电子信息专业英语（第二版）课件：Modern Digital Design

ModernDigitalDesignPassageAOverviewofModernDigitalDesignPassageBElectronicDesignwithFPGAsPassageCVHDLPassageAOverviewofModernDigitalDesign

Electroniccircuitdesignhastraditionallyfallenintotwomainareas:analogueanddigital.Thesesubjectsareusuallytaughtseparately,andelectronicsengineerstendtospecializeinonearea.Withinthesetwogroupingstherearefurtherspecializations,suchasradiofrequencyanaloguedesign;digitalintegratedcircuitdesign;and,wherethetwodomainsmeet,mixed-signaldesign.Inaddition,ofcourse,softwareengineeringplaysanincreasinglyimportantroleinembeddedsystems.

Digitalelectronicsisevermoresignificantinconsumergoods.Carshavesophisticatedcontrolsystems.Manyhomesnowhavepersonalcomputers.Productsthatusedtobethoughtofasanalogue,suchasradio,televisionandtelephones,areallbecomingdigital.DigitalcompactdiscshavealmostentirelyreplacedanalogueLPsforrecordedaudio.Withthesechanges,thelifetimesofproductshavelessened.Inaperiodoflessthanayear,newmodelswillprobablyhavereplacedallthedigitalelectronicproductsinyourlocalstore.

1.Designautomation

Tokeeppacewiththisrapidchange,electronicsproductshavetobedesigned

extremelyquickly.Analoguedesignisstillaspecialized(andwell-paid)profession.Digitaldesignhasbecomeverydependentoncomputer-aideddesign(CAD)-alsoknownas

designautomation(DA)orelectronicdesignautomation(EDA).TheEDAtoolsallowtwotaskstobeperformed:synthesis,inotherwordsthetranslationofaspecificationintoanactualimplementationofthedesign;andsimulation,inwhichthespecificationorthe

detailedimplementationcanbeexercisedinordertoverifycorrectoperation.［1］

SynthesisandsimulationEDAtoolsrequirethatthedesignbetransferredfromthedesigner’simaginationintothetoolsthemselves.Thiscanbedonebydrawingadiagramofthedesignusingagraphicalpackage.Thisisknownasschematiccapture.

Alternatively,thedesigncanberepresentedinatextualform,muchlikeasoftwareprogram.Textualdescriptionsofdigitalhardwarecanbewritteninamodifiedprogramminglanguage,suchasC,orinahardwaredescriptionlanguage(HDL).Overthepastthirtyyearsorso,anumberofHDLshavebeendesigned.TwoHDLsareincommon

usagetoday:VerilogandVHDL(VHSICHardwareDescriptionLanguage,whereVHSICstandsforVeryHighSpeedIntegratedCircuit).StandardHDLsareimportantbecausetheycanbeusedbydifferentCADtoolsfromdifferenttoolvendors.

InthedaysbeforeVerilogandVHDL,everytoolhaditsownHDL,requiringlaborioustranslationbetweenHDLs,forexampletoverifytheoutputfromasynthesistoolwithanothervendors’ssimulator.

2.Logicgates

Thebasicbuildingblocksofdigitalcircuitsaregates.Agateisanelectroniccomponentwithanumberofinputsand,generally,asingleoutput.Theinputsandtheoutputsarenormallyinoneoftwostates:logic0orlogic1.Theselogicvaluesarerepresentedbyvoltages(forinstance,0Vforlogic0and3.3Vforlogic1)orcurrents.Thegateitselfperformsalogicaloperationusingallofitsinputstogeneratetheoutput.Ultimately,ofcourse,digitalgatesarereallyanaloguecomponents,butforsimplicitywetendtoignoretheiranaloguenature.

Itispossibletobuyasingleintegratedcircuitcontaining,say,fouridenticalgates,asshowninFigure22.1.(Notethattwooftheconnectionsareforthepositiveandnegativepowersuppliestothedevice.Theseconnectionsarenotnormallyshowninlogicdiagrams.)

Adigitalsystemcouldbebuiltbyconnectinghundredsofsuchdevicestogether-indeedmanysystemshavebeendesignedinthatway.Althoughtheindividualintegratedcircuitsmightcostaslittleas10centseach,thecostofdesigningtheprintedcircuitboardforsuchasystemandthecostofassemblingtheboardareverysignificantandthisdesignstyleisnolongercost-effective.

Figure22.1ASmall-scaleIntegratedCircuit

Muchmorecomplicatedfunctionsareavailableasmass-producedintegratedcircuits,rangingfromflip-flopsthroughtomicroprocessors.Withincreasingcomplexitycomesflexibility-amicroprocessorcanbeprogrammedtoperformanear-infinitevarietyoftasks.Digitalsystemdesignthereforeconsists,inpart,oftakingstandardcomponentsandconnectingthemtogether.Inevitably,however,someaspectofthefunctionalitywillnotbeavailableasastandarddevice.Thedesigneristhenleftwiththechoiceofimplementingthisfunctionalityfromdiscretegatedorofdesigningaspecializedintegrated

circuittoperformthattask.Whilethislattertaskmayappeardaunting,itshouldbe

rememberedthatthecostofasystemwilldependtoagreatextentnotonthecostoftheindividual

componentsbutonthecostofconnectingthosecomponentstogether.

3.ASICSandFPGAS

Thedesignofahigh-performance,full-customintegratedcircuit(IC)is,ofcourse,adifficulttask.Infull-customICdesign,everything,downtoandincludingindividualtransistorsmaybedesigned(althoughlibrariesofpartsare,ofcourse,used).Formanyyears,however,ithasbeenpossibletobuildsemi-customintegratedcircuitsusinggatearrays.Agatearray,asitsnamesuggests,isanintegratedcircuitonwhichanarrayoflogicgateshasbeencreated.Thedesignofanapplication-specificintegratedcircuit(ASIC)

usingagatearraythereforeinvolvesthedefinitionofhowthegatesinthearrayshouldbeconnected.Inpracticalterms,thismeansthatoneortwolayersofmetalinterconnectmustbedesigned.Sinceanintegratedcircuitrequiressevenormoreprocessingstages,alltheprocessingstepsotherthanthefinalmetalizationcanbecompletedin

advance.Becausetheuncommittedgatearrayscanbeproducedinvolume,thecostofeachdeviceisrelativelysmall.

ThetermASICisoftenappliedtofull-customandsemi-customintegratedcircuits.Anotherclassofintegratedcircuitisthatofprogrammablelogic.Theearliestprogrammablelogicdevices(PLDs)wereprogrammablelogicarrays(PLAs).Likegatearrays,theseconsistofarraysofuncommittedlogic,butunlikemask-programmablegatearrays,theconfigurationofthearrayisdeterminedbyapplyingalarge(usuallynegative)voltagetoindividualconnections.ThegeneralstructureofaPLAisshowninFigure22.2.ThePLAhasanumberofinputs(A,B,C)andoutputs(X,Y,Z),anAND-planeandanOR-plane.Connectionsbetweentheinputsandtheproductterms(P,Q,R,S)andbetweentheproducttermsandoutputsareshown;theremainingconnectionshavebeen

removedaspartoftheprogrammingprocedure.SomePLAsmaybereprogrammedelectricallyorbyrestoringtheconnectionsbyexposingthedevicetoultravioletlight.

Figure22.2PLAStructure

Programmablearraylogic(PALs)extendtheideaofPLAstoincludeupto12flip-flops.Inrecentyears,programmabledeviceshavebecomemuchmorecomplexandincludeCPLDs(complexPLDs)andFPGAs(fieldprogrammablegatearrays).

4.Designflow

Mostdigitalsystemsaresequential,thatistheyhavestates,andtheoutputsdependonthepresentstate.Someearlydesignsofcomputerwereasynchronous;inotherwords,thetransitiontoanewstatehappenedassoonasinputshadstabilized.Formanyyears,digitalsystemshavetendedtobesynchronous.Inasynchronoussystem,thechangeofstateistriggeredbyoneormoreclocksignals.Inordertodesignreliablesystems,formaldesignmethodologieshavebeendefined.Thedesignofasynchronoussequentialdigitalsystemusingdiscretegateswouldthereforeproceedasfollows.

(1)Writeaspecification.

(2)Ifnecessary,partitionthedesignintosmallerpartsandwriteaspecificationforeachpart.

(3)Fromthespecificationdrawastatemachinechart.Thisshowseachstateofthesystemandtheinputconditionsthatcauseachangeofstate,togetherwiththeoutputsineachstate.

(4)Minimizethenumberofstates.Thisisoptionalandmaynotbeusefulinall

cases.

(5)AssignBooleanvariablestorepresenteachstate.

(6)Derivenextstateandoutputlogic.

(7)Optimizethenextstateandoutputlogictominimizethenumberofgatedneeded.

(8)Chooseasuitableplacementforthegatesintermsofwhichgatesshareintegratedcircuitsandintermsofwhereeachintegratedcircuitisplacedontheprintedcircuitboard.

(9)Designtheroutingbetweentheintegratedcircuits.

Ingeneral,steps(1)and(2)cannotbeavoided.Thisiswherethecreativityofthedesignerisneeded.Mostbooksondigitaldesignconcentrateonsteps(3)to(7).Steps(8)and(9)canbeperformedmanually,butplacementandroutingwasoneofthefirsttaskstobesuccessfullyautomated.Itispossibletosimulatethedesignatdifferentstagesifitisconvertedintoacomputer-readableform.Typically,inordertoperformtheplacementandrouting,aschematiccaptureprogramwouldbeusedataroundstep(7),suchthatthegate-levelstructureofthecircuitwouldbeentered.Thisschematiccouldbeconvertedtoaformsuitableforalogicsimulator.Afterstep(9)hadbeencompleted,thestructureofthecircuit,includinganydelaysgeneratedbytheresistanceandcapacitanceoftheinterconnect,couldbeextractedandagainsimulated.

TheimplementationofdigitaldesignsonASICsorFPGAsthereforeinvolvestheconfigurationofconnectionsbetweenpredefinedlogicblocks.Asnoted,wecannotavoidsteps(1)and(2),aboveandsteps(8)and(9)canbedoneautomatically.TheuseofanHDL,inthecaseofthisbookVHDL,meansthatthedesigncanbeenteredintoaCADsystemandsimulatedatstep(3)or(4),ratherthanstep(7).So-calledregistertransferlevel(RTL)synthesistoolsautomatesteps(6)and(7).Step(4)stillhastobedonebyhand.Step(5)canbeautomated,butnowtheconsequencesofaparticularstateassignmentcanbeassessedveryquickly.Behavioralsynthesistoolsarestartingtoappearthatautomatetheprocessfromaboutstep(2)onwards.Figure22.3showstheoveralldesignflowforRTLsynthesis-baseddesign.

Figure22.3TheOverallDesignFlowforRTLSynthesis-basedDesignNotes

［1］TheEDAtoolsallowtwotaskstobeperformed:synthesis,inotherwordsthetranslationofaspecificationintoanactualimplementationofthedesign;andsimulation,inwhichthespecificationorthedetailedimplementationcanbeexercisedinordertoverifycorrectoperation.

EDA工具可以完成两类任务:综合和模拟。综合是将设计指标转化为实际的设计实现;而模拟是对设计指标或详细的实现方案进行“演习”,以确保其运行正确。［2］Whilethislattertaskmayappeardaunting,itshouldberememberedthatthecostofasystemwilldependtoagreatextentnotonthecostoftheindividualcomponentsbutonthecostofconnectingthosecomponentstogether.

尽管选择进行专用集成电路设计可能让人畏缩,但应当记住:在很大程度上,系统成本取决于器件互连的成本,而不是单个器件的成本。

·it是形式主语,真正的主语是that…。not…but…是“不是……而是……”的意思。Exercises

1.PleasetranslatethefollowingphrasesintoEnglish.

(1)印刷电路板

(2)专用集成电路

(3)可编程逻辑电路

(4)乘积项

(5)紫外线

(6)现场可编程门阵列

2.PleasetranslatethefollowingphrasesintoChinese.

(1)flip-flops

(2)programmablelogicarrays(PLAs)

(3)mask-programmablegatearrays

(4)AND-plane

(5)OR-plane

(6)RTL(registertransferlevel)

3.Answerthequestions.

(1)WhatarethetwoHDLscommonlyusedtoday?

(2)Ingeneral,inthedesignflowwhichtwostepscannotbeavoided?

PassageBElectronicDesignwithFPGAs

CustomLogicOptions

Inmostdigitaldesigns,thecircuitrycanbeclassifiedbythefollowingcategories:

·Standardproducts.Theseproductsprovideafunctionalitythatisnotassociatedwithaspecificapplicationareabutcommontoabroadrangeofdevices.Typicalpartsinthiscategoryareprocessorsandmemories.

·ApplicationSpecificStandardProductsorASSPS.Theseproductsprovidefunctionalitythatisnotassociatedwithaspecificimplementation,butcommontoanapplicationarea.TypicalpartsinthiscategoryareMPEGdecoders.

·CustomLogic.Thislogicisassociatedwithaspecificapplicationandistheessenceofwhatdistinguishoneproductfromanother.Oftenthisisgluelogic,connectingstandardproductsorASSPswitheachother.

Thereareseveraloptionsonhowtoimplementcustomlogic,FPGAsbeingone

amongstthem.Theseoptionsarediscussedinthefollowingparagraphs.

Figure22.4showsthetypicaldigitalsystem.

Figure22.4TypicalDigitalSystem

ASICs

ASICistheabbreviationofApplicationSpecificIntegratedCircuit.Thisdenotesanintegratedcircuitthatisfullycustomizedtotherequirementsofaspecificapplication.Today’sASICsareusuallydesignedwithaStandardCellapproach.

Thismeans,thatthecircuitisnotdesignedontheleveloftransistors,butonthelevelofgates,flips-flops,andmemoryblocks.Thebasisforthisisalibraryofprimitives,whichisprovidedbythesiliconfoundry.ASICscanbecharacterizedbythefollowing

items:

·Lowestproductioncost;

·Highestpossibledesigndensity;

·Highestdevelopmenteffort.

GateArrays

GatearraysarecircuitsprovidingafixedSea-of-Gatesfabricthatisdefinedbythemanufacturer,andaninterconnectioninfrastructurethatiscustomizedaccordingtotheapplication’srequirements.［1］

GateArrayscanbecharacterizedbythefollowingitems:

·Lowproductioncost;

·Lowdesigndensity;

·Highdevelopmenteffort.

FPGAs

FPGAistheabbreviationofFieldProgrammableGateArray.Thisdenotesanintegratedcircuitthatisprogrammedinthefield,i.e.bythesystemmanufacturer.FPGAscanbecharacterizedbythefollowingitem:

·Highproductioncost;

·Lowdesigndensity;

·Lowdevelopmenteffort.

FPGAUsage

Inthepast,FPGAshavebeenanichemarket.Typicalapplicationswereprototypingandemulationsystems.［2］

Complexitywastoolowtoimplementrealproductsandpricingwastooexpensiveformoderatevolumes.Inthepastfewyears,FPGAtechnologymadelargesuccess,changingthewayFPGAsareuseddramatically:

·FPGAsarebeingusedinmainstreamproducts

-Networking;

-Telecom;

-DSP;

-Consumerelectronics.

·MoreFPGAdesignstartsthanASICdesignstarts.

CaseStudies

Justafewyearsago,FPGAswereanimportantbutperipheralnicheinthesemiconductorindustry.Knownfortheirflexibility,FPGAswerewidelyusedforASICemulation,glue-logicconsolidation,orasasolutionforapplicationswithvolatilityandchangingstandards.FPGAprocesstechnologyusuallylaggedtheindustrybyatleastoneprocessgeneration,andtheincrediblepowerofre-programmabilitywaspacedbyhighproductionprices,slowinternalperformance,andtoolsthatrequired10to20hourstocompile50,000gates.［3］

Today,FPGAsareoftentheheartofthesystem,beingdesignedintomainstreamaswellasstate-of-the-arthigh-volumeproducts.Marketdynamics,technologyinnovation,costmodels,andtoolshaveallbeenaffectedbythedeepsubmicronera.AllthesefactorshavemadeFPGAsaviablealternativetoASICdesign.FPGAsfacilitateflexibility,quicktime-to-marketandarebeingusedasproductionsolutionsineverythingfromnetworkingandindustrialtoconsumerapplications.Architecturallyandbydesign,FPGAsminimizeissuescommontodeepsubmicrontechnology,whilemaintainingtheASICabstraction.Inkeepingwiththeentiresemiconductorsegment,theindustry-leadingFPGAcompaniesaregainingmomentum,whilethesmallerplayersconsolidateorseekoutspecificniches.［4］

What’sfuelingthepopularityofFPGAtechnologytothepointthatit’snowreplacingbothgatearrayandstandard-cellASICsinmanyapplications?0Auniquecombinationofacceleratingproductcyclesneedsforhigherintegrationthanispossibleinagatearray,fastmigrationtodeepsubmicronprocesstechnologyandtheexplosionofnetworkedapplicationsarethekeys.

Set-top-box

Re-programmabilityisthedriver,allowingdesignerstotakegreaterrisksandinnovatemoreduringthedevelopmentphaseswithoutlosingtimetomarket.Italsoforgiveschangingstandardsorlast-minutefeaturechanges,andenablestruein-systemreconfigurability.Thisletstelecomdevelopersuseonechipformanyapplications,ratherthanaseparateASICforeachspecificcustomrequirement.Oneexamplecomesfromaset-top-boxmakerwhodiscoveredthatoneFPGAcouldreplacethreeASICs,eachonegeared

towardthecustomspecificationsofAsia,Europe,andNorthAmerica.

Theintellectualproperty(IP)portfolioofFPGAsuppliersisgrowingtoincludekeysystem-levelcores,includingPCI,DSP,microprocessorperipherals,andkeytelecommunicationscores.Designerscuttheirtimetomarketby“droppingin”cores,eliminatingtheneedtore-createthecoredesign.DesignersareincreasinglyusingFPGAsinproduction.Theirreasonsareasvariedastheirapplications,butallaredirectlyrelatedtothechangesthatthelogicindustryisundergoing.

FiberChannelSwitch

Inanotherexample,adesignerchoseanFPGAforaFiberChannelswitchingproject,becausehisASICplanscouldn’ttberealizedinareasonabletimeframe.ThisFiberChannelsystemisintendedtoincreasebandwidthfrom200Mb/sto1Gb/sforIBMprotocols.Thesystemhadtoallowmultiplexinganddemultiplexingtocreateasmoothertransitiontothehigherbandwidth.Originally,theplanwastoprototypetheentiresystemusing

FPGAs,thenmigratetoASICsonceinitialproductionturnedtovolume.TheASIC

designscamebackfromthefabricationprocesswitherrors,andatthesametime,the

FPGApriceswerecontinuingtodrop.Withtheirlimitedresources,thedesignerschoseto

pursuenewproductdevelopmentovercontinuingtoreducecost.Theyalsodidn’twanttolosethebenefitsofprogrammability.

Notes

［1］GatearraysarecircuitsprovidingafixedSea-of-Gatesfabricthatisdefinedbythemanufacturer,andaninterconnectioninfrastructurethatiscustomizedaccordingtotheapplication’srequirements.

门阵列提供生产商预置的大量固定门电路结构和可以根据应用需求定制的互联电路。

·providing…修饰circuits,其中afixedSea-of-Gatesfabric和aninterconnection

infrastructure是providing的宾语,两个that引导定语从句修饰前面两个宾语。［2］Inthepast,FPGAshavebeenanichemarket.Typicalapplicationswereprototypingandemulationsystems.过去FPGA占有的市场份额很小。其典型应用为原型系统和仿真系统。

·niche的两个主要引申义为:①特别适合某人兴趣、能力或性格的环境或活动;

②特别需求某个产品或某项服务的领域。［3］FPGAprocesstechnologyusuallylaggedtheindustrybyatleastoneprocessgeneration,andtheincrediblepowerofre-programmabilitywaspacedbyhighproductionprices,slowinternalperformance,andtoolsthatrequired10to20hourstocompile

50,000gates.

通常情况下,FPGA的生产工艺技术至少要滞后业界现有水平一代,但强大的重复编程能力受制于高昂的产品价格、较差的内部性能和需要10~20个小时编译50000个逻辑门的工具。［4］Inkeepingwiththeentiresemiconductorsegment,theindustry-leadingFPGAcompaniesaregainingmomentum,whilethesmallerplayersconsolidateorseekoutspecific

niches.

在跟随整个半导体行业发展的过程中，业界领导的各大FPGA公司不断获得发展动力，而中小公司或者联合起来，或者寻求专用领域进行开发。

·gainmomentum的意思是“获得动力”、“势头越来越大”，其同义词组为gathermomentum,反义词组为losemomentum。Exercises

1.PleasetranslatethefollowingphrasesintoChinese.

(1)ASIC

(2)intellectualproperty

(3)set-top-box

(4)CustomLogic

(5)full-custom

(6)semi-custom

(7)PCI

(8)MPEG

(9)ASSP

(10)JPEG

2.PleasetranslatethefollowingsentencesintoChinese.

(1)FPGAsfacilitateflexibility,quicktime-to-marketandarebeingusedasproductionsolutionsineverythingfromnetworkingandindustrialtoconsumerapplications.

(2)Auniquecombinationofacceleratingproductcyclesneedsforhigherintegrationthanispossibleinagatearray,fastmigrationtodeepsubmicronprocesstechnologyandtheexplosionofnetworkedapplicationsarethekeys.

(3)Oneexamplecomesfromaset-top-boxmakerwhodiscoveredthatoneFPGAcouldreplacethreeASICs,eachonegearedtowardthecustomspecificationsofAsia,Europe,andNorthAmerica.

(4)DesignersareincreasinglyusingFPGAsinproduction,theirreasonsareasvariedastheirapplications,butallaredirectlyrelatedtothechangesthatthelogicindustryisundergoing.

(5)ThisFiberChannelsystemisintendedtoincreasebandwidthfrom200Mb/sto1Gb/sforIBMprotocols.

(6)TheASICdesignscamebackfromthefabricationprocesswitherror,andatthesametime,theFPGApriceswerecontinuingtodrop.

PassageCVHDL

WhatIsVHDL?

VHDLisaprogramminglanguagethathasbeendesignedandoptimizedfordescribingthebehaviorofdigitalsystems.VHDLhasmanyfeaturesappropriatefordescribingthebehaviorofelectroniccomponentsrangingfromsimplelogicgatestocompletemicroprocessorsandcustomchips.FeaturesofVHDLallowelectricalaspectsofcircuitbehavior(suchasriseandfalltimesofsignals,delaysthroughgates,andfunctionaloperation)tobepreciselydescribed.TheresultingVHDLsimulationmodelscanthenbeusedasbuildingblocksinlargercircuits(usingschematics,blockdiagramsorsystem-levelVHDL

descriptions)forthepurposeofsimulation.

VHDLisalsoageneral-purposeprogramminglanguage:justashigh-levelprogramminglanguagesallowcomplexdesignconceptstobeexpressedascomputerprograms,VHDLallowsthebehaviorofcomplexelectroniccircuitstobecapturedintoadesignsystemforautomaticcircuitsynthesisorforsystemsimulation.LikePascal,CandC++,VHDLincludesfeaturesusefulforstructureddesigntechniques,andoffersarichsetofcontrolanddatarepresentationfeatures.Unliketheseotherprogramminglanguages,VHDLprovidesfeaturesallowingconcurrenteventstobedescribed.Thisisimportant

becausethehardwaredescribedusingVHDLisinherentlyconcurrentinitsoperation.

OneofthemostimportantapplicationsofVHDListocapturetheperformancespecificationforacircuit,intheformofwhatiscommonlyreferredtoasatestbench.TestbenchesareVHDLdescriptionsofcircuitstimuliandcorrespondingexpectedoutputsthatverifythebehaviorofacircuitovertime.TestbenchesshouldbeanintegralpartofanyVHDLprojectandshouldbecreatedintandemwithotherdescriptionsofthecircuit.

AStandardIanguage

OneofthemostcompellingreasonsforyoutobecomeexperiencedwithandknowledgeableinVHDLisitsadoptionasastandardintheelectronicdesigncommunity.UsingastandardlanguagesuchasVHDLvirtuallyguaranteesthatyouwillnothavetothrow

awayandrecapturedesignconceptssimplybecausethedesignentrymethodyouhavechosenisnotsupportedinanewergenerationofdesigntools.Usingastandardlanguagealsomeansthatyouaremorelikelytobeabletotakeadvantageofthemostup-to-datedesigntoolsandthatyouwillhaveaccesstoaknowledgebaseofthousandsofotherengineers,manyofwhomaresolvingproblemssimilartoyourown.

ABriefHistoryofVHDL

VHDL,whichstandsforVHSIC(VeryHighSpeedIntegratedCircuit)HardwareDescriptionLanguage,wasdevelopedintheearly1980sasaspin-offofahigh-speedintegratedcircuitresearchprojectfundedbytheU.S.DepartmentofDefense.DuringtheVHSICprogram,researcherswereconfrontedwiththedauntingtaskofdescribingcircuitsofenormousscale(fortheirtime)andofmanagingverylargecircuitdesignproblemsthatinvolvedmultipleteamsofengineers.Withonlygate-leveldesigntoolsavailable,itsoonbecameclearthatbetter,morestructureddesignmethodsandtoolswouldbeneeded.

Tomeetthischallenge,ateamofengineersfromthreecompanies(IBM,Texas

InstrumentsandIntermetrics)werecontractedbytheDepartmentofDefensetocompletethespecificationandimplementationofanew,language-baseddesigndescriptionmethod.ThefirstpubliclyavailableversionofVHDL,version7.2,wasreleasedin1985.In1986,theInstituteofElectricalandElectronicsEngineers(IEEE)waspresentedwithaproposaltostandardizethelanguage,whichitdidin1987aftersubstantialenhancementsandmodificationsweremadebyateamofcommercial,governmentandacademicrepresentatives.Theresultingstandard,IEEE1076-1987,isthebasisforvirtuallyeverysimulationandsynthesisproductsoldtoday.Anenhancedandupdatedversionofthelanguage,IEEE1076-1993,wasreleasedin1994,andVHDLtoolvendorshavebeenrespondingbyaddingthesenewlanguagefeaturestotheirproducts.

AlthoughIEEEStandard1076definesthecompleteVHDLlanguage,thereare

aspectsofthelanguagethatmakeitdifficulttowritecompletelyportabledesigndescriptions(descriptionsthatcanbesimulatedidenticallyusingdifferentvendors’tools).TheproblemstemsfromthefactthatVHDLsupportsmanyabstractdatatypes,butitdoesnot

addressthesimpleproblemofcharacterizingdifferentsignalstrengthsorcommonlyusedsimulationconditionssuchasunknownsandhigh-impedance.

SoonafterIEEE1076-1987wasadopted,simulatorcompaniesbeganenhancingVHDLwithnew,non-standardtypestoallowtheircustomerstoaccuratelysimulatecomplexelectroniccircuits.Thiscausedproblemsbecausedesigndescriptionsenteredintoonesimulatorwereoftenincompatiblewithothersimulationenvironments.VHDLwasquickly

becominganon-standard.

Togetaroundtheproblemofnon-standarddatatypes,anotherstandardwasdevelopedbyanIEEEcommittee.Thisstandard,numbered1164,definesastandardpackage(aVHDLfeaturethatallowscommonlyuseddeclarationstobecollectedintoanexternallibrary)containingdefinitionsforastandardnine-valueddatatype.Thisstandarddatatypeiscalledstd_logic,andtheIEEE1164packageisoftenreferredtoastheStandardLogicpackage.

TheIEEE1076-1987andIEEE1164standardstogetherformthecompleteVHDLstandardinwidestusetoday.(IEEE1076-1993isslowlyworkingitswayintotheVHDLmainstream,butitdoesnotaddsignificantnewfeaturesforsynthesisusers.)

Standard1076.3(oftencalledtheNumericStandardorSynthesisStandard)definesstandardpackagesandinterpretationsforVHDLdatatypesastheyrelatetoactualhardware.T