linux python interfaces · – familiarizes with linux & python • means pi can run very...

RaspberryPiandInterfacing

Linux

PythonInterfaces

ThePoint

•  Experimentso;enmeanmeasuringandrecordingdata–  sense–  digi<ze–  communicate

–  automate–  store–  analyze–  publish–  fameandglory?

Lecture4:Pi/Python/Interface UCSDPhys122 2

FocusonAccessible

•  Oceansofpossibili<esfordataacquisi<on/interface•  RaspberryPiis:

–  cheap(youcanhaveyourown)–  cheap(so;wareisfree)–  cheap(low-costaccoutrements,likeADC)

•  OtherRPibenefits:–  familiarizeswithLinux&Python

•  meansPicanrunveryadvanced/sophis<catedcode,ifneeded

–  supportsloadsofmoderninterfaces•  I2C,SPI,serial,GPIO,USB

–  canplay“nice”withresearch-gradeinterfaces•  telnet,ssh,othernetworkinterfaces


Linux(Unix)Environment

•  Command-lineinterface(terminalsession)•  Willwanttofindandworkthroughtutorials•  Essen<alcommands:

–  cd(andmeaningof.,..),mkdir,ls(andls -l),cp,rm,mv,pwd,viornano,less,head,tail,cat,grep, wc(wordcount),|(pipe),>(stuffintofile),<(sourcefromfile),chmod,passwd,exit,etc.

–  familiarizeyourselfwithatleastthese(andassociatedarguments/flags)

–  use“man”(manual)pagesfordetails:•  man mkdir

•  MaccomputershaveUnixfounda<on,soprevalentOS


RaspberryPiAccess

•  Pi4unitsinlab;oneperbench;“headless”•  Accessviasshorpuayonlabmachines•  hostname:bench1,bench2,etc.•  username:bench1,bench2,etc.(matchesunit/bench)

•  password:bench1,bench2,etc.–  temporary:suggestchanginga;eryou&partnerestablishyourbench(share/decidewithpartner)

–  command:passwd


PythonLanguage

•  PrevalentinPhysics/Astro•  Interpreted(slowerthancompiled)•  Easysyntax(highlevel,readable)•  Excep<onallygoodatstringparsing/handling•  Librariesprovidepowerfulfunc<onality

–  numpy:mathonvectors/arrays–  scipy:specialfunc<ons,op<miza<on–  matplotlib(pylab):plocng,alaMatLab–  boatloadsofothers(manyincludedinstandardinstalla<on:math,sys,os,<me,re,asastart)


PythonTutorials

•  Findingyourownresources,learnhowto:–  runinterac<velytoexploresyntax;usedir()andhelp()–  uselists,tuples,dic<onaries;listcomprehension–  performmath:import math;dir(math)–  create/invoke/runprogram(nextslide)–  controlflow:if/else;for/do/while–  formatprintstatements:%s,%d,%5.2f,etc.–  usecommandlinearguments:float(sys.argv[3]),e.g.–  readfromfile:open();for line in file_handle;close()–  writetofile:file_handle.write(formaaed_string)

•  Example:Google:pythonlistcomprehensiontutorial


ExamplePythonCrea<on/Execu<on

$ mkdir sandbox (create place to mess around)$ cd sandbox (navigate into directory)$ vi test.py (or edit using nano, emacs, etc)#!/usr/bin/env python #top line of file; invoke Pythonimport sys #so we can use command line arg.name = sys.argv[1] #not checking to verify exist.print “Hello, %s” % name #formats personalized output(save and quit)$ chmod +x test.py (do once: make file executable)$ ./test.py Tom (run with ./ and incl. argument)Hello, Tom (output)$ (prompt)


Interfaces

•  Amovingtarget,astechnologychanges–  serial(RS-232),USB,I2C,SPIarecommon

•  RaspberryPidoesthese,plusGPIO(Gen.Purp.Input/Output)–  GPIB,CAMAC,VME/VXI,PCIcards(DAQ)forlabenviron.


SerialCommunica<ons•  MostPCshaveaDB9maleplug

forRS-232serialasynchronouscommunica<ons–  we’llgettothesedefini<ons

later–  o;enCOM1onaPC

•  Inmostcases,itissufficienttousea2-or3-wireconnec<on–  ground(pin5)andeitheror

bothreceiveandtransmit(pins2and3)

•  Othercontrolsavailable,butseldomused

•  Datatransmiaedonebitata<me,withprotocolsestablishinghowonerepresentsdata

•  Slow-ish(mostcommonis9600bits/sec)


Lecture 4: Pi/Python/Interface UCSD Phys 122 11

TimeIsoftheEssence•  Ifprovidedseparateclockanddata,thetransmiaergivesthereceiver<ming

ononesignal,anddataonanother•  Requirestwosignals(clockanddata):canbeexpensive(butI2C,SPIdoesthis)

•  Datavaluesarearbitrary(norestric<ons)

•  Asdistanceand/orspeedincrease,clock/dataskewdestroys<ming

time

sample times centered in data bits

data

cl

ock sample on

rising edge of clock

slidecourtesyE.Michelsen


NoClock:DoYouKnowWhereYourDataIs?

interpolated sample times (bit centers)

transitions locate data

time

data



Asynchronous:UpCloseandPersonal•  Asynchronous

–  technicaltermmeaning“wheneverIfeellikeit”

•  Startbitisalways0.Stopbitisalways1.

•  Theline“idles”betweenbytesinthe“1”state.

•  Thisguaranteesa1to0transi<onatthestartofeverybyte•  A;ertheleadingedgeofthestartbit,ifyouknowthedatarate,you

canfindallthebitsinthebyte

interpolated sample times (bit centers)

transition locates data

time

star

t 1

0

bit 0

bit 1

bit 2

bit 3

bit 4

bit 5

bit 6

bit 7

stop

idle idle one byte



CanWeTalk?

•  Ifweagreeon4asynchronouscommunica<onparameters:–  Datarate:Speedatwhichbitsaresent,inbitsperseconds(bps)–  Numberofdatabits:databitsineachbyte;usually8

•  oldstuffo;enused7–  Parity:Anerrordetec<ngmethod:None,Even,Odd,Mark,Space

–  Stopbits:numberofstopbitsoneachbyte;usually1.•  Rarely2or(morerarely)1.5:justaminimumwait<me:canbeindefinite

star

t

bit 0

bit 1

bit 2

bit 3

bit 4

bit 5

bit 6

parit

y

stop

1

idle idle 9600, 7E2

star

t

bit 0

bit 1

bit 2

bit 3

bit 4

bit 5

bit 6

bit 7

stop

idle idle ASCII “A” = 0x41

9600, 8N1

1 bit @ 9600 bps = 1/9600th sec

stop

2


Note:LSBsentfirst


RS-232:mostcommonimplementa<on•  RS-232isanelectrical(physical)specifica<onforcommunica<on

–  idle,or“mark”stateislogic1;•  -5to−15V(usuallyabout−12V)ontransmit•  -3to−25Vonreceive

–  “space”stateislogic0;•  +5to+15V(usually~12V)ontransmit•  +3to+25Vonreceive

–  thedeadzoneisfrom−3Vto+3V(indeterminatestate)•  Usuallyusedinasynchronousmode,definedbyparametersonprev.slide

–  soidlesat−12;startjumpsto+12;stopbitat−12–  sinceeachpacketisframedbystart/stopbits,guaranteedatransi<onatstart–  parity(ifused)worksasfollows:

•  evenparityguaranteesanevennumberofonesinthetrain•  oddparityguaranteesanoddnumberofonesinthetrain

•  UART:UniversalAsynchronousReceiver/Transmiaer–  commonterm/labelforaserialinterface


GPIB(IEEE-488)

•  An8-bitparallelbusallowingupto15devicesconnectedtothesamecomputerport–  addressingofeachmachine(eitherviamenuordip-switches)

determineswho’swho–  candaisy-chainconnectors,eachcable2morlessinlength

•  Extensivehandshakingcontrolsthebus–  computercontrolswhocantalkandwhocanlisten

•  Manytest-and-measurementdevicesequippedwithGPIB–  commonmeansofcontrollinganexperiment:posi<oning

detectors,measuringorsecngvoltages/currents,etc.•  Canbereasonablyfast(1Mbit/sec)


DataAcquisi<on•  APCI-cardfordataacquisi<onisa

veryhandything

•  Theonepicturedatright(Na<onalInstrumentsPCI-6031E)has:–  64analoginputs,16bit–  2DACs,16bitanalogoutputs–  8digitalinput/output–  100,000samplespersecond–  on-board<mers,counters

•  Breakoutbox/boardrecommended

RPiInterface

•  40-pinheaderonsideofRPi

•  serialisorange(UART)•  I2Cislightblue•  SPIispurple•  GPIOisgreen

–  andcanalsouseanypinlabeledGPIOxx


SPI:SerialPeripheralInterface

•  4lines(plusgroundreference,asalways)–  clock(CLK)–  data“in”(MISO:masterin,slaveout)

–  data“out”(MOSI:masterout,slavein)–  chipenable(CE#_N:usuallyac<velow)

•  RPihastwoCElines•  some<mescalledchipselect(CS)orslaveselect(SS)

•  SynchronousForm

•  Namingresolvesambiguityaboutdatadirec<on–  TX/RXalwaysconfusing:accordingtowhichdevice?


SPIScheme

Lecture4:Pi/Python/Interface UCSDPhys122 20fromsparkfun.com

Mul<pleDevices


fromsparkfun.com

DeviceonlylistenswhenitsCE/CS/SSlineispulledlow

AlsoPossibletoDaisyChain


Eachdevicepassesmessageontonext;commonforLEDstrings

fromsparkfun.com

ExamplefromLTC2141(ADC)datasheet


Notes: MSBfirst;MOSI=SDI(slavedatain);MISO=SDO(slavedataout) looksatSDI(MOSI)orSDO(MISO)onupwardclocktransi<on R/Whighmeansread;low(notebar)meanswrite firstwriteaddress,theneitherreadorwritedata chipenableassertedlowforwholeexchange

ExampleRegisteronLTC2141


Tosetregister4toABPand2’scomp.,wouldwrite0x04,0x05overSPI


Aquicknoteonhexadecimal


Hexadecimal,con<nued

•  Onceitiseasyforyoutorecognizefourbitsata<me,8bitsistrivial:–  01100001is0x61–  10011111is0x9f

•  CanbehandybecausetheASCIIcodeisbuiltaroundhex:–  ‘A’is0x41,‘B’is0x42,…,‘Z’is0x5a–  ‘a’is0x61,‘b’is0x62,…,‘z’is0x7a–  ‘^A’(control-A)is0x01,‘^B’is0x02,‘^Z’is0x1A–  ‘0’is0x30,‘9’is0x39

CorePythonSPICode


import spidev # module with SPI cmds

spi = spidev.SpiDev() # instantiate devicespi.open(0,0) # selects CE0spi.max_speed_hz = 122000 # 122 kHz*

def readRegister(regAddr):address = 0x80 | regAddr # sets read bitresp = spi.xfer2([address,0x00]) # xfer2 keeps CE lowreturn resp[1] # result is in second byte

def writeRegister(regAddr,data):spi.xfer2([regAddr,data]) # simply write (write bit low)

writeRegister(0x04,0x05) # sets register 4 to 0x05result = readTegister(0x04) # if want to confirm reg. 4 setting

*op<onsforspeedare:7629,15200,30500,61000,122000,244000,488000,976000,1953000,3900000,7800000,15600000,31200000,62500000,125000000

I2C:Inter-IntegratedCircuit•  PronouncedI-squared-CorI-two-C•  Twosignallines(plusground):

–  clock(SCL)–  data(SDA;bi-direc<onal)

–  StartswhenSDApulledlowwhileSCLs<llhigh–  stoPswhenSDApulledhighwhileSCLrestoredtohigh–  dataread/validwhileSCLhigh(updatedwhenSCLlow)–  datalinecancontainread/writeandacknowledgebits


ARealExampleforLab3:ADS1015

•  TexasInstr.ADS1015–  12-bitADC,4channels–  VDD2.0to5.5V

–  I2CInterface•  DeviceaddressdependsonwhatADDRconnectsto:


ADDRPinto: FullAddress(7bit)

GND 1001000

VDD 1001001

SDA 1001010

SCL 1001011


• CanconfigureinputsvariouswaysusingMUX(closetwoswitches)• Variablegain(range)viaPGA(programmablegainamplifier)• I2Cforinterface• Op<onalcomparatorac<ontocontrolALERTpin


Fourframes(bytesplusR/Wandacknowledge):targetaddress;registertoaccess;thentwobytesofdata

Notes: firstframeinstructswhetherreadorwrite(herewrite) ACKpulledlowmeansdeviceconfirmscommunica<on MSBfirst,LSBlast

couldbe“runt”


Firstwriteaddressregister(2frames);Thenre-addressasread,andread2bytes

MSBfirst;ACKpulledlowifconfirmedcomm.

couldbe“runt”

RegisterMapping

•  We’lljustcareaboutfirsttworegisters(00and01)

•  12-bitconversionregister(00)arrangedin2bytesas:–  D11D10D9D8D7D6D5D4andD3D2D1D00000

•  Configura<onregisterispreaybusy…


Configura<onRegister

•  ADS1015datasheettakes2pagestodetailop<ons–  controlsOpera<ngState(e.g.,startconversion)– MUX:4single-endedor2differen<almeasurements–  setsvoltagerangeforconversion(Prog.GainAmplifier)–  singleshotorcon<nuousMODE–  DataRate(ifcon<nuoussampling)–  COMParatoropera<onforcontrollingALERTopera<on


ExamplePython


import smbus # module for i2c

i2cbus = smbus.SMBus(1) # instantiate: can name whatever

ADDR = 0x48 # default 1001000 if ADDR->GND

# write to config register (1) default valuesi2cbus.write_i2c_block_data(ADDR,1,[0x85,0x83])

# read from conversion register (0) 2 bytes and combinedata = i2cbus.read_i2c_block_data(ADDR,0,2)val_twos_comp = (data[0] << 4) + ((data[1] & 0xf0) >> 4)

Resultwillbesingledifferen<alconversionofA0minusA1in±2.048Vrange

Alltheworkisinfiguringouthowtomanipulatetheconfigregistertogettheresultsyouwant(insinglemode,eachconversionneedsaconfigurecommand)

RefertoADS1015datasheetforfulldetailsonregisterconfigura<onop<ons

Resultisin2’scomplement

•  Binaryrepresenta<onforsignedintegers–  makesbinarymatheasy/natural(singlesetofrules)

•  Posi<venumberslook“normal”–  00000000=0;00000001=1;01001101=77

•  Nega<venumbershavetheMSB“lit”,thenotherbitsinverted,thenadd1–  Ex:−3;startwith00000011;MSB!1andinvertothers(11111100),thenadd1:11111101

–  now−3addedto+3inbinarywillgive100000000(zeroifignoringoverflowbit)


Recovering2’scomplementvalue

•  Mustspecifynumberofbitsinrepresenta<on–  inpreviousslide,used8;forADS1015,it’s12

•  TheifstatementchecksMSB–  <<isle;-shi;bysome#ofplaces;&isbit-wiseANDopera<on

•  Example:00010110<<2becomes01011000•  Example:01101101&10101010becomes00101000(only1ifbothbits1)

•  WhenMSBislit(notequalzero)–  subtractoff100000000(in8-bitexample)

•  Our−3example:11111101isliterally253inunsignedbinary–  subtract256(100000000)andle;with-3

•  Perhapsyouseethe“complement”aspect–  the“other”partof2N,oncethenega<vepartisremoved


def twos(val,bits): # bits in represent. if (val & (1 << (bits - 1))) != 0: # check if MSB=1 val = val - (1 << bits) # subtract 2^bits return val

Applica<onforLab3

•  We’llreadmul<pletemperaturesensors–  RTDs(resis<vetemperaturedevices)–  signalcondi<oning(turnresistanceintovoltage)–  analog-to-digitalconversion(ADS1015)–  interfacetoRaspberryPi–  programmingPythontocollectandstoredata



Temperaturemeasurement•  Avarietyofwaystomeasuretemperature

–  thermistor–  RTD(Resis<veTemperatureDevice)–  AD-590(currentpropor<onaltotemperature)–  thermocouple

•  BoththethermistorandRTDareresis<vedevices–  thermistornotcalibrated,nonlinear,cheap,sensi<ve–  pla<numRTDsaccurate,calibrated,expensive

•  We’llusepla<numRTDsforthispurpose–  small:veryshort<meconstant–  accurate;noneedtocalibrate–  canmeasurewithsimpleohm-meter–  R=1000.0+3.85×(T−0°C)

•  so20°Cwouldread1077.0Ω•  “tempco”of0.385%per°C(3.85Ω/°C)

Problem:MeasuringResistance

•  TheADC(ADS1015)readsavoltage,notaresistance•  HowcanwemeasurearesistanceusingtheADC?

–  howdowedoitright/well?–  whatissuesmightarise?


CurrentSource

•  Providestable1.00mAtoRTD,so1.00kΩ!1.00V–  afinerangeformeasuringusingADC–  if5Vrange,getapprox.1mVresolu<onat12bits

•  1mVisat1mAcorrespondsto1ΩchangeinRTD

•  translatestoabout0.25degrees,andnotlimi<ngfactor•  RTDcalibra<on,andsubtlegradientstendtobelargererrors


Implementa<on

•  LM334currentsource–  resistorsconfigurecurrentoutput

•  datasheetFigs13&15–  diodeperformstemperaturecompensa<on(holdclosetoLM334)socurrentsteadyasambienttemperaturechanges

–  RTDaaachedinseriesandvoltagemeasurementattopendgoestoADC


InnerWorkingsoftheLM334

•  VRheldto~64mV–  acrossRSETgivesISET–  stronglineartemp.dep.

–  214µV×T(K)


MeanwhileISET/IBIASRa<oWell-Behaved

•  At1mA,ara<oof~17

•  Resultofmathisthat:–  ISET=VR/RSET×n/(n−1)–  nisra<o–  VRis214µV×T(K)

•  about64mVatroomT

–  ISET=227µV×T(K)/RSET–  sotoget1mAat300K:

•  RSETwantstobe68Ω


DiodeCompensa<on

•  The“tempco”oftheLM334is0.227mV/C–  0.33%perdegree;RTDis0.385%perdegree–  samesign,soalmostdoublesdV/dTofambientrise

•  Typicaldiodeshaveatempcoaboutten<meshigher,andoppositesign(−2.5mV/C)

•  Theresistorra<oisroughly10×toeffectcompensa<on–  seedatasheetforassociatedcalcula<ons

•  Reliesonsimilartemperatureforbothcomponents–  thereforegoodtoputclosetogether,touching,evenencase


Lab3Flow

•  LogontoPi;resetgroup/benchpassword•  MessaroundwithLinux/Unix•  MessaroundwithPython•  EstablishI2Ccommunica<ontoADS1015

–  includingoscilloscopeverifica<on•  BuildbreadboardRTDcurrentsource•  MakeprogramtocollectRTDdata

•  Expandtomul<pleRTDchannels–  canbreadboardorusepre-builtmodules


Announcements

•  IfnoUnix/Linuxfamiliarity–  encouragedtolookatLab3beforeWed.–  findtutorials,andexploreessen<alcommandslistedearlier–  idealifyoucantryonterminal

•  MacTerminal;canuselabPiaswell

•  IfnoPythonfamiliarity–  encouragedtolookatLab3beforeWed.–  findtutorials,andlearntowriteandexecutesimpleprograms–  idealifabletorunPythoninterac<vesessionandalsotry

execu<ngprograms•  MacTerminal;canuselabPiaswell

•  Lab3willbecombinedwithLab4forsinglewrite-up,dueOct.30


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