外文文献翻译-microcomputer systems

本科毕业设计（论文）外文翻译专业电气工程及其自动化所在学院电子信息工程学院二零一二年六月MICROCOMPUTERSYSTEMSELECTRONICSYSTEMSAREUSEDFORHANDINGINFORMATIONINTHEMOSTGENERALSENSETHISINFORMATIONMAYBETELEPHONECONVERSATION,INSTRUMENTREADORACOMPANYSACCOUNTS,BUTINEACHCASETHESAMEMAINTYPEOFOPERATIONAREINVOLVEDTHEPROCESSING,STORAGEANDTRANSMISSIONOFINFORMATIONINCONVENTIONALELECTRONICDESIGNTHESEOPERATIONSARECOMBINEDATTHEFUNCTIONLEVELFOREXAMPLEACOUNTER,WHETHERELECTRONICORMECHANICAL,STORESTHECURRENTANDINCREMENTSITBYONEASREQUIREDASYSTEMSUCHASANELECTRONICCLOCKWHICHEMPLOYSCOUNTERSHASITSSTORAGEANDPROCESSINGCAPABILITIESSPREADTHROUGHOUTTHESYSTEMBECAUSEEACHCOUNTERISABLETOSTOREANDPROCESSNUMBERSPRESENTDAYMICROPROCESSORBASEDSYSTEMSDEPARTFROMTHISCONVENTIONALAPPROACHBYSEPARATINGTHETHREEFUNCTIONSOFPROCESSING,STORAGE,ANDTRANSMISSIONINTODIFFERENTSECTIONOFTHESYSTEMTHISPARTITIONINGINTOTHREEMAINFUNCTIONSWASDEVISEDBYVONNEUMANNDURINGTHE1940S,ANDWASNOTCONCEIVEDESPECIALLYFORMICROCOMPUTERSALMOSTEVERYCOMPUTEREVERMADEHASBEENDESIGNEDWITHTHISSTRUCTURE,ANDDESPITETHEENORMOUSRANGEINTHEIRPHYSICALFORMS,THEYHAVEALLBEENOFESSENTIALLYTHESAMEBASICDESIGNINAMICROPROCESSORBASEDSYSTEMTHEPROCESSINGWILLBEPERFORMEDINTHEMICROPROCESSORITSELFTHESTORAGEWILLBEBYMEANSOFMEMORYCIRCUITSANDTHECOMMUNICATIONOFINFORMATIONINTOANDOUTOFTHESYSTEMWILLBEBYMEANSOFSPECIALINPUT/OUTPUTI/OCIRCUITSITWOULDBEIMPOSSIBLETOIDENTIFYAPARTICULARPIECEOFHARDWAREWHICHPERFORMEDTHECOUNTINGINAMICROPROCESSORBASEDCLOCKBECAUSETHETIMEWOULDBESTOREDINTHEMEMORYANDINCREMENTEDATREGULARINTERVALSBUTTHEMICROPROCESSORHOWEVER,THESOFTWAREWHICHDEFINEDTHESYSTEMSBEHAVIORWOULDCONTAINSECTIONSTHATPERFORMEDASCOUNTERSTHEAPPARENTLYRATHERABSTRACTAPPROACHTOTHEARCHITECTUREOFTHEMICROPROCESSORANDITSASSOCIATEDCIRCUITSALLOWSITTOBEVERYFLEXIBLEINUSE,SINCETHESYSTEMISDEFINEDALMOSTENTIRELYSOFTWARETHEDESIGNPROCESSISLARGELYONEOFSOFTWAREENGINEERING,ANDTHESIMILARPROBLEMSOFCONSTRUCTIONANDMAINTENANCEWHICHOCCURINCONVENTIONALENGINEERINGAREENCOUNTEREDWHENPRODUCINGSOFTWARETHEFIGURE11ILLUSTRATESHOWTHESETHREESECTIONSWITHINAMICROCOMPUTERARECONNECTEDINTERMSOFTHECOMMUNICATIONOFINFORMATIONWITHINTHEMACHINETHESYSTEMISCONTROLLEDBYTHEMICROPROCESSORWHICHSUPERVISESTHETRANSFEROFINFORMATIONBETWEENITSELFANDTHEMEMORYANDINPUT/OUTPUTSECTIONSTHEEXTERNALCONNECTIONSRELATETOTHERESTTHATIS,THENONCOMPUTERPARTOFTHEENGINEERINGSYSTEMFIG11THREESECTIONSOFATYPICALMICROCOMPUTERALTHOUGHONLYONESTORAGESECTIONHASBEENSHOWNINTHEDIAGRAM,INPRACTICETWODISTINCTTYPESOFMEMORYRAMANDROMAREUSEDINEACHCASE,THEWORDMEMORYISRATHERINAPPROPRIATESINCEACOMPUTERSMEMORYISMORELIKEAFILINGCABINETINCONCEPTINFORMATIONISSTOREDINASETOFNUMBEREDBOXESANDITISREFERENCEDBYTHESERIALNUMBEROFTHEBOXINQUESTIONMICROCOMPUTERSUSERAMRANDOMACCESSMEMORYINTOWHICHDATACANBEWRITTENANDFROMWHICHDATACANBEREADAGAINWHENNEEDEDTHISDATACANBEREADBACKFROMTHEMEMORYINANYSEQUENCEDESIRED,ANDNOTNECESSARILYTHESAMEORDERINWHICHITWASWRITTEN,HENCETHEEXPRESSIONRANDOMACCESSMEMORYANOTHERTYPEOFROMREADONLYMEMORYISUSEDTOHOLDFIXEDPATTERNSOFINFORMATIONWHICHCANNOTBEAFFECTEDBYTHEMICROPROCESSORTHESEPATTERNSARENOTLOSTWHENPOWERISREMOVEDANDARENORMALLYUSEDTOHOLDTHEPROGRAMWHICHDEFINESTHEBEHAVIOROFAMICROPROCESSORBASEDSYSTEMROMSCANBEREADLIKERAMS,BUTUNLIKERAMSTHEYCANNOTBEUSEDTOSTOREVARIABLEINFORMATIONSOMEROMSHAVETHEIRDATAPATTERNSPUTINDURINGMANUFACTURE,WHILEOTHERSAREPROGRAMMABLEBYTHEUSERBYMEANSOFSPECIALEQUIPMENTANDARECALLEDPROGRAMMABLEROMSTHEWIDELYUSEDPROGRAMMABLEROMSAREERASABLEBYMEANSOFSPECIALULTRAVIOLETLAMPSANDAREREFERREDTOASEPROMS,SHORTFORERASABLEPROGRAMMABLEREADONLYMEMORIESOTHERNEWTYPESOFDEVICECANBEERASEDELECTRICALLYWITHOUTTHENEEDFORULTRAVIOLETLIGHT,WHICHARECALLEDELECTRICALLYERASABLEPROGRAMMABLEREADONLYMEMORIES,EEPROMSTHEMICROPROCESSORPROCESSESDATAUNDERTHECONTROLOFTHEPROGRAM,CONTROLLINGTHEFLOWOFINFORMATIONTOANDFROMMEMORYANDINPUT/OUTPUTDEVICESSOMEINPUT/OUTPUTDEVICESAREGENERALPURPOSETYPESWHILEOTHERSAREDESIGNEDFORCONTROLLINGSPECIALHARDWARESUCHASDISCDRIVESORCONTROLLINGINFORMATIONTRANSMISSIONTOOTHERCOMPUTERSMOSTTYPESOFI/ODEVICESAREPROGRAMMABLETOSOMEEXTENT,ALLOWINGDIFFERENTMODESOFOPERATION,WHILESOMEACTUALLYCONTAINSPECIALPURPOSEMICROPROCESSORSTOPERMITQUITECOMPLEXOPERATIONSTOBECARRIEDOUTWITHOUTDIRECTLYINVOLVINGTHEMAINMICROPROCESSORTHEMICROPROCESSORPROCESSESDATAUNDERTHECONTROLOFTHEPROGRAM,CONTROLLINGTHEFLOWOFINFORMATIONTOANDFROMMEMORYANDINPUT/OUTPUTDEVICESSOMEINPUT/OUTPUTDEVICESAREGENERALPURPOSETYPESWHILEOTHERSAREDESIGNEDFORCONTROLLINGSPECIALHARDWARESUCHASDISCDRIVESORCONTROLLINGINFORMATIONTRANSMISSIONTOOTHERCOMPUTERSMOSTTYPESOFI/ODEVICESAREPROGRAMMABLETOSOMEEXTENT,ALLOWINGDIFFERENTMODESOFOPERATION,WHILESOMEACTUALLYCONTAINSPECIALPURPOSEMICROPROCESSORSTOPERMITQUITECOMPLEXOPERATIONSTOBECARRIEDOUTWITHOUTDIRECTLYINVOLVINGTHEMAINMICROPROCESSORTHEMICROPROCESSOR,MEMORYANDINPUT/OUTPUTCIRCUITMAYALLBECONTAINEDONTHESAMEINTEGRATEDCIRCUITPROVIDEDTHATTHEAPPLICATIONDOESNOTREQUIRETOOMUCHPROGRAMORDATASTORAGETHISISUSUALLYTHECASEINLOWCOSTAPPLICATIONSUCHASTHECONTROLLERSUSEDINMICROWAVEOVENSANDAUTOMATICWASHINGMACHINESTHEUSEOFSINGLEPACKAGEALLOWSCONSIDERABLECOSTSAVINGSTOEMADEWHENARTICLESAREMANUFACTUREDINLARGEQUANTITIESASTECHNOLOGYDEVELOPS,MOREANDMOREPOWERFULPROCESSORSANDLARGERANDLARGERAMOUNTSOFMEMORYAREBEINGINCORPORATEDINTOSINGLECHIPMICROCOMPUTERSWITHRESULTINGSAVINGINASSEMBLYCOSTSINTHEFINALPRODUCTSFORTHEFORESEEABLEFUTURE,HOWEVER,ITWILLCONTINUETOBENECESSARYTOINTERCONNECTANUMBEROFINTEGRATEDCIRCUITSTOMAKEAMICROCOMPUTERWHENEVERLARGERAMOUNTSOFSTORAGEORINPUT/OUTPUTAREREQUIREDANOTHERMAJORENGINEERINGAPPLICATIONOFMICROCOMPUTERSISINPROCESSCONTROLHERETHEPRESENCEOFTHEMICROCOMPUTERISUSUALLYMOREAPPARENTTOTHEUSERBECAUSEPROVISIONISNORMALLYMADEFORPROGRAMMINGTHEMICROCOMPUTERFORTHEPARTICULARAPPLICATIONINPROCESSCONTROLAPPLICATIONSTHEBENEFITSLFFITTINGTHEENTIRESYSTEMONTOSINGLECHIPAREUSUALLYOUTWEIGHEDBYTHEHIGHDESIGNCOSTINVOLVED,BECAUSETHISSORTLFEQUIPMENTISPRODUCEDINSMALLERQUANTITIESMOREOVER,PROCESSCONTROLLERSAREUSUALLYMORECOMPLICATEDSOTHATITISMOREDIFFICULTTOMAKETHEMASSINGLEINTEGRATEDCIRCUITSTWOAPPROACHESAREPOSSIBLETHECONTROLLERCANBEIMPLEMENTEDASAGENERALPURPOSEMICROCOMPUTERRATHERLIKEAMOREROBUSTVERSIONLFAHOBBYCOMPUTER,ORASAPACKAGEDSYSTEM,SIGNEDFORREPLACINGCONTROLLERSBASEDONOLDERTECHNOLOGIESSUCHASELECTROMAGNETICRELAYSINTHEFORMERCASETHESYSTEMWOULDPROBABLYBEPROGRAMMEDINCONVENTIONALPROGRAMMINGLANGUAGESSUCHASTHEONESTO9BEINTRODUCEDLATER,WHILEINTHEOTHERCASEASPECIALPURPOSELANGUAGEMIGHTBEUSED,FOREXAMPLEONEWHICHALLOWEDTHEFUNCTIONOFTHECONTROLLERTOBEDESCRIBEDINTERMSOFRELAYINTERCONNECTIONS,INEITHERCASEPROGRAMSCANBESTOREDINRAM,WHICHALLOWSTHEMTOBEALTEREDTOSUITCHANGESINAPPLICATION,BUTTHISMAKESTHEOVERALLSYSTEMVULNERABLETOLOSSLFPOWERUNLESSBATTERIESAREUSEDTOENSURECONTINUITYOFSUPPLYALTERNATIVELYPROGRAMSCANBESTOREDINROM,INWHICHCASETHEYVIRTUALLYBECOMEPARTOFTHEELECTRONICHARDWAREANDAREOFTENREFERREDTOASFIRMWAREMORESOPHISTICATEDPROCESSCONTROLLERSREQUIREMINICOMPUTERSFORTHEIRIMPLEMENTATION,ALTHOUGHTHEUSELFLARGESCALEINTEGRATEDCIRCUITSTHEDISTINCTIONBETWEENMINIANDMICROCOMPUTERS,PRODUCTSANDPROCESSCONTROLLERSOFVARIOUSKINDSREPRESENTTHEMAJORITYOFPRESENTDAYMICROCOMPUTERAPPLICATIONS,THEEXACTFIGURESDEPENDINGONONESINTERPRETATIONOFTHEWORDPRODUCTVIRTUALLYALLENGINEERINGANDSCIENTIFICUSESOFMICROCOMPUTERSCANBEASSIGNEDTOONEOROTHEROFTHESECATEGORIESBUTINTHESYSTEMWEMOSTSTUDYPRESSUREANDPRESSURETRANSMITTERSPRESSUREARISESWHENAFORCEISAPPLIEDOVERANAREAPROVIDEDTHEFORCEISONENEWTONANDUNIFORMLYOVERTHEAREAOFONESQUAREMETERS,THEPRESSUREHASBEENDESIGNATEDONEPASCALPRESSUREISAUNIVERSALPROCESSINGCONDITIONITISALSOACONDITIONOFLIFEONTHEPLANETWELIVEATTHEBOTTOMOFANATMOSPHERICOCEANTHATEXTENDSUPWARDFORMANYMILESTHISMASSOFAIRHASWEIGHT,ANDTHISWEIGHTPRESSINGDOWNWARDCAUSESATMOSPHERICPRESSUREWATER,AFUNDAMENTALNECESSITYOFLIFE,ISSUPPLIEDTOMOSTOFUSUNDERPRESSUREINTHETYPICALPROCESSPLANT,PRESSUREINFLUENCESBOILINGPOINTTEMPERATURES,CONDENSINGPOINTTEMPERATURES,PROCESSEFFICIENCY,COSTS,ANDOTHERIMPORTANTFACTORSTHEMEASUREMENTANDCONTROLOFPRESSUREORLACKOFITVACUUMINTHETYPICALPROCESSPLANTISCRITICALTHEWORKINGINSTRUMENTSINTHEPLANTUSUALLYINCLUDESIMPLEPRESSUREGAUGES,PRECISIONRECORDERSANDINDICATORS,ANDPNEUMATICANDELECTRONICPRESSURETRANSMITTERSAPRESSURETRANSMITTERMAKESAPRESSUREMEASUREMENTANDGENERATESEITHERAPNEUMATICORELECTRICALSIGNALOUTPUTTHATISPROPORTIONALTOTHEPRESSUREBEINGSENSEDINTHEPROCESSPLANT,ITISIMPRACTICALTOLOCATETHECONTROLINSTRUMENTSOUTINTHEPLACENEARTHEPROCESSITISALSOTRUETHATMOSTMEASUREMENTSARENOTEASILYTRANSMITTEDFROMSOMEREMOTELOCATIONPRESSUREMEASUREMENTISANEXCEPTION,BUTIFAHIGHPRESSUREOFSOMEDANGEROUSCHEMICALISTOBEINDICATEDORRECORDEDSEVERALHUNDREDFEETFROMTHEPOINTOFMEASUREMENT,AHAZARDMAYBEFROMTHEPRESSUREORFROMTHECHEMICALCARRIEDTOELIMINATETHISPROBLEM,ASIGNALTRANSMISSIONSYSTEMWASDEVELOPEDTHISSYSTEMISUSUALLYEITHERPNEUMATICORELECTRICALANDCONTROLINSTRUMENTSINONELOCATIONTHISMAKESITPRACTICALFORAMINIMUMNUMBEROFOPERATORSTORUNTHEPLANTEFFICIENTLYWHENAPNEUMATICTRANSMISSIONSYSTEMISEMPLOYED,THEMEASUREMENTSIGNALISCONVERTEDINTOPNEUMATICSIGNALBYTHETRANSMITTERSCALEDFROM0TO100PERCENTOFTHEMEASUREMENTVALUETHISTRANSMITTERISMOUNTEDCLOSETOTHEPOINTOFMEASUREMENTINTHEPROCESSTHETRANSMITTEROUTPUTAIRPRESSUREFORAPNEUMATICTRANSMITTERISPIPEDTOTHERECORDINGORCONTROLINSTRUMENTTHESTANDARDOUTPUTRANGEFORAPNEUMATICTRANSMITTERIS20TO100KPA,WHICHISALMOSTUNIVERSALLYUSEDWHENANELECTRONICPRESSURETRANSMITTERISUSED,THEPRESSUREISCONVERTEDTOELECTRICALSIGNALTHATMAYBECURRENTORVOLTAGEITSSTANDARDRANGEISFROM4TO20MADCFORCURRENTSIGNALORFROM1TO5VDCFORVOLTAGESIGNALNOWADAYS,ANOTHERTYPEOFELECTRICALSIGNAL,WHICHISBECOMINGCOMMON,ISTHEDIGITALORDISCRETESIGNALTHEUSEOFINSTRUMENTSANDCONTROLSYSTEMSBASEDONCOMPUTERORFORCINGINCREASEDUSEOFTHISTYPEOFSIGNALSOMETIMESITISIMPORTANTFORANALYSISTOOBTAINTHEPARAMETERSTHATDESCRIBETHESENSOR/TRANSMITTERBEHAVIORTHEGAINISFAIRLYSIMPLETOOBTAINONCETHESPANISKNOWNCONSIDERANELECTRONICPRESSURETRANSMITTERWITHARANGEOF0600KPATHEGAINISDEFINEDASTHECHANGEINOUTPUTDIVIDEDBYTHECHANGEININPUTINTHISCASE,THEOUTPUTISELECTRICALSIGNAL420MADCANDTHEINPUTISPROCESSPRESSURE0600KPATHUSTHEGAINBESIDEWEMUSTMEASURETEMPERATURETEMPERATUREMEASUREMENTISIMPORTANTININDUSTRIALCONTROL,ASDIRECTINDICATIONSOFSYSTEMORPRODUCTSTATEANDASINDIRECTINDICATIONSOFSUCHFACTORSASREACTIONRATES,ENERGYFLOW,TURBINEEFFICIENCY,ANDLUBRICANTQUALITYPRESENTTEMPERATURESCALESHAVEBEENINUSEFORABOUT200YEARS,THEEARLIESTINSTRUMENTSWEREBASEDONTHETHERMALEXPANSIONOFGASESANDLIQUIDSSUCHFILLEDSYSTEMSARESTILLEMPLOYED,ALTHOUGHMANYOTHERTYPESOFINSTRUMENTSAREAVAILABLEREPRESENTATIVETEMPERATURESENSORSINCLUDEFILLEDTHERMALSYSTEMS,LIQUIDINGLASSTHERMOMETERS,THERMOCOUPLES,RESISTANCETEMPERATUREDETECTORS,THERMOSTATS,BIMETALLICDEVICES,OPTICALANDRADIATIONPYROMETERSANDTEMPERATURESENSITIVEPAINTSADVANTAGESOFELECTRICALSYSTEMSINCLUDEHIGHACCURACYANDSENSITIVITY,PRACTICALITYOFSWITCHINGORSCANNINGSEVERALMEASUREMENTSPOINTS,LARGERDISTANCESPOSSIBLEBETWEENMEASURINGELEMENTSANDCONTROLLERS,REPLACEMENTOFCOMPONENTSRATHERTHANCOMPLETESYSTEM,FASTRESPONSE,ANDABILITYTOMEASUREHIGHERTEMPERATUREAMONGTHEELECTRICALTEMPERATURESENSORS,THERMOCOUPLESANDRESISTANCETEMPERATUREDETECTORSAREMOSTWIDELYUSEDDESCRIPTIONTHEAT89C51ISALOWPOWER,HIGHPERFORMANCECMOS8BITMICROCOMPUTERWITH4KBYTESOFFLASHPROGRAMMABLEANDERASABLEREADONLYMEMORYPEROMTHEDEVICEISMANUFACTUREDUSINGATMELSHIGHDENSITYNONVOLATILEMEMORYTECHNOLOGYANDISCOMPATIBLEWITHTHEINDUSTRYSTANDARDMCS51INSTRUCTIONSETANDPINOUTTHEONCHIPFLASHALLOWSTHEPROGRAMMEMORYTOBEREPROGRAMMEDINSYSTEMORBYACONVENTIONALNONVOLATILEMEMORYPROGRAMMERBYCOMBININGAVERSATILE8BITCPUWITHFLASHONAMONOLITHICCHIP,THEATMELAT89C51ISAPOWERFULMICROCOMPUTERWHICHPROVIDESAKPAMAKPKMAKR27610642HIGHLYFLEXIBLEANDCOSTEFFECTIVESOLUTIONTOMANYEMBEDDEDCONTROLAPPLICATIONSFUNCTIONCHARACTERISTICTHEAT89C51PROVIDESTHEFOLLOWINGSTANDARDFEATURES4KBYTESOFFLASH,128BYTESOFRAM,32I/OLINES,TWO16BITTIMER/COUNTERS,AFIVEVECTORTWOLEVELINTERRUPTARCHITECTURE,AFULLDUPLEXSERIALPORT,ONCHIPOSCILLATORANDCLOCKCIRCUITRYINADDITION,THEAT89C51ISDESIGNEDWITHSTATICLOGICFOROPERATIONDOWNTOZEROFREQUENCYANDSUPPORTSTWOSOFTWARESELECTABLEPOWERSAVINGMODESTHEIDLEMODESTOPSTHECPUWHILEALLOWINGTHERAM,TIMER/COUNTERS,SERIALPORTANDINTERRUPTSYSTEMTOCONTINUEFUNCTIONINGTHEPOWERDOWNMODESAVESTHERAMCONTENTSBUTFREEZESTHEOSCILLATORDISABLINGALLOTHERCHIPFUNCTIONSUNTILTHENEXTHARDWARERESETPINDESCRIPTIONVCCSUPPLYVOLTAGEGNDGROUNDPORT0PORT0ISAN8BITOPENDRAINBIDIRECTIONALI/OPORTASANOUTPUTPORT,EACHPINCANSINKEIGHTTTLINPUTSWHEN1SAREWRITTENTOPORT0PINS,THEPINSCANBEUSEDASHIGHIMPEDANCEINPUTSPORT0MAYALSOBECONFIGUREDTOBETHEMULTIPLEXEDLOWORDERADDRESS/DATABUSDURINGACCESSESTOEXTERNALPROGRAMANDDATAMEMORYINTHISMODEP0HASINTERNALPULLUPSPORT0ALSORECEIVESTHECODEBYTESDURINGFLASHPROGRAMMING,ANDOUTPUTSTHECODEBYTESDURINGPROGRAMVERIFICATIONEXTERNALPULLUPSAREREQUIREDDURINGPROGRAMVERIFICATIONPORT1PORT1ISAN8BITBIDIRECTIONALI/OPORTWITHINTERNALPULLUPSTHEPORT1OUTPUTBUFFERSCANSINK/SOURCEFOURTTLINPUTSWHEN1SAREWRITTENTOPORT1PINSTHEYAREPULLEDHIGHBYTHEINTERNALPULLUPSANDCANBEUSEDASINPUTSASINPUTS,PORT1PINSTHATAREEXTERNALLYBEINGPULLEDLOWWILLSOURCECURRENTIILBECAUSEOFTHEINTERNALPULLUPSPORT1ALSORECEIVESTHELOWORDERADDRESSBYTESDURINGFLASHPROGRAMMINGANDVERIFICATIONPORT2PORT2ISAN8BITBIDIRECTIONALI/OPORTWITHINTERNALPULLUPSTHEPORT2OUTPUTBUFFERSCANSINK/SOURCEFOURTTLINPUTSWHEN1SAREWRITTENTOPORT2PINSTHEYAREPULLEDHIGHBYTHEINTERNALPULLUPSANDCANBEUSEDASINPUTSASINPUTS,PORT2PINSTHATAREEXTERNALLYBEINGPULLEDLOWWILLSOURCECURRENT,BECAUSEOFTHEINTERNALPULLUPSPORT2EMITSTHEHIGHORDERADDRESSBYTEDURINGFETCHESFROMEXTERNALPROGRAMMEMORYANDDURINGACCESSESTOEXTERNALDATAMEMORYTHATUSE16BITADDRESSESINTHISAPPLICATION,ITUSESSTRONGINTERNALPULLUPSWHENEMITTING1SDURINGACCESSESTOEXTERNALDATAMEMORYTHATUSE8BITADDRESSES,PORT2EMITSTHECONTENTSOFTHEP2SPECIALFUNCTIONREGISTERPORT2ALSORECEIVESTHEHIGHORDERADDRESSBITSANDSOMECONTROLSIGNALSDURINGFLASHPROGRAMMINGANDVERIFICATIONPORT3PORT3ISAN8BITBIDIRECTIONALI/OPORTWITHINTERNALPULLUPSTHEPORT3OUTPUTBUFFERSCANSINK/SOURCEFOURTTLINPUTSWHEN1SAREWRITTENTOPORT3PINSTHEYAREPULLEDHIGHBYTHEINTERNALPULLUPSANDCANBEUSEDASINPUTSASINPUTS,PORT3PINSTHATAREEXTERNALLYBEINGPULLEDLOWWILLSOURCECURRENTIILBECAUSEOFTHEPULLUPSPORT3ALSOSERVESTHEFUNCTIONSOFVARIOUSSPECIALFEATURESOFTHEAT89C51ASLISTEDBELOWPORT3ALSORECEIVESSOMECONTROLSIGNALSFORFLASHPROGRAMMINGANDVERIFICATIONRSTRESETINPUTAHIGHONTHISPINFORTWOMACHINECYCLESWHILETHEOSCILLATORISRUNNINGRESETSTHEDEVICEALE/PROGADDRESSLATCHENABLEOUTPUTPULSEFORLATCHINGTHELOWBYTEOFTHEADDRESSDURINGACCESSESTOEXTERNALMEMORYTHISPINISALSOTHEPROGRAMPULSEINPUTPROGDURINGFLASHPROGRAMMINGINNORMALOPERATIONALEISEMITTEDATACONSTANTRATEOF1/6THEOSCILLATORFREQUENCY,ANDMAYBEUSEDFOREXTERNALTIMINGORCLOCKINGPURPOSESNOTE,HOWEVER,THATONEALEPULSEISSKIPPEDDURINGEACHACCESSTOEXTERNALDATAMEMORYIFDESIRED,ALEOPERATIONCANBEDISABLEDBYSETTINGBIT0OFSFRLOCATION8EHWITHTHEBITSET,ALEISACTIVEONLYDURINGAMOVXORMOVCINSTRUCTIONOTHERWISE,THEPINISWEAKLYPULLEDHIGHSETTINGTHEALEDISABLEBITHASNOEFFECTIFTHEMICROCONTROLLERISINEXTERNALEXECUTIONMODEPSENPROGRAMSTOREENABLEISTHEREADSTROBETOEXTERNALPROGRAMMEMORYWHENTHEAT89C51ISEXECUTINGCODEFROMEXTERNALPROGRAMMEMORY,PSENISACTIVATEDTWICEEACHMACHINECYCLE,EXCEPTTHATTWOPSENACTIVATIONSARESKIPPEDDURINGEACHACCESSTOEXTERNALDATAMEMORYEA/VPPEXTERNALACCESSENABLEEAMUSTBESTRAPPEDTOGNDINORDERTOENABLETHEDEVICETOFETCHCODEFROMEXTERNALPROGRAMMEMORYLOCATIONSSTARTINGAT0000HUPTOFFFFHNOTE,HOWEVER,THATIFLOCKBIT1ISPROGRAMMED,EAWILLBEINTERNALLYLATCHEDONRESETEASHOULDBESTRAPPEDTOVC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