Laser Communications SystemWord格式.docx

Laser Communications SystemWord格式.docx

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HighLevelDesign

Thesystemcansendarbitrarydataat27.7kbpsbaud.Thesystemillustratedaboveissetuptosendvoicedata.Themicrophoneinputisconditioned（amplifiedandbiased）sothatthefulleightbitrangeoftheanaloguetodigitalconverterisutilized.Afairamountoftimewasspentoptimizingtheamplifierforthemicinputsoastominimizepoppingbutmaximizequality.

OncethedigitalsignalisobtainedbytheADC,theMCUpassesthesignaltotheUART.TheUARTsetsatransmitpinhighorlowaccordingtotheserialprotocol.Someconditioningisappliedtothissignalaswellinordertoensureconstantcurrenttothelaser.

Onthereceiverside,thesignalisreadbyaphototransistor.Thesignalcomingoffthetransistorisputthroughacomparatortogenerateappropriatehighandlowsignals.TheUARTreadsthesesignalsandgeneratesabyteaccordingtotheserialprotocol.Thisbyteisappliedtoaport,sentthroughadigitaltoanalogueconverter,andappliedtoaspeaker.

Hardware

Adescriptionofthehardwareandcircuitryusedinthisprojectfollows.Resistor,capacitoranddiodevaluesareommittedbecausetheyneedtotuneddependingonthehardwareused.Thespecificuseforeachcomponentisdocumented.

MicrophoneAmplifier

Thefirststepintransmittingsoundistodigitizesoundwaves.ForthisweusedanelectretmicrophonepurchasedfromRadtronics.FrequentshoppersatTito'

splacedowntownknowthatfindingspecsheetsforproductsthereisimpossible.Themicrophonehesoldushadthreeleads,whichafterconsiderableangstwedecidedwereforpower,ground,andsignal.Thesignalcomingoffthemicwasfartoolowtoberead（withanydegreeofprecision）bytheanalogtodigitalconverter.Soofcourse,anamplifierisneeded.BeforethesignalisputthroughtheamplifierhoweveritisfirstputthroughacapicitortoremoveDC,andthenthroughavoltagedividertoappropriatelybiasthesignal.ALF353op-ampisusedtoboostthesignal,thegainisadjustedbytheresistorsandforthemysterymicrophonethegainisaround50-100（dependingonhowmuchpoppingandhowmuchqualityyouwant）.

LaserDriver

AftertheA/Dconvertertranslatesthemicsignalinto8bits,theMCUgeneratestheappropriatebitstosend（includingstartandstopbits）andappliesthemtothelaserdrivercircuita5Vand0Vsignals.TheBJTinthiscircuitturnsonat5Vandprovidesthepropercurrentaccordingtothediodeandresitorvalues.

Receiver

Aphotodiodedetectsthelaserpulsesinadifferent（distant）location.Thissignalisputthroughacomparatorinordertogeneratesolid5Vand0Vvalueswhichareappliedtothereceivepinonthemicrocontroller.

OncethesignalisputthroughtheDAC（notshown,it'

sasimpleladder）,itisboostedandlowpassfiltered（toimprovesoundquality）.

Software

HalfDuplexSoftwareUART

InordertosendandreceivebothtextandaudiodatawedecidedtoimplementasoftwareprotocolwhichemulatedtheworkingsofahardwareUART.Thismeantthatwewouldbesendinginformationindigitalformoveraserialdatalink.Thelaserwouldessentiallyconstitutethe“wire”inthislinkconnectingourtwosoftwareUARTs.WecouldeasilyreservetwopinsofaporttobethereceiveandtransmitpinsforoursoftwareUART（inthefinalversionofthecodepinsD.0andD.1forreceiveandtransmitrespectively）.Whatremainedwasanimplementationofareceivesubroutineandatransmitsubroutine.Thedevelopmentofthesesubroutinesisdescribedbelow.

UART101

AUniversalAsynchronousReceiverTransmitter（UART）operatesonthepremisethatonecantransmitan8-bit（sometimes9-bit）datamessagethroughasinglewirelinkwithouttimesynchronizationbetweensourceanddestination.TheresultantsinglewirelinkisknownasaSerialDataLinkbecausethebitsofthe8-bitmessagearetransmittedinseries.TheasynchronouspartoftheUART（whichiswhatattractedusmosttotheideaofemulatingaUARTinoursoftware）isachievedbysendingknownbitstodenotethebeginningandendofthedatamessage.Thesebitsarecalledstartandstopbitsandare0and1intheUARTprotocolrespectively.WhentheUARTisnottransmittingdataitmaintainsalogicalhigh

（1）onthewire.Thus,whenalogicallow（0）startbitistransmitted,thereceiverknowsthatamessageisbeingtransmittedandstarts“payingattention”.The8bitsfollowingthestartbitarethecontentsofthemessage.The8bitsarefollowedalwaysbyalogicalhigh

（1）stopbit（andinmorecomplicatedimplementationsaparitybitwhichweshallignoreforourpurposes）.InthehardwareimplementationoftheUARTonmostAtmelchipstheUARTsamplesreceivedbits3timesinthemiddleofthebitandusesa“majorityrule”processtodeterminethevalueofthebit.Bycontrast,oursoftwareUARTwillsamplereceivedbitsonlyonceinthemiddletodeterminetheirvalue.

Also,anotherkeydifferencebetweenoursoftwareUARTandmosthardwareUARTsisthatitisahalfduplexUART.Thismeansthatitcanonlytransmitorreceiveatanygiventimeratherthanbothtransmitandreceiveatthesametime（fullduplex）.AfullduplexsoftwareUARTwouldbemuchmorecomplicatedandisunnecessaryforourapplication.

ThemeasureoftherateatwhichbitsaretransmittedbyaUARToveraSerialDataLinkisknownasabaudrate（theterm“baud”doesnotstandforanythingbutratherisashortenedformofthenameofEmileBaudot,apioneerinearlytelegraphsystemswhoseworkpavedthewayfortheModulatorDemodulator（Modem））.Moreover,adatarateof300baudisanalogoustoarateof300bitspersecond（bps）.Thisdatarateisimportantfortworeasons:

ItdetermineshowlongthetransmittingUARTmustmaintainabit’svalueonthewire,anditdetermineshowlongthereceivingUARTmustwaitbetweensamplingbits.ThesetwolengthsoftimeareactuallythesameandwillbeimplementedinoursoftwareUARTwithasoftwaredelaycall（referredtohereafterasa“delay”）.

Tocalculatethelengthoftimeweneedthedelaytolastwemustknowthatgettingintoeachofthetransmitandreceivesubroutinesanddoingthenecessarylogicaloperationstochangebitvaluesortodeterminereceivedbitvaluestakeatotalofabout1μs（or16operations）andsowemustsubtractthisamountfromourendresult.Inthefollowingcalculationswewilldenoteourenddelayas“delay”andourdesiredbaudas“baud”:

Wefirstmustcalculatethetimelengthofabitatthedesiredbaud:

Thengettheperiodinμs:

Wethenroundthisresultandsubtract1μstoaccountforoverheadandlogicaloperations.Thisisourfinaldelay:

Thisdelayisdenotedas“buadnum”inthefinalcode.Wealsorequire“halfbuadnum”forthereceiver（asaruleofthumbbuadnum/2,roundresultupifbuadnumisodd）.Anytimingerrorsassociatedwiththesecalculationsonlypersistfor8bitsbecausethemessagesareonly8bitslong.Sucherrorswouldbe“reset”atthebeginningofthenexttransmission,andasaresultwenoticedthatthesoftwareUARTwasnotterriblysensitivetoerrorsontheμslevel（especiallyatlowerbauds）.Thesediscussionsofdelaycalculationswillmakemoresensetothereaderafterreadingthedescriptionsofthetransmitandreceivesubroutines.Atableofsomecommondelaysisgivenbelow.

BAUD

baudnum

halfbaudnum

110

9090

4545

9600

103

52

27777

35

18

38400

25

13

57600

16

8

115200

4

Itshouldbenotedthat（althoughthecodeisnowherenearthesame）manyoftheconceptsweusedincreatingoursoftwareUARTcamefromtheAtmelApplicationNoteonHalfDuplexCompactSoftwareUARTfortheAVR8-bitRISCmicrocontrollerfamily（AppNoteAVR305at）.ThecodeprovidedwiththisAppNotewaswrittenentirelyinassemblerlanguageandsowewerepresentedwiththetaskofimplementingaCversionwhichmimickeditsoperation.Whatfollowsisadescriptionofourendproduct.

TheTransmitSubroutineput（）

Totransmitabyte（8bits）overtheserialdatalinkwedevelopedasubroutinecalledput.Thissubroutinetakesthe8bitsofdatainthecharactervariableTxbyteandshiftseachbitoutontotheserialdatalinkoneatatimestartingwiththeLSB.ThedatabytedesiredtobesentshouldbeplacedintoTxbyteandthenputmustbecalledinordertosendth