Basicstructure
ThefigureontherightshowsthebasicstructureofROM.ROMismainlycomposedofaddressdecoder,memorybank,readoutlineandsenseamplifier.ROMisamemoryaddressedbyaddress.TheaddressdecoderoftheROMisacombinationofANDgatesgiventheaddressofthestorageunittobeaccessedbytheCPU,andtheoutputisthesmallestitemofalladdressinput(fulldecoding).Afterthen-bitaddresscodeisdecoded,thereare2nresults,andthedriverselects2nwords,thatis,W=2n.ThememorybankisaW*mtwo-dimensionalarray(wordbitstructure)arrangedbyelementssuchasfuses,diodes,ortransistors,withatotalofWwords,eachwithmbits.ThememorybankisactuallyacombinationofORgates,andthenumberofROMoutputlinesisthenumberofORgates.Sinceitonlyreadsinformationwhenitworks,thereisnoneedtosetawritecircuit,whichmakesitsstorageunitandreadoutcircuitrelativelysimple.
Workingprocess
TherightfigureshowstheworkingprocessofROM.TheCPUsendstheaddressofthememoryunittobeaccessedviatheaddressbus,andtheaddressdecoderselectsawordaccordingtotheinputaddresscodeThen,itdrivesthebitlinesofthewordline,readsthebinarycodestoredineachstoragebitoftheword,sendsittothereadlineoutput,andthensendsittotheCPUviathedataline.
Features
Thefeatureofread-onlymemoryisthatitcanonlyreadbutnotwriteinformation.Usuallyabasicinput/outputsystemissolidifiedintheROMofthecomputermotherboard,calledBIOS(BasicInputOutputSystem).Itsmainfunctionistocompletethepower-onself-checkofthesystem,theinitializationofeachfunctionalmoduleinthesystem,thebasicinput/outputdriverofthesystemandthebootoperatingsystem.
Type
TherearemanytypesofROM,andeachread-onlymemoryhasitsowncharacteristicsandscopeofapplication.Fromitsmanufacturingprocessandfunctions,ROMhasfivetypes,namelymask-programmedread-onlymemoryMROM(Mask-programmedROM),programmableread-onlymemoryPROM(ProgrammableROM),erasableprogrammableread-onlymemoryMemoryEPROM(ErasableProgrammableROM),electricallyerasableprogrammableread-onlymemoryEEPROM(ElecricallyErasableProgrammableROM)andfasterasableread-writememory(FlashMemory).
Maskprogrammingread-onlymemory
Theinformationstoredinthemaskread-onlymemory(MaskROM)is"written"bythemanufacturerduringthemaskingprocess.Inthemanufacturingprocess,thedataisburnedinthecircuitwithaspecialmask(Mask),sometimescalled"MaskROM".Themanufacturingcostofthismemoryisrelativelylow,anditisoftenusedincomputers.Boottostart.MOStransistorsaresetattheintersectionsoftherowandcolumnlines.Inthelastmaskprocessduringmanufacturing,theMOStransistorsarecontrolledaccordingtothespecifiedcodinglayouttodeterminewhethertheMOStransistorsareconnectedtotherowandcolumnlines.Theconnectedonesaresetto1(or0),andtheunconnectedonesaresetto0(or1).Oncethiskindofmemoryismanufacturedbythemanufacturer,theusercannotmodifyit.
ThemainadvantageofMROMisthatthestoragecontentisfixed,theinformationstillexistsafterpowerfailure,andthereliabilityishigh.Thedisadvantageisthattheinformationcannotbemodifiedoncewritten(manufactured),itisveryinflexibleandhasalongproductioncycle,andthedependencebetweentheuserandthemanufacturerislarge.
Programmableread-onlymemory
ProgrammableROM(ProgrammableROM,PROM)allowsuserstowritetheinformationtheyneedatonetimethroughadedicateddevice(programmer),Itisgenerallyprogrammableonce,andeachstorageunitofthePROMmemoryisall1orall0whenitleavesthefactory.Whentheuserusesit,usetheprogrammingmethodtomakethePROMstoretherequireddata.
TherearemanytypesofPROM,whichrequireelectricityandlighttowriteandstoreprogramsandinformation.Butitcanonlybewrittenonce,andtheinformationwrittenforthefirsttimeispermanentlystored.Forexample,bipolarPROMhastwostructures:oneisthefuseblowntype,andtheotheristhePNjunctionbreakdowntype.Theycanonlyberewrittenonce,andoncetheprogrammingiscompleted,theircontentispermanent.Duetopoorreliabilityandone-timeprogramming,itisrarelyused.TheprogramanddatainPROMarewrittenbytheuserusingspecialequipment,oncewritten,theycannotbechangedandstoredpermanently.PROMhasacertaindegreeofflexibility,suitableforsmallbatchproduction,andisoftenusedinindustrialcontrolmachinesorelectricalappliances.
ProgrammableErasableProgrammableReadOnlyMemory
ProgrammableErasableProgrammableReadOnlyMemory(EPROM)canbeprogrammedmultipletimes.ItisakindofThemainwritableandreadablememory.ItisakindofROMthatisconvenientforuserstowriteaccordingtotheirneeds,andcanerasethewrittencontentbeforerewriting.Thestoredinformationcanbeprogrammedbytheuserbypoweringon,ortheoriginalstoredinformationcanbeerasedusingmethodssuchasultravioletlightsourceorpulsecurrent,andthenthenewinformationcanberewrittenwithawriter.EPROMismoreconvenient,flexibleandeconomicalthanMROMandPROM.ButEPROMusesMOStube,whichisslower.
Themethodoferasingremotestoragecontentscanbeasfollows:electricmethod(calledelectricrewriteableROM)orultravioletirradiationmethod(calledlightrewriteableROM).TheopticallyrewritableROMcanusehighvoltagetoprogramandwritedata.Whenerasing,exposethecircuittoultravioletlight,andthedatacanbeclearedandreused.Usuallyaquartztransparentwindowisreservedonthepackageshelltofacilitateexposure.
ElectricallyErasableProgrammableRead-OnlyMemory
ElectricallyErasableProgrammableRead-OnlyMemory(EEPROM)isakindofEEPROMthatcanbewrittenatanytimeThereisnoneedtoerasetheoriginalcontentofthememory.Thewriteoperationtakesmuchlongerthanthereadoperation.EEPROMcombinestheadvantagesofnoteasytolosedataandflexiblemodification.Onlyordinarycontrol,addressanddatabusesarerequiredformodification.TheoperatingprincipleofEEPROMissimilartoEPROM,buttheerasureisdonebyusingahighelectricfield,sothereisnoneedforatransparentwindow.EEPROMismoreexpensivethanEPROM,hasalowerintegrationlevel,andahighercost.Itisgenerallyusedtosavetheparametersofthesystemsettings,theinformationstoredontheICcard,andthecontrollerintheTVorairconditioner.Butbecauseitcanbemodifiedonline,itsreliabilityisnotasgoodasEPROM.
Fast-eraseread-writememory
Fast-eraseread-writememory(FlashMemory)isahigh-density,non-volatileread/writedeviceinventedbyIntelinthemid-1990s.WritingsemiconductormemoryhasthecharacteristicsofEEPROMandRAM.Itisabrand-newstoragestructure,commonlyknownasflashmemory.Itwasfirstintroducedinthemiddleandlate1980s,andthepriceandfunctionofflashmemoryarebetweenEPROMandEEPROM.LikeEEPROM,flashmemoryuseselectricallyerasabletechnology,andtheentireflashmemorycanbeerasedinonetoseveralseconds,whichismuchfasterthanEPROM.Inaddition,itcanerasecertainblocksinthememoryinsteadoftheentirechip.However,flashmemorydoesnotprovidebyte-levelerasure.LikeEPROM,flashmemoryusesonlyonetransistorperbit,soitcanachievethesamehighdensityasEPROM(comparedtoEEPROM).The"flashmemory"chipusesasinglepowersupply(3Vor5V)tosupplypower.Thespecialvoltagerequiredforerasingandprogrammingisgeneratedinsidethechip,soitcanbeerasedandprogrammedonline."Flash"isalsoatypicalnon-volatilememory.Undernormaluse,theelectronsstoredinthefloatinggatecanbestoredfor100yearswithoutloss.
Currently,flashmemoryhasbeenwidelyusedtomakeallkindsofmobilestorage,suchasUSBflashdrivesandmemorycardsusedbydigitalcameras/camcorders.
OneTimeProgrammableReadOnlyMemory(OTPROM)ThewritingprincipleisthesameasEPROM,butinordertosavecosts,afterprogrammingandwritingItisnolongererased,sonotransparentwindowisset.Scopeofuse
BecauseROMhasthefeatureofnotlosinginformationafterpowerfailure,itcanbeusedasaBIOSchipforcomputerstartup.EPROM,EEPROMandFlashROM(NORFlashandNANDFlash),theperformanceisthesameasROM,butcanberewritten,generallyreadfasterthanwrite,writerequiresahighervoltagethanread,(read5Vwrite12V)butFlashcanbereadunderthesamevoltageWrite,andlargecapacityandlowcost,suchasUdiskMP3widelyused.Inacomputersystem,RAMisgenerallyusedasmemory,andROMisgenerallyusedasfirmwaretostoresomehardwaredrivers.
Productionprinciple
ROMaddressdecoderisacombinationofANDgates,anditsoutputisthesmallestitemofalladdressinput.ThedecodercanberepresentedastheANDarrayshowninthefigureontheright.The"dot"markedattheintersectionofthehorizontalandverticallinesofthearrayinthefigureindicatestheconnectionof"AND".ThememorycellbodyisactuallyacombinationofORgates,andtheoutputnumberofROMisthenumberofORgates.EachminimumitemofthedecodermaybetheinputoftheORgate,butwhetheracertainminimumitemcanbetheinputoftheORgatedependsonthestorageinformation,sothememorycellbodycanberegardedasanORarray.Fromtheaboveanalysis,wecanlookatthestructureofROMfromanotherperspective:itiscomposedoftwoarrays-"AND"gatearrayand"OR"gatearray.Thecontentof"OR"issetbytheuser,soitisProgrammable,andtheANDarrayisusedtoformallthesmallestitems,soitisnotprogrammable.