計算機與控制自動化外文翻譯

Preface

Duringtheearlyyearsofmicroprocessors,therewerefewengineerswitheducationandex

perienceintheapplicationsofmicroprocessortechnology.Nowthatmicroprocessorsandm

icrocontrollcrshavebecomepervasiveinsomanydevices,theabilitytousethemhasbcc

omealmostarequirementtbrmanytechnicalpeople.

Todaythemicroprocessorandthemicrocontrollerhavebecometwoofthemostpowerfult

oolsavailabletothescientistandengineer.Microcontmllcrshavcbeenembeddedinsoma

nyproductsthatitiseasytooverlookthefaetthattheygreatlyoutnumberpersonalcomp

liters.MillionsofPCsareshippedeachyear,butbillionsofmicrocontrollersshipannually.

Whileagreatdealofattentionisgiventopersonalcomputers,thevastmajorityofnew

designsarefbrembeddedapplications.ForeveryPCdesigner,therearethousandsofdesign

ersusingmicrocontrollersinembeddedapplications.Thenumberofembeddeddesignsisg

rowingquickly.Thepurposeofthisbookistogivethereaderthebasicdesignandanaly

sisskillstodesignreliablemicrocontrollerormicroprocessorbasedsystems.Theemphasis

inthisbookisonthepracticalaspectsofinterfacingtheprocessortomemoryandI/Od

evices,andthebasicsofinterfacingsuchadevicetotheoutsideworld.

Amajorgoalofthisbookistoshowhowtomakedevicesthatarcinhcrcntlyrcliablcby

design.Whilealotofattentionhasbeengivento“qualityimprove-ment,“themajority

oftheemphasishasbeenplacedontheprocessestharoccurafterthedesignofaproduct

iscomplete.Designdeficienciesareasig-nificantproblem,andcanbeexceedinglydiffi

culttoidentifyinthefieldThesetypesofqualityproblemscanbeaddressedinthedesig

nphasewithrelativelylittleeffort,andwithfarlessexpensethanwillbeincurredlateri

ntheprocess.Unfortunately,therearemanyhardwaredesignersandorganiza-tionsthat,fo

rvariousreasons,donotunderstandthesignificanceandex-penseofanunreliabledesign.

Thedesignmethodologypresentedinthistextisintendedtoaddressthisproblem.

Learningtodesignanddevelopamicrocontrollersystemwithoutanypracticalhands-onex

pcricnceisabitliketryingtolearntorideabikefromrcadingbook.Thus,anothergoal

istoprovideapracticalexampleofacompleteworkingproduct.Whatappearseasyon

papermayproveextremelydifficultwithoutsomerealworldexperienceandsomepotentia

llypainfulcrashes.

Inordertodoitright,it'sbesttoexamineandusearealdesign.Ontheotherhand,the

currentstateofthetechnology(surfacemountedpackaging,etc.)canmakethepracticals

ideproblematic.Inordertoaddressthisproblem,aspecialeducationalSystemDevelopme

ntKitisavailabletoaccompanythisbook(8031SDK).AHthedocumentationtoconstruct

anSDKisavailableonthecompanionCD-ROM.Thisinfo,alongwithupdatedinfonnati

onandapplicationexamples,isalsoavailableonthewebsiteforthisbook:http://www.hte.

com/echdbook.AlltheinformationneededtobuildtheSDKisavailablethere,aswellas

informationonhowtoordertheSDKassembled

andtested.

WhilesearchingforanappropriatetextforoneofthecoursesIteachinernbeddedcompu

terengineering,Iwasunabletolocateabookthatcoveredthetopicadequately.Anearlie

rversionofthisbookwaswrittentoaccom-panythatcourseandhassinceevolvedinto

whatyouseehere.ThecourseisofferedattheUniversityofCalifornia,SanDiegoExte

ndedStudies,andistitled“EmbeddedControllerHardwareDcsign.^^Thesamecoursesma

yalsobetakeninanon-lineformatusingtheInternet,andcanbefoundathttp://vvww.hte.

com/uconline/ecdThegoalsofthecourseandthebookareverymuchthesame:todescri

betherightwaytodesignembeddedsystems.

Whilenopriorknowledgeofmicrocontrollersormicroprocessorsisrequired,thereadersh

ouldalreadybefamiliarwithbasicelectronics,logic,andbasiccomputerorganization.Cha

pteroneisintendedasareviewofthosebasicconcepts.Nextthereisageneraloverview

ofmicrocontrollerarchitecture,andaspecificmicrocontrollerchiparchitecture,the8051f

amily,isintroducedanddetailed.The8051waschosenbecauseitcanbeinterfacedtoex

ternalmemory,hassimpletimingspecs,iswidelyusedandavailablefromanumberofma

nufacturers.Theconceptsofworst-casedesignandanalysisarcdcscribcdalongwithtcchni

quesforhardwareinterfacing.Agoodembeddeddesignrequiresfamiliaritywiththeunderl

yingmemorytechnology,includingROM,SRAM,EPROM,FlashEPROM,EEPROMstora

gemechanismsanddevices.Theprocessorbusinterfaceisthencoveredingeneralform,

alongwithanintroductiontothe8051'sbusinterface.Mostembeddeddesignscanalsobe

netitfromtheuseofuserprogrammablelogicdevices(PLD).Thissubjectistoocomplex

forin-depthcoveragehere,soPLDtechnologyiscoveredfromarelativelyhighlevel.T

hecentralthemeofdesigninganembeddedsystemthatcanbeproventobereliableisill

ustratedwithasimpleembeddedcon-troller.Theiterativenatureofthedesignprocessis

shownbyexample,andseveraldesignalternativesareevaluated.Withthecentralpartof

thedesigncompleted,theremainingchapterscoverthevarioustypesofI/Ointerfaces,bus

operations,andacollectionofinformationthatisseldomincludedintheusualsources,b

utisoftenhandeddownfromoneengineertoanother.

Ihopethatyouwillfindthisbooktobeuseful,andwelcomeanyobservationsandcontri

butionsyoumayhave.Ifyoushouldfindanyerrorsinthetext,orifyouknowofsome

goodembeddeddesignresources,pleasefeelfreetocontactmedirectlybye-mail:ken.ar

nold@

CHAPTERONE

ReviewofEleclronicsFundamenlals

Whyaremicroprocessorsandmicrocontrollersdesignedintosomanydifferentdevices?W

hiletherearcmanydryandpracticalreasons,Isuspectoneofthestrongestmotivationsf

orusingamicroprocessorissimplythatitisalotmorefun.Overthepastfewdecades

oftheso-called“computerrevolution/5Ihaveseenmanyproductsandprojectsthatcould

havebeenhandledwithoutresortingtoamicroprocessor.Yetthereisalwaysatendencyt

orationalizethechoiceofamicro-basedsolutionbyeconomicortechnicalargumentstos

upportthedecision.Infact,mostofthereallyexcellentproductsweresuccessfultoagre

atextentbecausetheywerefuntodevelop.Manyofthebestproductideashaveoccurred

whensomeonewas“playing“withsomethingtheywereintcrcstcdin.Inmyownexperien

ce,IhavefoundlearningsomethingnewismucheasierandmoreeffectivewhenIam"j

ustplayingaround”ratherthantryingtolearninastructuredwayoragainstadeadline.

Studiesofvariouscduca-tionalmethodsalsoindicate“coachedcxploration^^ismoreeffect

ivethanthetraditionalmethods.Theseandotherobservationsleadmetotheconclusionth

atthebestwaytolearnaboutamicrocontrollerisby“playing”withone.

Nobook—nomatterhowwellwritten-canpossiblymotivateandcducateyouaswellas

buildingandplayingwithamicrocontroller.Thebestwaytolearntheconceptsinthisbo

okistobuildasimplemicrocontroller.Evenititiscapableofnothingmorethanblinkin

galight,itwillprovideaconcreteexampleofthemicrocontrollerasatoolthatcanbef

untouse.Toeasethiseffort,acompanionsystemdevelopmentkit(SDK),isavailableto

accompanylhistext.Itincorporatesthefunctionsofastand-alonesingleboardcomputed

SBC),andanin-circuitemulator(ICE).Italsoservesasasampleembeddedcontrollerde

sign.ThedesignisincludedontheCD-ROMandwebsiteforthisbook,soanyonecan

reproduceanduseitasalearningtool.Byapplyingtheguidelinessetforthinthisbook

torealworldhardware,youcanlearntodesignreliableembeddedhardwareintootherpr

oducts.InformationonobtainingtheSDKcanbefoundinthePreface.

Objectives

Severaldifferentskillsarerequiredforsuccessfulembeddedhardwaredesign.Hereareso

meofthethingsyouwillknowhowtodowhenyoufinishthisbook:

*Interpretdesignrequirementsforthedesignofanembeddedcontroller.

*Readandunderstandthemanufacturer'sspecificationsheet.

*SelectappropriateICsforthedesign.

*InterfacetheCPU,memory,andI/Odevicestoacommonbus.

*DesignsimpleI/O(inpuCoutput)interfaces.

*Definethedecodingandinterconnectionofthemajorcomponents.

*Performaworst-caseanalysisofthetimingandloadingofallsignals.

*Understandthesoftwaredevelopmentcycleforamicrocontroller.

*Debugandtestthehardwareandsoftwaredesigns.

Thesetasksrepresentthemajorskillsrequiredinthesuccessfulapplicationofanembedde

dmicro.Inaddition,otherabilities——suchasthedesignandimplementationofsimpleuser

programmablelogic-willbecoveredasrequiredtosupporttheproficientapplicationoft

hetechnology.

EmbeddedMicrocomputerApplications

Thereisanincrediblediversityofapplicationsfbrembeddedprocessors.Mostpeopleare

awareofthehighlyvisibleapplications,buttherearemanylessapparentuses.Manyoft

heprojectsmystudentshavechosenturnedouttobeofpracticaluseintheirwork.How

ever,theyhavecoveredtheentirerangefromtheeconomicallypracticaltotheblatantlya

bsurd.Onepracticalexamplewastheuseofamicroprocessortomonitorandcontrolthe

ratioofingredientsusedinmixingconcrete.Aboutayearafterthestudentimple-mentedt

hesystem,hewrotetoinformmethatthesystemhadsavedhiscom-panybetweentwo

andthreemilliondollarsayearbyreducingthenumberof“badbatches”ofconcretetha

thadtobejackhammeredoutandreplaced.

Anotherexamplewasthatofastudentwhosuspendedaballbyairflowgener-atedbya

fanandprovidedclosedloopcontroloftheball'spositionwiththemicroprocessor.The

onlythingthatmanyofthestudentprojectsreallyhadincommonwastheuseofamicr

ocontrollerasatool.Someoftheactualcommercialapplicationsofembeddedcomputerc

ontrolsthattheauthorhasbeendirectlyinvolvedwithinclude:

*Abeltmeasuresaperson'sheartrateandrespirationthatsignalsanalarmwhensafeli

initsareexceeded.Aradiosignalisthentransmittedtoainicrocontrollerinapocketpage

rtodisplaythetypeofproblemandtheidentityofthebelt.

*Anenvironmentalsystemcontrolstheheatingventilatingandaircondi-tioninginoneo

rmorelargebuildingstominimizepeakenergydemands.

*Asystemthatmeasuresandcontrolstheprocessofetchingawaytheunwantedportions

ofmaterialfromthesurfaceofanintegratedcircuitbeingmanufactured.

*Thefarecollectionsystemusedtomonitorandcontrolentrytoarapidtransitsystemb

asedontheaccountbalancestoredonthemagneticstripeonacard.

*Determinationofexactgeographicpositionontheearthbymeasuringthetimeofarriva

Iofradiosignalsreceivedfromnavigationalbeacons.

*Anintelligentphonethatreceivesradiosignalsfromsmokealarms,intru-sionsensors,

andpanicswitchestoalertacentralmonitoringstationtopotentialemergencysituations.

*Afuelcontrolsystemthatmonitorsandcontrolstheilowoffueltoaturbinejetengin

e.

Selectingaparticularprocessorforagivenapplicationisusuallyafunctionofthedesigne

r'sfamiliaritywithaparticulararchitecture.Whiletherearemanyvariationsinthedetails

andspecificfeatures,therearetwogeneralcategoriesofdevices:microprocessorsandmi

crocontrollers.Thekeydifferencebetweenamicroprocessorandamicrocontrolleristhata

microprocessorcontainsonlyamoryandI/OonthechipinadditiontoaCPU.Microcont

rollersaregenerallyusedtbrdedicatedtasks.Microcomputerisageneraltermthatapplies

tocompletecom-putersystemsimplementedwitheitheramicroprocessorormicrocontrol

Icr.

MicrocomputerandMicrocontrollerArchitectures

Microprocessorsaregenerallyutilizedforrelativelyhighperformanceappli-cationswhere

costandsizearenotcriticalselectioncriteria.Becausemicro-processorchipshavetheire

ndrefunctiondedicatedtotheCPUandthushaveroomformorecircuitrytoincreaseexe

cutionspeed,theycanachieveveryhigh-levelsofprocessingpower.However,microproces

sorsrequireexternal

memoryandI/Ohardware.MicroprocessorchipsareusedindesktopPCsandworkstations

wheresoftwarecompatibility,performance,generality,andflexibilityareimportant.

Bycontrast,microcontrollerchipsareusuallydesignedtominimizethetotalchipcountan

dcostbyincorporatingmemoryandI/Oonthechip.Theyarcoftcn"'applicationspccializ

ed“attheexpenseofflexibility.Insomecases,themicrocontrollerhasenoughresources

on-chipthatitistheonlyICrequiredforaproduct.Examplesofasingle-chipapplication

includethekeyfobusedtoarmasecuritysystem,atoaster,orhand-heldgames.Theh

ardwareinterfacesofbothdeviceshavemuchincommon,andthoseofthemicrocontroller

saregenerallyasimplifiedsubsetofthemicroprocessor.Theprimarydesigngoalsforeach

typeofchipcanbesummarizedthisway:

*microprocessorsaremostflexible

*microcontrollersaremostcompact

TherearealsodifferencesinthebasicCPUarchitecturesused,andthesetendtoreflectth

eapplication.MicroprocessorbasedmachinesusuallyhaveavonNeumannarchitecturewit

hasinglememoryforbothprogramsanddatatoallowmaximumflexibilityinallocation

ofmemory.Microcontrollerchips,ontheotherhand,frequentlyembodytheHarvardarch

itccturc,whichhasseparatcmemoriesforprogramsanddata.Figure1-1illustratesthisdif

ference.

Figure1-1:AtleftisthevonNeumannarchitecture;atrightistheHarvardarchitecture.

OneadvantagetheHarvardarchitecturehasforembeddedapplicationsisduetothetwot

ypesofmemoryusedinembeddedsystems.Afixedprogramandconstantscanbestored

innon-volatileROMmemorywhileworkingvariabledatastoragecanresideinvolatile

RAM.Volatilememorylosesitscontentswhenpowerisremoved,butnon-volatileROM

memoryalwaysmaintainsitscontentsevenafterpowerisremoved.TheHarvardarchitectur

ealsohasthepotentialadvantageofaseparateinter-faceallowingtwicethememorytra

nsferratebyallowinginstructionfetchestooccurinparallelwithdatatransfers.Unfortuna

tely,inmostHarvardarchi-tecturemachines,thememoryisconnectedtotheCPUusing

abusthatlimitstheparallelismtoasinglebus.Atypicalembeddedcomputerconsistsof

theCPU,memory,andI/O.Theyaremostoftenconnectedbymeansofasharedbusfor

communication,asshowninFigure1-2.

TheReeil

World

L________________________________________________

Theperipheralsonamicrocon-trollerchiparetypicallytimers,counters,serialorparallel

data

ports,andanalog-to-digitalanddigital-lo-analogconvertersthatareintegrateddirectlyonthe

chip.Theperformanceoftheseperipheralsisgenerallylessthanthatofdedicatedperipheral

chips,whicharcfrequentlyusedwithmicroprocessorchips.However,havingthebusco

nnec-tions,CPU,memory,andI/Ofunctionsononechiphasseveraladvantages:

*Fewerchipsarerequiredsincemostfunctionsarealreadypresentontheprocessorchip.

*Lowercostandsmallersizeresultfromasimplerdesign.

*Lowerpowerrequirementsbecauseon-chippowerrequirementsaremuchsmallerthane

xternalloads.

*Fewerexternalconnectionsarerequiredbecausemostaremadeon-chip,andmostofth

cchipconnectionscanbeusedfbrI/O.

*MorepinsonthechipareavailablefbruserI/Osincetheyaren'tneededfbrthebus.

*Overallreliabilityishighersincetherearefewercomponentsandinterconnections.Ofc

oursetherearedisadvantagestoo,including:

*Reducedflexibilitysinceyoucan*leasilychangethefunctionsdesignedintothechip.

*ExpansionofmemoryorI/Oislimitedorimpossible.

*Limiteddatatransferratesduetopracticalsizeandspeedlimitsforasingle-chip.

*LzowerperformanceI/Obecauseofdesigncompromisestofiteverythingononechip.

DigitalHardwareConcepts

InadditiontotheCPU,memory,andI/Obuildingblocks,otherlogiccircuitsmayalsobe

equired.Suchlogiccircuitsarefrequentlyreferredtoasgluelogicbecausetheyareused

toconnectthevariousbuildingblockstogether.Themostdifficultandimportanttaskth

ehardwaredesignerfacesistheproperselectionandspecificationofthis“gluelogic."D

evicessuchasregisters,buffers,driversanddecodersarefrequentlyusedtoadaptthecont

rolsignalsprovidedbytheCPUtothoseoftheotherdevices.WhileTTLgatelevellog

icisstillinuseforthispurpose,theprogrammablelogicdevice(PLD)hasbe-comean

importantdeviceinconnectingthebuildingblocks.Contemporarymicrocontrollerdesigner

sneedtoacquirethefollowingskills:

*Interpretationofmanufacturersspecifications

*Detailed,worstcasetiminganalysisanddesign

*Worstcasesignalloadinganalysis

*Designofappropriatesignalandlevelconversioncircuits

*Componentevaluationandselection

*Programmablelogicdeviceselectionanddesign

Thegluelogicusedtojointheprocessor,memories,andI/Oisultimatelycomposedof1

ogicgates,whicharethemselvescomposedalmostentirelyoftransistors,diodes,resistors,

andinterconnectingwires.Inordertounder-standthebasicoperationofthegluelogic,

wearegoingtobeginatthecom-ponentlevelwithareviewofbasicelectronicsconcept

s.Theseconceptswill

bepresentedasfluidflowanalogies.

Voltage,Current,andResistance

InFigure1-3,abatteryprovidesavoltagesourceforelectricity,muchlikeapumpprovide

sapressuresourceforafluid.Voltage,orpressure,isrequiredtoproducccurrcntflowint

hecircuit.

VoltageSourcePressureis

analagous

toVoltage

Figure1-3:Voltageinanelectricalcircuitis

analogoustopressureinafluid.

Thevoltagesourceprovidesthepressure“motivation,“ifyouwill,fbrcurrentflow.Resista

neepro-videsalimitingconstraintontheamountofcurrentthatwillactuallyflow.The

resistorwillallowacurrenttoflowthroughitthatisproportionaltothevoltageacrossi

t,andinverselyproportionaltotheresistancevalue.Higherresistanceislikeasmallerap

ertureforthefluidtoflowthrough.Theresistanceresultsinavoltage,orpressuredrop,

acrosstheresistanceaslongascurrentisflowingintheresistor.Figure1-4illustratesth

is.

Restricts

ResistorCurrent

Restrictionof

CurrentFlow

Figure1-4:Resistanceinanelectricalcircuitis

analogoustoarestrictioninthe"owofafluid.

Thewiringconnectingthecom-ponentsinacircuitislikethepipingconnectingplumbin

g

componentsthatletafluidflow.Theflowofcurrentinthecircuitiscontrolledbythem

agnitudeofthevoltage(pressure)andtheresistance(pressuredrop)inthecircuit.InFigur

e1-5,thebatteryprovidesavoltagetoforcecurrentthroughtheresistor.Themagnitude

ofthevoltage(V)generatedbythebatteryisdevelopedacrosstheresistor,andthemag

nitudeoftheresis-tance(R),determinethecurrent(I),Notethe“return“currentpathi

softenshownas“ground,“whichisthereferencevoltageusedastheuzerovolts”p

oint.Inthiscase,currentflowsfromthepositivebatteryterminal,throughthewire,then

theresistor,thenthroughthe“ground”connectiontotheminusterminalofthebatter

y.Thisisusuallynotthesameasearthground,

whichprovidesaconnectiontoastakeorpipeliterallystuckintheground.Thcmagnitud

eofthecurrentinthiscaseisI=V/Rbyre-arrangingtheequationV=I*R,ass

howninFigure1-5.ThisisknownasOhm'slaw.Anotherwaytolookatitisthatwh

cncvcrcurrentflowsthrougharesistonthcrcisadropinvoltageacrosstheresistorduet

otherestrictionincurrent..

Cixrent(0

through

R?$t$tor(K)

cases

voila,

drop(V)

V=IR

PowerdtssipakdinKesKtor

isp=PR=VI=￥

Figure}-5.Vo/faqcacrossRisequaltocurrentnmitfjlicdbyresistance.

theirperformanceintherealworldandaresubjecttootherlimitations,suchasoperating

temperature,powerlimits,etc.Currentflowsonlythroughacompletecircuit,andinmost

cases(forapositivepowersupply)currentflowsfromthepowersourcethroughthecirc

uitryandreturnstothepowersupplythroughthecommon“ground”connection.Current

flowingthroughanyresistanceresultsinthedissipationofpowerasheat.Thepowerdi

ssiatedisP=IA2R=V*I=VA2/R.Notethatvoltageissometimesdenotedbytheva

riableVandsometimesbyE,forelectromotiveforce.

Allpracticalcomponentshavesomeresistance.Realbatterieshaveaninternalresistance,

forexample,whichprovidesanupperlimittothecurrentthebatterycansupplytoane

xternalcircuit.Realwireshaveresistanceaswell,sotheactualperformanceofacircuit

willdeviatesomewhatfromtheidcaLTheseeffectsarcobviousinsomecases,butnotin

others.Inanautomobilestartingcircuit,it'snotsurprisingthatthebattery,supplying1

2voltstoastarterwithinternalresistanceontheorderof0.01to0.1ohms,wiJresult

incurrentsofhundredsofamperesinordertostarttheengine.Ontheotherhand,while

consultingwithaprominentnotebookcomputermanufacturer,

Iuncoveredadesignerrorresultinginaninternalcurrentofhundredsofamperesflowin

ginthecircuitforafewnanoseconds.Obviously,thiswreakedhavocontheoperationo

fthecomputer,andgeneratedagreatdealofclcctro-magncticnoise!

Oneofthethingsyouwilllearninthisbookishowtoavoidthosekindsofmistakes.It

'salsoimportanttorememberthatpowerisdissipatedinanyresistancepresentintheci

rcuit.Thepowerisproportionaltothevoltagetimesthecurrentacrosstheresistance,wh

ichisdissipatingthepower.Inthelasttwoexamples,theamountofpowerdissipatedin

stantancouslyisquitehighwhilethecurrentisflowing.Whenthecurrentpulseisonlya

fewnanosecondslong,however,itmaynotbeobvious,sincetherewon'tbemuchheat

generated.

Diodes

Thediodeisasimpleseiniconductordeviceactingasa“oneway”currentvalve.Itonly

letscurrentflowinonedirection.Figure1-6illustrateshowthediodeoperateslikea

“one-way“fluidvalve.(Puristspleasenote:Thisbookdoesnotuseelectroncurrentflow.

Allelectricalcurrent

flowwillbeupositivevoruconventionalnCurrentflow,meaningcurrent)

CurrentDiodeisanalqous

.____toaone-wayvalve.

/、Currentcanonly

flowinonedirection.

CurrentFl6V$

Valve

-Closed"

NoCurrent

Rows

riigurr)-6.Adiodetocicctricityisa^aioqoustoavafvcinthe

(lowotatkiid.

Transistors

Theflowanalogycanalsobeusedtomodelhowatransistoroperatesinalogiccircuit.

Thetransistorisanamplifier.Itusesasmallamountofenergytocontrolalargerener

gysource,justasavalvecontrolsahigh-pressurewatersource.Therearetwokindsof

transistors:bipolarandfield-effecttransistors(FETs).Wewilllookatbipolartransistorsfir

st;theseamplifycurrent.Asmallamountofcurrentflowsinthecontrolcircuit(thetran

sistorbase-emittercircuit)toturnthetran-sistoron.Thiscontrolcurrentisamplified(mu

Itipliedbythegainorbetaofthetransistor)andallowsalargercurrenttoflowintheo

utputcircuit(thecollector-emittercircuit).Onceagain,thedeviceisnotperfectbecauseo

ftheresistance,current,gain,andleakagelimitationsofrealtransistors.Bipolartransistor

scomeintwopolari-ties,NPNandPNP,withthedifferencebeingthedirectioninwhic

hcurrentflowsfornormaloperation.AbipolarPNPtransistorisshownandmodeledin

Figure1-7.

FigureJ?8.CjperafiocotabipolarNPNtramiitor.

FigureJ?7:Opcrohor?ofabipolarPNPiransistor.

MechanicalSwitches

Mechanicalswitchesareusefulfordirectinputtodigitalcircuits.Oneofthemoreconven

ientversionsisabankofrockerswitchespackagedintoamodulethatcanfitintothesa

melocationasastandardchip.Thedualin-linepackage,orDIRswitchisoneofthee

asiestwaystoaddmultipleswitchestoamicro-controllerdesign.Themechanicalswitch

hasextremelylow“on"Resistanceandhigh“off"resistance,unlikemostsemicondu

ctorswitches.Figure1-9showsatypicalDIPswitchandtheschematicsymbolforit.

figureb欠8-posrttonD(Psmfchandschematicequivdent.

TransistorSwitchON

Transistorscanbeconfiguredtofunctionasswitches.AscanbeseeninFigureI-10,an

NPNtransistoroperatingasacurrentcontrolledswitchcanbeusedtobuildasimplein

vcrtcr.Itchangesalogiconeonitsinputtoalogiczeroatitsoutput,andviceversa.I

nthiscase,logiconeisrepresentedasapositivevoltage,andalogiczeroisrepresented

byzerovolts.Thelogicone

input(positiveinputvoltage)issuppliedthrougharesistorfromthepowersupplyvoltag

etothetransistorbaseterminal,resultinginasmallbasecontrolcurrentintothebase.

TransistorInverterTransistorInverter

InputI->Output0EquivalentCircuit

TranilstorSwitch"ON"EquivalentCircuit

riqiirrI-10.Thetransistorinverter;irput=JandtransistorON.Thetransistor

ONconfigurationisatidtandtherouivafoifctrcuifisatnqnf.

Thetransistorisusedbecauseithasgainallowingalargeroutputcurrenttoflowascon

trolledbyaweakerinput.Whenthetransistoristurnedonasmuchasitcanbe,theco

Hectoremittercircuitlooksalmostlikeashortcircuit,effectivelyconnectingtheoutputt

ogroundorzerovolts.Thisgivesalogiczeroonthecollectoroutput.Whenthetransist

orcollectorisshortedtoground,currentflowsfromthesupplythroughtheresistorandi

ntothetransistorcollectortoground.Thetransistorissaidtosinktheresistorcurrentint

oground.Ifthereisanexternalload,suchasanotherinverterorgate,connectedtothe

collectoroutp