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InterfacingtoExternalDevicesNotesand/orReference6.111October18,2016
HugeAmountofSelf-ContainedDevices
• Sensors• A-to-Dconverters• D-to-A• Memory• Microcontrollers• Etc…
• Weneedability/fluencytoextractinfofromandworkwiththem
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CaseStudy
• 9axisIMU(InertialMeasurementUnit)• Accelerometer• Gyroscope• Magnetometer
• OneoftheonlyrealMEMS(MicroElectroMechanical Systems)applicationsthathasgonefull-scale(othersmightbeTI’sDMD,gyroscopes,microphones,somemicrofluidics,Siresonators,Piezoelectrics fromInkjets,etc…)
Accelerometers
• FirstMEMSaccelerometer:1979• Positionofaproofmassiscapacitively sensedanddecodedtoprovideaccelerationdata
Proof MassSpringSpring
MeasureCapacitanceviaImpedanceDivider𝑎" → Δ𝑑
SEMoftwo-axisaccelerometer
𝑑
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UsesofAccelerationMeasurements:• Accelerationcanbeusedtodetectmotion• (pedometer,dropdetection):
• Usegravityandtrigtofindorientation:
𝜃' = tan,-𝑎"𝑎.
−𝑎"
−𝑎.
𝑔
𝑎1 = 𝑎.2 + 𝑎'2 + 𝑎"2�
Accelerometerdirections+X,+Y,+Z
Chip
Problems• Accelerometershavehugeamountsofhigh-frequencynoise• Tofix,usuallyLowPassFiltertherawsignal• ThiscutsdownonfrequencyresponsethoughL
𝜃'[𝑛] = 𝜃'[𝑛 − 1]𝛽+ 1 − 𝛽 tan,-𝑎"[𝑛 − 1]𝑎.[𝑛 − 1]
0 < 𝛽 < 1
𝜃' Angleestimatearoundyaxis
FilterCoefficient𝑎.𝑎"
Xacceleration
zacceleration
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BringinGyroscopes
• ProvideDirectAngularVelocitywhichwecanintegratetogetangle• Verylittlehigh-frequencynoise,butlotsoflowfrequencynoise (Gyrosdriftlikecrazy)
Gyroreadingsare“around”theaxistheyreferto(useright-hand
rule):time(seconds)
angle(arb.units)
Angularv
elocity
(arb.units)
GyroOperationProof Mass
SpringSpring
MeasureCapacitanceviaImpedanceDivider
Resonating
MeasureCapacitancevia
ImpedanceDivider
RotationofDevice
• ResonatingProofMass• ElectrostaticDrive• PiezoelectricDrive
• Turningout-of-plane:• Proof-massfightsturn• Detectdeviationviacapacitance
• Dothisforallthreeaxes
Changesincapacitancemeasuredatdifferentpoints
Scalenotaccurate/nordesigndetails
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• BecauseofDrift(lowfrequencynoise/offset)youwanttoavoiddoingmuchlong-termintegration• Havingbetalessthanunityensuresanyanglethatcomesfromgyroreadingwilleventuallydisappear,butinshorttermitwilldominate• Dependingontimestep:
HowtouseGyroReadings:
𝜃' 𝑛 = 𝛽𝜃' 𝑛 − 1 + 𝑇𝑔'[𝑛 − 1]
0 < 𝛽 < 1 FilterCoefficient
𝑇 TimeStep
𝑔' Gyroyreading
𝛽 ≈ 0.95 startingpoint
Whattodo?
• Usingonlyaccelerometer,leavesusblindtomotion/changeintheshorttermbutfineinthelong-term
• Usingonlygyroscope,leavesusblindinthelongterm,butgoodintheshortterm
• Whattodo?
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Mergethesignals
• ComplementaryFilter:
• CouldalsodoKalman Filter(LQE)ifdesired(orothers)
𝜃' 𝑛 = 𝛽 𝜃' 𝑛 − 1 + 𝑇𝑔'[𝑛 − 1] + 1 − 𝛽 tan,-𝑎"[𝑛 − 1]𝑎.[𝑛 − 1]
0 < 𝛽 < 1 FilterCoefficient
𝑇 TimeStep
𝑔' Gyroyreading 𝑎.𝑎"
Xacceleration
zacceleration𝛽 ≈ 0.95goodstartingpoint
HowtogetAccesstothesignalsinfirstplace?
• Someaccelerometersareanalogout(canthereforereadthemwithanA-to-Dconverter)(ADXL335,forexample)• Thesehavelimitedfunctionality…andalsoitisanalogsothere’sthewholenoiseissue....whichisnotnice• Mostflavorsofsensorsaredigital
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MPU-9250• 3-axisAccelerometer(16-bitreadings)• 3-axisGyroscope(16-bitreadings)• 3-axisMagneticHallEffectSensor(Compass)(16bitreadings)• SPIorI2Ccommunication(!)…noanalogout• On-chipFilters(programmable)• On-chipprogrammableoffsets• On-chipprogrammablescale!• On-chipsensorfusionpossible(withquaternionoutput)!• Interrupt-out(forlow-powerapplications!)• On-chipsensorfusionandothercalculations(candoorientationmathon-chiporpedometry even)• Socheaptheyusuallyaren’tevencounterfeited!J
Board:$8.00fromEbayChip:$5.00inbulk
CommonDevice-DeviceCommunicationProtocols
• Parallel(notsomuchanymore)
• Serial(UART)(stillcommoninsomecommunicationandGPSdevices)
• SPI(SerialPeripheralInterface)verycommon
• I2C(Inter-IntegratedCircuitCommunication)verycommon
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Serial(UART)• StandsforUniversalAsynchronousReceiverTransmitter• Requiresagreementahead-of-timebetweendevicesregardingthingslikeclockrate(BAUD),etc…• Twowirecommunication• Cannotreallyshare
• (everypairofdevicesneedsownpairoflines)• Dataratereally<115.2Kbps
TX/RX
RX/TX
Device 1 Device 2
SPI
• StandsforSerial-PeripheralInterface• FourWires:
• MOSI:Master-Out-Slave-In• MISO:Master-In-Slave-Out• SCK:Clock• CE/CS(ChipEnableorChipSelect)
• SCKremovesneedtoagreeaheadoftimeondatarate(fromUART)• HighDataRates:(1MHzupto~70MHzclock(bits))
MOSIMISOMaster Slave
SCKCE0/CS0
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SPI
• CanshareMOSI/MISOBus• Additionofmultipleslavesrequiresadditionalselectwires• Hardware/firmwareforSPIisprettyeasytoimplement:• Wiresareuni-directional• Classic“duh”sortofapproachtodigitalcommunication,butveryrobust.
MOSIMISOMaster Slave1
SCKCE0/CS0
Slave2
CE1/CS1
i2C
• StandsforInter-IntegratedCircuitcommunication• Inventedin1980s• TwoWire,OneforClock,onefordata(bothdirections)• Usually100kHzor400kHzclock(newerversionsgoto3.4MHz)
SDA
SCL
Master Slave
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Oni2CMultipleDevicesRequireSame#ofWires
SDA
SCL
Master Slave1
Slave2
• DevicescomewiththeirownIDnumbers(originallya7bitvaluebutmoremodernoneshave10bits)…allowspotentiallyupto2^7devicesor2^10onabus(theoreticallyanyways)
• ID’sarespecifiedatbuild,usuallyseveraltochoosefromandyouselectthembypullingexternalpinsHIorLOW
Moretostory(needpull-upresistors)
4.7kΩ
3.3V
SDA
SCL
Master Slave1
3.3V
4.7kΩ
• i2Cusesanopendrain• MeaningbothMasterandSlaveareeither:• LOW• High-Impedance
• Needexternalpull-upresistors
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Tri-State
• inout cannotbeareg ever,ever…itisclosertoawire...usualwaytoworkwiththemisthefollowing:
inout sda;
reg sda_val;
assign thing = sda_val? 1’bz: 1’b0;
Inverilog…
Asaresult:
Mode Master Slave
MasterTransmit HiZ (HI) orLOW HiZ (listening)
Slave ACK/NACK HiZ (listening) HiZ (HI)orLOW
SlaveTransmit HiZ (listening) HiZ (HI)orLOW
MasterACK/NACK HiZ (HI)orLOW HiZ (listening)
SDAin
VGS
4.7kΩ
3.3V
SDA
inout sda;reg sda_val;assign sda = sda_val? 1’bz: 1’b0;
Wanna writetoSDA?
sda_val <= 0; //or 1 if desired :wq
Wanna readtoSDA?
sda_val <= 1;//wait clock cycle…some_reg <= sda; //read from input
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i2COperation
• DataisconveyedonSDA(EitherfromMasterorSlavedependingonpointduringcommunication)• SCLis50%dutycycle• SDAgenerallychangesonfallingedgeofSCL(isn’trequired)• SDAsampledatrisingedgeofSCL• MasterisinchargeofsettingSCLfrequencyanddrivingit
MeaningsI:(Start,Stop,Sampling)
SCL:
SDA:
MasterClaimsBus(START)BypullingSDALOWwhileSCLisHIIdleState
SDAandSCLsitHI
DatafromSDAsampled@posedge ofSCL
Data/StateonSDAtransitions@negedge ofSCL*
*notspecifiedbutprobablyeasiestspottodo
MasterReleasesBus(STOP)BypullingSDAHIwhileSCLisHI
HI
LO
HI
LO
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MeaningsIIAddress
• FirstthingsentbyMasteris7bitaddress(10bitinmoremoderni2C…hassomeleading11111’sinit..don’t worryaboutthat)
• Ifadeviceonthebuspossessesthataddress,itacknowledges(ACK/NACK=0)anditbecomestheslave
• Allotherdevices(otherthanMaster/Slave)willignoreuntilSTOPsignalappearslateron.
MeaningsIII(Read/WriteBit)
• Aftersendingaddress,aRead/WriteBitisspecifiedbyMasteronSDA:• IfWrite(0)isspecified,thenextbytewillbearegistertowriteto,andfollowingbyteswillbeinformationtowriteintothatregister• IfRead(1)isspecified,theSlavewillstartsendingdataout,withtheMasteracknowledgingaftereverybyte(untilitwantsdatatonotbesentanymore)
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MeaningsIV(ACK/NACK)
• Afterevery8bits,itisthelistener’sjobtoacknowledgeornotacknowledgethedatajustsent(calledanACK/NACK)• TransmitterpullsSDAHIandlistensfornextreading(@posedge ofSCL):• IfLOW,thenreceiveracknowledgesdata• IfremainsHI,noacknowledgement
• Transmitter/Receiveractaccordingly
MeaningsV• ForMastertowritetoSlave:
• START• SendDeviceAddress(withWritebit)• Sendregisteryouwanttowriteto• Senddata…untilyou’resatisfied• STOP
• ForMastertoreadfromSlave:• START• SendDeviceAddress(withWritebit)• Sendregisteryouwanttoreadfrom• ReSTART communication• SendDeviceAddress(WithReadbit)• Readinbits• Afterevery8bits,itisMaster’sjobtoacknowledgeSlave…continuedacknowledgementleadstocontinueddataoutbySlave.
• Not-Acknowledgesays“nomoredatatoSlave”• STOPleadstoMasterceasingallcommunication
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Implementingi2ConFPGAwithMPU9250:• Mademasteri2CcontrollerinVerilog• UsedMPU9250Datasheet:42pages(basicfunctionality,timingrequirements,etc…)• MPU9250RegisterMap:55pages
State-MachineImplementationofi2CMaster
• Continuouslyreads2bytesstartingatthe0x3Bregister(Xaccelerometerdata)• PrintoutvalueinhexinLEDs• 34States• Clockedat200kHz,andcreates100kHzSCL• ChangeSDAonfallingedgeofSCL• SampleSDAonrisingedgeofSCL
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State-MachineImplementationofi2CMaster• Redundantstates(repeatedREAD/WRITE,ADDRESS,ACK/NACK,etc…)
• ARMmanualdescribes~20stateFSM
• Includedcodeonsiteforreference/startingpoint
• Diagram:onnextpageforreference
…200morelines
IDLE START1ADDRESS1
ADDRESS2
READWRITE1
REGISTER1
REGISTER2
ACKNACK1A
ACKNACK1C
IDLE
ACKNACK2AACKNACK2C
IDLE
STOP
READ2
READ1
START2
ADDRESS3
ADDRESS4
READWRITE2
ACKNACK3A
ACKNACK3C
IDLE
READ3
READ4ACK4
NACK
NACK
ACK
NACK
ACK
NACKACK
7x
7x8x
8x
8x
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CommunicationPart
1 0 1 1 0 1 0 0 0 0 0 0 0 1 1 1 0 1 1 0 1 0 0 1 1 0 1 0 0 0 1 0 0 1 1 0 1 1 1 0 0
01010101010101010101010101010101010101110101010101010101010101010101010101010
SDA
SCL
VCC
GND
Nexys4MPU9250
…SCL
SDA
CommunicationPart
1 0 1 1 0 1 0 0 0 0 0 0 0 1 1 1 0 1 1 0 1 0 0 1 1 0 1 0 0 0 1 0 0 1 1 0 1 1 1 0 0
01010101010101010101010101010101010101110101010101010101010101010101010101010
SDA
SCL
VCC
GND
Nexys4 MPU9250
DeviceAddress(0x68)Write=0
Acknowledge=0
DeviceRegister(0x3B)
Acknowledge=0
DeviceAddress(0x68)
Read=1
DataReadInStart
…SCL
SDA
ReStart
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CommunicationPart
1 0 1 1 0 1 0 0 0 0 0 0 0 1 1 1 0 1 1 0 1 0 0 1 1 0 1 0 0 0 1 0 0 1 1 0 1 1 1 0 0
01010101010101010101010101010101010101110101010101010101010101010101010101010
SDA
SCL
VCC
GND
Nexys4 MPU9250
…SCL
SDA
“Iclaimthisbus”
“Hey,0x68…”
“Iwanna tellyousomething”
“I’mhere.Soundsgood”
“Lookatyour0x6Bregister”
“OK”
“Differentthought”
“Hey,0x68…”
“Readtomefromwhereyou’relooking”
“Forsure”
“0x6D”
”More,please”
MPU9250(Slave)DialogNexys4(Master)Dialog
CommunicationinReal-Life: DatabeingsentfromMPU9250
TriggeredonleavingIDLEstate
SCL=Purple
SDA=Yellow
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Running andreadingXacceleration:
16’hFD88=16’b1111_1101_1000_1000(2’scomplement)Flipbitstogetmagnitude:16’b0000_0010_0111_0111=-315Full-scale(default+/- 2g)-315/(2**15)*2g=-0.02gJmakessense
16’h4088=16’b0100_0000_1000_1000(2’scomplement)Leavebitstogetmagnitude:16’b0100_0000_1000_1000=+16520Full-scale(default+/- 2g)-16520/(2**15)*2=+1.01gJmakessense!
Horizontal: Vertical:
HOOKUP
Clock-Stretching(Coolpartofi2C!!!)
SCL:
• NormallyMasterdrivesSCL,butsinceMasterdrivesSCLhighbygoinghiZ,itleavestheoptionopenforSlavetostepinandpreventSCLfromgoinghighbysettingSCLLOW
MasterwantedtopullSCLHIbutslavepreventsbygoingLOW
(redneverhappens)
OnceSlavegoesHiZ again,MasterpicksbackuponSCL
• AllowsSlaveawaytobuytime/slowdownthings(ifitrequiresmultipleclockcyclestoprocessincomingdataand/orgenerateoutput)
😻
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FinalThoughts…WhataboutSPIorSerial?
• Ifyoucanimplementi2C,theothersareeasier.• SPIisalsoalittlelessstandardized• Generallywithcommunicationprotocols,themorewires,theeasiertheprotocol/lessoverhead• SPI(fourwires)• SerialTX/RX(littlebitmorecomplicated,butnottoobad)
• Checkouttheexamplei2Ccodefromthislecture…seeifyoucanaddclock-stretching!(notrequired)
