Fast Prototyping 24-Bit Sigma-Delta ADC With Energia Ecosystem

34 Pages·2015·2.98 MB·English

by  Texas Instruments, Incorporated

TI Designs
Fast Prototyping 24-Bit Sigma-Delta ADC With Energia
Ecosystem
TIDesigns DesignFeatures
ThisTI DesignshowcasesaSigma-DeltaADC • EnergiaControlofADS1220
BoosterPack™forTILaunchPad™andhowto
• EnergiaLibrariestoConvertData
proceedtoafastprototypingevaluationwithEnergia
• EnergiaLibraryfor ADCCodetoConvert
leadinganalogperformancesevaluationwithin
Temperature
minutes.
• 24-Bit ΔΣADC:ADS1220
DesignResources
• PhysicalFormFactorinBoosterPackfor
CompatibilityWithallTILaunchPadDevices
TIDA-00647 DesignFolder
• Scalable FromGenerictoSpecificEvaluation
ADS1220 ProductFolder
WithinMinutes
TIDA-00648 ToolsFolder
TIDA-00339 ToolsFolder FeaturedApplications
• FactoryAutomationAndProcessControl
• SensorsAndFieldTransmitters
ASKOurE2EExperts
Booster-Pack side
Sinepnustor ADS1220
t
Generic Sigma-Delta fast prototying
LaunchPad <-> Booster Pack Connectors
Debug
USB to PC StateMachine
FR5969t
Control and debug side
LaunchPad side
AnIMPORTANTNOTICEattheendofthisTIreferencedesignaddressesauthorizeduse,intellectualpropertymattersandother
importantdisclaimersandinformation.
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SystemOverview www.ti.com
1 System Overview
1.1 System Introduction
Forsignalswithrelativelylowbandwidth,upto100kHz,Delta-Sigmaconvertersofferhigher
performancesthanothertopologies[1][2].ThisTIDesignleveragesEnergiaanditsrapidprototyping
capabilitiestoevaluatehardware,suchasahigh-performanceADC,withinminutes.
Table1.DesignSolutionstoIndustryProblems
INDUSTRYCHALLENGE TIDA-00468SOLUTION
Reducingtimetomarketwindows Fastprototypingsolutionforlowpower24-bitSDADC
Showcasehowtousetheplatformforuniversaltemperature
Beclosetoendapplicationforrelevantevaluation
inputevaluation
1.2 Key System Specifications
Table2.KeySystemSpecifications
PARAMETER DETAILS
2-wireRTDinput Section7.1
3-wireRTDinput Section7.2
4-wireRTDinput Section7.3
Thermocoupleinput Section7.4
Universaltemperatureinput Section7.5
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2 System Design Theory
2.1 Delta-Sigma
TheDSconverterismadeofthreemainblocks:theDSmodulator, thedigitalfilter,anddecimation.The
internalDSmodulatorshowninFigure1 coarselysamplestheinput signalataveryhighrateinto a1-bit
stream.Thedigitalanddecimationfiltersthentakethissampleddataandconvert itintoahigh-resolution,
slowerdigitalcode.Whilemostconvertershaveonesamplerate,theDSconverterhastwo—theinput
samplingrate(f )andtheoutputdatarate(f ).
S D
Sample Rate(fS)
Analog ∆Σ
Input Modulator
fS/fD=Decimation Ratio
Data Rate(fD)
Digital Digital
Decimator
Filter Output
Digital/Decimation Filter
Figure1.BlockDiagramofDSADC
2.1.1 ΔΣModulator
Themodulatorcanbemodeledeitherinatimedomain(Figure2)orinafrequencydomain.
Inthetimedomain,theinputsignal(xi)andtheoutput ofa1-bitDAC(x4)aredifferentiatedtogenerate an
intermediateanalogvoltage(x2),whichisintegratedintoanotheranalogvalue(x3) andcomparedto a
thresholdvalue.Theoutputvalue(x4)isclockedoutoftheADCaswellastotheDACfor differentiation
againstthe inputvalue(keepinginmindthat theinput valueisconsideredalmostconstant against
changesofx4).
After acertaintime,thevaluex3reachesthereferenceandx4changessign, asdoesx2andthe slopeof
x3.SuchamodulationschemeprovesthequantizationnoiseisasdescribedinFigure2.Higherorderof
modulationswithbetternoiseshapingperformancescanbedesigned, butthesedesignsusuallycome at
theexpenseoftheresponse.
Difference Integrator Comparator Latch
Analog + (cid:139) D Q Bitstream
in – out
1-bit DAC
Clock A
D
Figure2.BlockDiagramofFirst-OrderSigma-DeltainTimeDomain
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e
Sigma i
Delta (Integrator)
x + 1-Sample 1-Bit y
i Delay ADC i
Analog – Output to
Input Digital Filter
Magnitude Signal
1-Bit
DAC
Frequency
f
S
Quantization
Noise
Figure3.First-OrderDSModulatorinFrequencyDomain
2.1.2 DigitalFilter
ThedigitalfiltermentionedinFigure1 canbeimplementedasshownonFigure4;alternatively, more
advancedfilterssuchassyncfilterscanbeusedaswelldependingonthedesiredoutcome.
Themainfunctionofthefilteristoremovethequantizationnoiseinthehigher frequenciespresentat the
outputofthe1-bitADC(seeFigure4).Sincethenoiseisnotflat,removingthehigher frequencycontent,
willincreasethesignaltonoiseratio.
Input Delay Delay Delay
b b b b
1 2 3 i
Σ Σ Σ
Output
Figure4.First-Order,Low-PassAveragingFilter
2.1.3 Decimator
Thedecimatorrolemostlyprovidesamanageabledataoutputrate.
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2.1.4 Summary
Figure5.OverviewofSignalsatDifferentStagesofSigma-Delta
2.2 TI Rapid Prototyping Environment
TheTI rapidprototypingenvironmentismadeofthefollowingthreekeycomponents:
• Energia:thegraphicaluserinterfacerunningonallmainoperatingsystems
• Sketch:thenamethatEnergiausesfor aprogram;itistheunit ofcodethat isuploadedtoand runson
aLaunchPadboard
• LaunchPad:theboardonwhichresidesthemicrocontrollersfromTexasInstruments
• BoosterPack:theadd-onboardsconnectedtotheLaunchPad, extendingthefunctionalitiesof theMCU
(andexercisedthankstoasketch)
CAUTION
While simplifying the design, this rapid prototyping environment does not
compromise on performances: the program running on the LaunchPad has
been processed by Energia then compiled using the standard C++ compiler
beforebeingloaded.
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2.2.1 Energia
Energiaisanopen-sourceintegrateddevelopmentenvironment(IDE)for rapidprototypingbringing the
wiringframework(theframeworkunderlyingotherrapidprototypingenvironment)totheTexasInstruments
LaunchPadevaluationkit.
Thewiringframeworkisanopen-sourceprogrammingframeworkfor microcontrollers.Thisframework
allowscross-platformsoftwaretocontroldevicesattachedtoawiderangeofmicrocontrollers.
Thewiring-basedEnergiaIDEisanapplicationwritteninJava,derivedfromtheIDEmadefor the
processingprogramminglanguage(forinstance,thename"sketch"isderivedfromtheprocessingIDE).
TheEnergiaIDEcomeswithaC/C++librarycalled"Energia", whichmakescommoninput/output
operationsmucheasier.
EnergiaprogramsarewritteninC/C++, thoughmost ofthecomplexityofC/C++codeishiddenbythe
EnergiaIDE.Forinstance,onlytwofunctionsneedtobedefinedfor aprogram torun(thoughmore
functionscanbedefined):
• Setup():afunctionthatrunsonceatthestartoftheprogram,whichcanbeusedtodefineinitial
environmentsettings
• Loop():afunctioncalledrepeatedlyuntiltheboardispoweredoff
2.2.2 LaunchPadEcosystem
LaunchPadsaremicrocontrollerdevelopmentkitsfromTexasInstrumentsthat focuseasyevaluations:
• SimpleUSBinterfacetoPC(nomoreneedfor additionaldebuggertoconnecttotheJTAGportasthe
logicisalreadyontheLaunchPadboard)
• Standardizedinterfaceforextensionboards(calledBoosterPacks)
LaunchPadscomplytotheelectricalinterfacespecificationsshowninFigure6,whichisavailable asa
PDF here:http://www.ti.com/ww/en/launchpad/dl/boosterpack-pinout-v2.pdf
All through-holes on 100-mil grid
1800 mil
3.3 V 1 1600 mil 20 GND
Analog In PWM Out GPIO (!)
UARTRX( MCU) SPI CSWireless GPIO (!)
TX( MCU) Emulator GPIO**
GPIO (!) RST
Analog In MOSI
SPI
SPI CLK MISO
GPIO (!) SPI CS Display GPIO (!)
I2C * SCL SPI CS Other GPIO (!)
SDA 10 11 GPIO (!)
20-pin MCU
5 V 21 40 PWM Out GPIO (!)
GND PWM Out GPIO (!)
Analog In LaunchPad PWM Out GPIO (!)
Analog In PWM Out GPIO (!)
Analog In Timer Capture GPIO (!)
Analog In Timer Capture GPIO (!)
Analog In WS 200 mil 200 mil GPIO (!)
Analog In SCLK GND GND GPIO (!)
I2S
Analog Out SDout GND GND GPIO (!)
Analog Out SDin 30 5 V 3.3 V 31 GPIO (!)
40-pin
Figure6.ElectricalInterfaceSpecifications
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2.2.3 BoosterPackEcosystem
BoosterPacksareplug-inmodulesthatfitontopofLaunchPads.Theseinnovativetoolsplugintoa
consistentandstandardizedconnectorontheLaunchPadandallowdeveloperstoexploredifferent
applicationsenabledbyanyTImicrocontroller.
BoosterPacksareavailablefromTexasInstruments,fromthirdparties,andfromthecommunity.
BoosterPacksincludefunctionssuchascapacitivetouch,wirelesscommunication,sensorreadings,LED
lightingcontrol,andmore.BoosterPacksareavailablein20-and40-pinvariants,andmultiple
BoosterPackscanplugintoaLaunchPadtoenhancethefunctionalityofadesign.
2.2.4 LaunchPad,BoosterPack,andEnergiaAbstraction
BoosterPackscanbemechanicallyandelectricallyconnectedasdescribedinFigure7 andFigure8.
BoosterPack 3
Sketch.ino
BoosterPack 3
BoosterPack 1
LaunchPad
Figure7.MechanicalandElectricalConnections Figure8.SimpleUSBConnectionFromLaunchPadto
BetweenLaunchPadandBoosterPack PC
TheEnergīaframeworkbindstheentirerapidprototypingenvironmenttogether:
• Editingandsavingthesourcecode(calledsketch)
• Compilingandlinkingthesourcetobinaries
• LoadingthebinariesoverUSBtothetarget (LaunchPad)
• Displayingthemessages(serialportoverUSB)fromtheLaunchPad
2.2.5 IDE
TheIDE providesasimplifieddevelopmentwindowthat isidealfor engineerswhoarenotfamiliarwith
softwaredevelopmenttoolssuchasCodeComposerStudio™(CCS). Engineers(shouldtheybe
hardwareengineers,systemarchitects,orsoon)canthendevelopthelogicinsmallfilescalleda
"sketch", whicharethenloadedontheLaunchPadfor execution.
Energīaprovideslotofthemorebasicandcorefunctionalitiesneededfor introductoryapplicationsand
projects. Onceproofofconceptapplicationsarerealized, developmentwilllikelyleverageCCS,afull-
blownprofessionaldevelopmentenvironment that introducesmorecapabilitiesandfunctionalitysuch as
fulldebug.
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2.2.6 CompilingandLoading
Figure9 summarizeswhatEnergīadoesinthebackgroundbeforeloadingthecompiledcodeonthe
LaunchPad.
9
When you press (verify) in Energia, this is what happens in the
background
r
o
s
es er
oc pil
r m
p
- o
e c
pr C er
a C bl
Sketch.ino gi G m Sketch.out
er 0 se er
n 3 s k
E 4 a n
SInk Eetncehr gfiilae ItDypEed MSP TI TI li Btooni nbtoae r Lyloa fauiledn ecrdhe Padayd
x
EnergflD libraries
Figure9.EnergiaBuildProcess
Morespecifically,thefollowingeventsmust happenfor theEnergīacodetogetontotheLaunchPad
board:
First,theEnergīaenvironmentperformssomesmalltransformationstomakesurethecodeiscorrectinC
orC++(twocommonprogramminglanguages)includingthe"Energialibraries"(#include“Energia.h”),
whichcontainallthedefinitionsneededfor thestandardEnergīacore.If asketchcontainsmultiple files
(tabs),alltabswithoutanextensionareconcatenatedtogether toformamainone.Tabswith.c,.cpp,or
.hwillbecompiledseparatelywhileotherswillberenamed.ThenEnergīasearchesfor functiondefinitions
andcreatesprototypesforthem.ThenEnergīaaddsthetarget main.cxxfiletothebottom ofthe sketch,
whichincludessomeofthetargetspecificsettings(apart ofwhat allowstherapidprototypingasthecode
canstart fromaknowngoodsetup).
Theobjectfilesthengetpassedtoacompiler(MSPGCC), whichturnsthehumanreadablecodeinto
machinereadableinstructions(orobjectfiles).Then, thecodegetscombinedwith(linkedagainst)the
standardEnergīalibrariesthatprovidebasicfunctionslikedigitalWrite()orSerial.print().Theresult isa
singleIntelhexfile,whichcontainsthespecificbytesthat needtobewrittentotheprogram memoryof the
chiponthe LaunchPad.
The.cxxfilecanthenbeuploadedtotheboard.It istransmittedovertheUSBorserialconnection
throughthebootloaderalreadyonthechiporwithexternalprogramminghardwareduetothemspflasher.
2.2.7 Running
Oncethefirmwarehasbeenuploadedandstoredinthenon-volatilememory(NVM),whichcanbeFlash
memoryorFRAMmemory,thedebuglogicontheLaunchPadwillissueahardwarereset,whichwillthen
launchthe newlyprogrammedfirmware.
2.2.8 Debugging
WithEnergīa,thedefaultdebugconsistsmostlyinUARTtracesgoingovertheUSBintotheterminalof
theEnergīaGUI.
Formoreadvancedfeatures,itisrecommendedtoimportsketchesintoCCS(seeSection2.2.9).
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2.2.9 ImportingtoCCSv6
EnergīasketchescanbeimportedtoCCS.
Importingsketchesallowsuserstoreuseandscaletheworkdoneduringtheearlyprototypingstagesby
thesoftwareteams.
Formoredetails,seetheEnergiasite [8] orSection11wherethetutorialwascapturedfor future.The
onlycodechangeistoincludetheEnergialibrarytobeabletoleveragetheEnergiaAPI.
2.2.10 ScalabilityandReuse
SketchesaswellasthebinariesgeneratedwithEnergiacanbeloadedonboardsotherthanLaunchPads
withCCSinconjunctionwithanemulator.
TheonlyconstraintisthattheallocationofpinsremainsthesamethanontheoriginalLaunchPadon
whichthecodewasinitiallydevelopedandvalidated.
Beyondbinaryreuse,sourcecodecanalsobereusedandintegratedinsoftwareprojectstoallowreuseof
thefunctionalitiesvalidatedbytheprototypingteambythesoftwareteam.
2.2.10.1 AccessingMapFileforMemoryFootprintEvaluation
Anotheraspectwhichcanbeofinterestwhentransitioningthecodefromafast prototypingenvironment
toasoftwareproductionenvironmentisrelatedtothememoryfootprint usedbytheframework.While the
toolsdonotkeeptheactual .mapfile,itispossibletore-runthetoolchainandbyaddingtherelevant
flagstokeepthemapfile.inthelogwindowduringthecompile, findthefollowingoutput(oftenline8and
9fromthebottom):
<Energia install folder>hardware\tools\msp430\bin\msp430-gcc -Os -Wl,-gc-sections,-
u,main -mmcu=msp430f5529 -o
<User folder>\AppData\Local\Temp\build2484947497227615311.tmp\sketch_aug18a.cpp.elf
<User folder>\AppData\Local\Temp\build2484947497227615311.tmp\sketch_aug18a.cpp.o
<User folder>\AppData\Local\Temp\build2484947497227615311.tmp\core.a -L
<User folder>\AppData\Local\Temp\build2484947497227615311.tmp -lm
Takethisandadd:-Wl,-Map "x.map"
orrunintheoutputfolder(forexample, <User
folder>\AppData\Local\Temp\build2484947497227615311.tmp):
<Energia install folder>\hardware\tools\msp430\bin\msp430-size.exe -t
sketch_aug18a.cpp.elf
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3 System Description
Thisdesignfocusesonbringingafastprototypingsolutionfor lowpower ΣΔ ADCtotheEnergia,
LaunchPad,orBoosterPackecosystem.
ItconsistsofanADS1220mountedonaBoosterPackmechanicalandelectricalboardasdescribedin
Figure10:
Booster-Pack side
Sensor
ADS1220
input
t
Generic Sigma-Delta fast prototying
LaunchPad <-> Booster Pack Connectors
Debug
USB to PC StateMachine
t
FR5969
Control and debug side
LaunchPad side
Figure10.BlockDiagram
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