Rom (read only memory)

Read-only memory (ROM), also known as firmware, is an integrated circuit programmed with specific data when it is manufactured. ROM chips are used not only in computers, but in most other electronic items as well. In this section, you will learn about the different types of ROM and how each works.

Read-only memory (ROM) is a type of non-volatile memory used in computers and other electronic devices. Data stored in ROM can only be modified slowly, with difficulty, or not at all, so it is mainly used to store firmware (software that is closely tied to specific hardware and unlikely to need frequent updates) or application software in plug-in cartridges.

1. ROM (Read Only Memory)
Sub : Digital Electronics
SIGMA INSTITUTE OF TECHNOLOGY AND ENGINEERING
COMPUTER ENGG. (B.E)(SEM –3)
Prepared by :
1.HEMIN PATEL (150500107025)
2.HARSHAL PATEL (150500107024)
3.VANDIT PATEL (150500107029)
Guided by :
HARDIK SOLANKI

2. Read-onlymemory(ROM),alsoknownasfirmware,isanintegratedcircuit
programmedwithspecificdatawhenitismanufactured.ROMchipsareused
notonlyincomputers,butinmostotherelectronicitemsaswell.Inthissection,
youwilllearnaboutthedifferenttypesofROMandhoweachworks.
Read-onlymemory (ROM)isatypeofnon-volatilememory used
incomputersandotherelectronicdevices.DatastoredinROMcanonlybe
modifiedslowly,withdifficulty,ornotatall,soitismainlyusedto
storefirmware(softwarethatiscloselytiedtospecifichardwareandunlikelyto
needfrequentupdates)orapplicationsoftwareinplug-incartridges.
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3. Thesimplesttypeofsolid-stateROM isasoldasthesemiconductor
technologyitself.Combinationallogicgatescanbejoinedmanuallytomapn-
bitaddressinputontoarbitraryvaluesofm-bitdataoutput(alook-uptable).Withthe
inventionoftheintegratedcircuitcamemaskROM.MaskROMconsistsofagrid
ofwordlines(theaddressinput)andbitlines(thedataoutput),selectivelyjoinedtogether
withtransistorswitches,andcanrepresentanarbitrarylook-uptablewitharegular
physicallayoutandpredictablepropagationdelay.
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4. Informationonthedataoutputlinedependsonlyontheinformationontheaddressinput
lines.
-->CombinationalLogicCircuit
m x n ROM
(m=2
k
)
k address input lines
n data output lines
X
0=A’B’ + B’C
X
1=A’B’C + A’BC’
X
2=BC + AB’C’
X
3=A’BC’ + AB’
X
4=AB
X
0=A’B’C’ + A’B’C + AB’C
X
1=A’B’C + A’BC’
X
2=A’BC + AB’C’ + ABC
X
3=A’BC’ + AB’C’ + AB’C
X
4=ABC’ + ABC
CanonicalMinterms
Add Output
ABC X
0X
1X
2X
3X
4
10000
11000
01010
00100
00110
10010
00001
00101
000
001
010
011
100
101
110
111
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5. Characteristics :
Perform read operation only, write operation is not possible
Information stored in a ROM is made permanent during production, and cannot be
changed
Organization
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6. SinceROM(atleastinhard-wiredmaskform)cannotbemodified,itis
reallyonlysuitableforstoringdatawhichisnotexpectedtoneed
modificationforthelifeofthedevice.Tothatend,ROMhasbeenused
inmanycomputerstostorelook-uptablesfortheevaluationof
mathematicalandlogicalfunctions.Thiswasespeciallyeffective
whenCPUswereslowandROMwascheapcomparedtoRAM.
Notably,thedisplayadaptersofearlypersonalcomputersstoredtables
ofbitmappedfontcharactersinROM.Thisusuallymeantthatthetext
displayfontcouldnotbechangedinteractively.Thiswasthecasefor
boththeCGAandMDAadaptersavailablewiththeIBMPCXT.
TheuseofROMtostoresuchsmallamountsofdatahasdisappeared
almostcompletelyinmoderngeneral-purposecomputers.However,
FlashROMhastakenoveranewroleasamediumformass
storageorsecondarystorageoffiles.

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8. 8

9. ROM
-Storeinformation(function)duringproduction
-Maskisusedintheproductionprocess
-Unalterable
-Lowcostforlargequantityproduction-->usedinthefinalproducts
PROM(ProgrammableROM)
-StoreinfoelectricallyusingPROMprogrammerattheuser’ssite
-Unalterable
-HighercostthanROM->usedinthesystemdevelopmentphase
->Canbeusedinsmallquantitysystem
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10. EPROM(ErasablePROM)
-StoreinfoelectricallyusingPROMprogrammerattheuser’ssite
-Storedinfoiserasable(alterable)usingUVlight(electricallyinsomedevices)and
rewriteable
-HighercostthanPROMbutreusable-->usedinthesystemdevelopmentphase.Not
usedinthesystemproductionduetoerasability
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11. Programmable ReadOnlyMemory(PROM)isaread-onlymemorythatcan
beprogrammed directlybytheuserusingaspecialPROMprogrammer.Itisalso
sometimescalledafusible-linkPROM.
Aprogrammable read-onlymemory(PROM)orfieldprogrammable read-only
memory(FPROM)orone-timeprogrammablenon-volatilememory(OTPNVM)isaform
ofdigitalmemorywherethesettingofeachbitislockedbyafuseorantifuse.Itisoneof
thetypeofROM(read-onlymemory).Thedatainthemispermanentandcannotbe
changed.PROMsareusedindigitalelectronicdevicestostorepermanentdata,usually
lowlevelprogramssuchasfirmware(microcode).
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12. ThekeydifferencefromastandardROMisthatthedataiswrittenintoaROMduring
manufacture,whilewithaPROMthedataisprogrammedintothemaftermanufacture.
Thus,ROMstendtobeusedonlyforlargeproductionrunswithwell-verifieddata,while
PROMsareusedtoallowcompaniestotestonasubsetofthedevicesinanorder
beforeburningdataintoallofthem.
PROMsaremanufacturedblankand,dependingonthetechnology,canbe
programmedatwafer,finaltest,orinsystem.BlankPROMchipsareprogrammedby
pluggingthemintoadevicecalledaPROMprogrammer.Theavailabilityofthis
technologyallowscompaniestokeepasupplyofblankPROMsinstock,andprogram
thematthelastminutetoavoidlargevolumecommitment
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13. Thesetypesofmemoriesarefrequentlyusedinmicrocontrollers,videogameconsoles,
mobilephones,radio-frequencyidentification(RFID)tags,implantablemedicaldevices,
high-definitionmultimediainterfaces(HDMI)andinmanyotherconsumerandautomotive
electronicsproducts.
AtypicalPROMcomeswithallbitsreadingas"1".Burningafusebitduringprogramming
causesthebittoreadas"0".Thememorycanbeprogrammed justonceafter
manufacturingby"blowing"thefuses,whichisanirreversibleprocess.
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14. AnEPROM(rarelyEROM),orerasableprogrammableread-onlymemory,isatypeof
memorychipthatretainsitsdatawhenitspowersupplyisswitchedoff.Computer
memorythatcanretrievestoreddataafterapowersupplyhasbeenturnedoffand
backoniscallednon-volatile.Itisanarrayoffloating-gatetransistorsindividually
programmedbyanelectronicdevicethatsupplieshighervoltagesthanthosenormally
usedindigitalcircuits.
Onceprogrammed,anEPROMcanbeerasedbyexposingittostrongultravioletlight
source(suchasfromamercury-vaporlight).EPROMsareeasilyrecognizablebythe
transparentfusedquartzwindowinthetopofthepackage,throughwhich
thesiliconchipisvisible,andwhichpermitsexposuretoultravioletlightduringerasing.
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15. Forlargevolumesofparts(thousandsofpiecesormore),mask-programmedROMsarethe
lowestcostdevicestoproduce.However,theserequiremanyweeksleadtimetomake,since
theartworkforanICmasklayermustbealteredtostoredataontheROMs.Initially,itwas
thoughtthattheEPROMwouldbetooexpensiveformassproductionuseandthatitwouldbe
confinedtodevelopmentonly.Itwassoonfoundthatsmall-volumeproductionwas
economicalwithEPROMparts,particularlywhentheadvantageofrapidupgradesof
firmwarewasconsidered.
Somemicrocontrollers,frombeforetheeraofEEPROMsandflashmemory,usean
on-chipEPROMtostoretheirprogram.Suchmicrocontrollersincludesomeversions
oftheIntel8048,theFreescale68HC11,andthe"C"versionsofthePIC
microcontroller.LikeEPROMchips,suchmicrocontrollerscameinwindowed
(expensive)versionsthatwereusefulfordebuggingandprogramdevelopment
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16. EEPROM(alsowrittenE
2
PROMandpronounced"e-e-prom","double-e-prom"or"e-
squared-prom")standsforElectricallyErasableProgrammableRead-OnlyMemoryandis
atypeofnon-volatilememoryusedincomputersandotherelectronicdevicestostore
relativelysmallamountsofdatabutallowingindividualbytestobeerasedand
reprogrammed.
EEPROMsareorganizedasarraysoffloating-gatetransistors.EEPROMscanbe
programmedanderasedin-circuit,byapplyingspecialprogrammingsignals.Originally,
EEPROMswerelimitedtosinglebyteoperationswhichmadethemslower,butmodern
EEPROMsallowmulti-bytepageoperations
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17. Italsohasalimitedlifeforerasingandreprogramming,nowreachingamillionoperations
inmodernEEPROMs.InanEEPROMthatisfrequentlyreprogrammedwhilethecomputeris
inuse,thelifeoftheEEPROMisanimportantdesignconsideration.
Unlikemostotherkindsofnon-volatilememory,anEEPROMtypicallyallowsbytestobe
read,erased,andre-writtenindividually.EPROMsareerasedbyexposingachiptoultra-
violetlighttoeraseitsentirecontents.FlashEPROMsareelectricallyerasedand
programmedbutonlyasgroupsofbytes,rangingfromtenstotensofthousandsofbytes
fordifferentdevices.
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18. EachEEPROMdevicetypicallyhasitsownsetofOP-Codeinstructionsmappedtodifferent
functions.CommonoperationsonSPIEEPROMdevicesare:
I.WriteEnable(WRENAL)
II.WriteDisable(WRDI)
III.ReadStatusRegister(RDSR)
IV.WriteStatusRegister(WRSR)
V.ReadData(READ)
VI.WriteData(WRITE)
OtheroperationssupportedbysomeEEPROMdevicesare:
I.Program
II.SectorErase
III.ChipErasecommands
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19. WhatisthedifferencebetweenPROMandEPROM?
•PROMistheoldertechnologywhilebothPROMandEPROMarenon-volatilememory
devices.
•PROMscanbeprogrammed onlyoncewhileEPROMsarereusableandcanbe
programmedmultipletimes.
•TheprocessintheprogrammingofPROMSisirreversible;hencethememoryispermanent.In
EPROMsmemorycanbeerasedbyexposuretoUVlight.
•EPROMshaveafusedquartzwindowinthepackagingtoallowthis.PROMsareenclosedin
completeplasticpackaging;thereforeUVhasnoeffectonPROMs
•InPROMsdataiswritten/programmedontothechipbyblowingthefusesateachbitusing
muchhighervoltagesthantheaveragevoltagesusedindigitalcircuits.EPROMSalsousehigh
voltage,butnotenoughtoalterthesemiconductorlayerpermanently.
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20. WhatisthedifferencebetweenEEPROMandEPROM?
•EPROMshavetobeerasedwithexposuretoUVlightandEEPROMscanbeerased
electronically.
•EPROMshaveaQuartzwindowinthepackagetoexposethechiptoUVlightand
EEPROMsarecompletelyencasedinanopaqueplasticcase.
•EPROMistheoldertechnology.
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21. AprogrammablelogicdeviceworkslikeaROMbutisamoreefficientsolutionfor
implementingsparseoutputfunctions.(Notallmintermsaredecoded.)
Therearetwotypesofprogrammablelogicdevices:
1)PLA(ProgrammableLogicArray)
2)PAL(ProgrammableArrayLogic)
WesuggestedearlierthataROMhadadecoderinsideit.Youcouldvisualizethisas:
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22. TheimageabovealsodefinestwotermswewillusetodistinguishbetweenPLAandPAL
devices:
•ANDArray-thisistheportionofthedevicethatdecodestheinputs.TheANDarray
determinesthemintermsdecodedbythedevice.AROMdecodesallpossibleminterms.
•ORArray-thisistheportionofthedevicethatcombinesthemintermsforthedefinition
ofafunction.
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23. PLA:
•NowwearereadytodefineaPLA.
•APLAisaprogrammablelogicdevicewithaprogrammableANDarrayandaprogrammable
ORarray.
•APLAwithninputshasfewerthan2
n
ANDgates(otherwisetherewouldbenoadvantageover
aROMimplementationofthesamesize).APLAonlyneedstohaveenoughANDgatesto
decodeasmanyuniquetermsasthereareinthefunctionsitwillimplement.
•BecausewecancontroltheANDarrayandthereisalimittothenumberoftermsthatcanbe
specifiedintheANDarray,itmaybemoreeconomicaltosimplifythefunctionbefore
implementingitwithaPLA.IfyoudosimplifythefunctionandintendtoimplementwithaPAL
deviceyoushouldalsokeepinmindthatproducttermscanbesharedbetweenfunctions.
(ProductsharingiswhentwofunctionsshareaproducttermdecodedbytheANDarray.For
example,intheimagebelowtheproducttermABissharedbetweenF
0
andF
1
.)
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24. Example:ImplementthefunctionsF
0F
1weintroducedaboveusingaPLAwith2inputs,3product
terms,and2outputs.
•Theunprogrammed PLAfromthemanufacturelookslike:
•AfterprogrammingforthetwofunctionsF
0F
1thestateofthePLAis:
•
•NoticethatweonlyneedthreeANDgates
becausethereareonlythreeuniquemintermsin
thefunctionsF
0andF
1.Also,noticethatsincewe
havecontrolovertheORarraywecansharethe
mintermABinthedefinitionsofbothfunctions.
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25. PAL:
•APALisaprogrammablelogicdevicewithaprogrammableANDarrayandafixedOR
array.
•APALhasafixedORarray.Forexample,hereiswhatanunprogrammed PALmightlook
likestraightfromthemanufacture:
•AfixedORarraymakesthedevicelessexpensivetomanufacture.Ontheotherhand,
havingafixedORarraymeansyoucan'tshareproducttermsbetweenfunctions.
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26. Example:ImplementthefunctionsF
0F
1weintroducedaboveusingthePALgivenabove.
ForthisimplementationwewillneedtosimplifythefunctionsF
0F
1becausethePALwe
aregivenhasanoutputfunctionthatcanaccommodate onlyoneproductterm.The
simplifiedformofthefunctionsare:
F
0=A
F
1=A'B'+AB
•AfterprogrammingforthetwofunctionsF
0F
1thestateofthePALis:
So,insummary:
•APLAdevicehasaprogrammableANDandprogrammableORarray
•APALdevicehasaprogrammableANDandfixedORarray
•(YoucouldalsosaythataROMhasafixedANDandprogrammableORarray)
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27. –Bill Gates, founder, Microsoft
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