as, in turn, duties are the unique bond of society. Public conscience has never been obliterated, however much it has often been obscured. The legislators of antiquity were not in a condition well to understand the nature and relations of highest divinity, but such revelations as were in their possession they employed to consolidate the social edifice, by placing religion in the family, and in the state, as a part of the domestic constitution and general government. In a manner, they caused the laws of heaven to descend and become attached to all the events of human life, and every variety of civil compacts. They even submitted inanimate

objects, as woods, waters, and the boundary-stones of their patrimonies, to celestial supervision; and, it would seem, strove to multiply their gods to an infinite extent, prompted by that instinctive consciousness which every where links the finite creature to his eternal Creator. "Let one attempt to build a city in the air," said Plutarch, "rather than expect to found and long preserve a state from which the gods are driven." Instructed by all preceding experience, and universal tradition, ancient wisdom comprehended thoroughly that there was no national perpetuity save as religion contributed that divine force, foreign to the works of men, and indispensable to the creation of durable institutions. Aristotle recognized in this the common law, and Cicero declared it to be the source of all obligations, the base, support, and main regulator, of states constituted according to nature, and under the direction of supreme intelligence. Plato taught that in every Republic, the first endeavor should be to establish true religion, and to place the welfare of all youth under executive protection. When this was least regarded at Rome, as under the first Cæsars, all the bonds of society were at once loosened, and the empire subsequently suffered complete dissolution under the blows of those barbaric nations who were sent of God to overthrow an atheistic people, and prepare the way for a diviner faith. It is a sad prudence which, to obtain a few minutes of false peace, would sacrifice the future of faith and the life of society.

Jesus Christ changed neither religion, nor laws, nor duties; but by developing and consummating the primitive law in his own person, and through his disciples, he elevated a religious society into a body politic, the first perfect commonwealth, wherein he designed that all families should ultimately become one family, governed by his own legislation alone, himself their only chief.

LEO X.: OR,

THE AGE OF SCIENTIFIC INVENTION.

PROLOGUE OF MOTTOES.

"The entire succession of men, through the whole course of ages, must be regarded as one man, always living and incessantly learning."—B

"It is hard to find a whole age to imitate, or what century to propose for our example. Some have been far more approvable than others: but virtue and vice, panegyrics and satires, scatteringly to be found in all history, sets down not only things laudable but abominable; things which should never have been, or never have been known. So that noble patterns must be fetched here and there from single persons rather than whole nations, and from whole nations rather than any one."—S

.

"Always with a change of era, there had to be a change of practice and outward relations brought about, if not peaceably, then by violence, for brought about it had to be; there could be no rest come till then. How many eras and epochs not noted at the moment, which, indeed, is the blessedest condition of epochs, that they come quietly, making no proclamation of themselves, and are only visible long after. A Cromwell Rebellion, a French Revolution, striking on the horologe of time, to tell all mortals what a clock it has become, are too expensive, if one could help it."—THOMAS CARLYLE.

"Stand up: I myself also am a man."—ACTS x. 26.

PART THIRD.

LEO X. AGE OF SCIENTIFIC INVENTION.

CHAPTER I. LITERATURE.

THE fall of the western empire was a strange phenomenon. The Roman people did not only abandon the government in its struggles against the barbarous invaders, but when left to themselves, did not attempt any resistance on their own behalf. During the whole protracted conflict, the nation endured all the scourges of war, devastation, and famine, and suffered an entire change in its character and condition, without acting, remonstrating, or even appearing. Their passive submission to inevitable destiny at the great crisis of changeful progress was most complete. We do wrong to regard the middle age as a blank in human history, a useless void between the refinement of antiquity and the freedom of modern times. No vital element of civilization actually died, though all may have fallen into deep sleep, from which they awoke in a wonderful and sublime manner after a thousand years. The substantial portion of antique knowledge and civilization never was forgotten, nor was its better spirit disused, but through subsequent and superior invention has re-appeared in many of the best and noblest productions of modern genius. The fullness of creative fancy characterized the period between the Trojan adventurers and the times of Solon and Pericles, the fountain-head of that variety, originality, and beauty, which marked the unrivaled productions of a later era. What that primary growth was to the richest harvest of Greece, the early centuries of mediæval literature were to all the diversified wealth of modern Europe. The frigid tempestuousness of winter essentially precedes the silent process of vernal vegetation, just as spring must go before the rich maturity of autumnal fruit. When the sources of life were drying up in the immense body of Rome, the fountain of northern energy broke upon the mighty

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