Flash Memory

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NAND vs.I'loRflashmemory cells

howitworks, Elash momoryis on6olth€ mostLrs€ful, I andubiqlitous,t€ch nv€niionsorth€ lasi 25 yea6. lt's becomoth6 podablestorag6m€diaoi cholcofor emald€vio€ssuch aBUSBthumbdnv€s,dlgitalcam€fas, o€ll phon€s,anddigitalm€daplay€rc, and t lookspols€dto f€plac6thospinnlnghard drivosin lapiops. AlthoughToshba Inv€rt6dth6 t€ohnotogy,Ints wasthe ilrst oompanyto niroduc€ a commercidflash momorychip, andit cur l€rtly holdethe lion'sshar€oi th6 mafk€i (tollowed bySamsung). Thbt€chnology wasdubb€d"tlash"bocauselh€ prccossof oiaEingitg conbnts 6vok€dlmsg€eotnash phoiogEphy.Flashmsmoryls a nonvolatilE (m6anhgn do63not requllselectdcalpow6r to r€laintho intormailonslorcd li it)typo ot (Eeclically E|asebl€Prcgrammable EEPBOIV Fsad-ony M€mod lhai can b6 6ras€dand prcgEmmedi la€e blocks.Othff typ€sof EEPFIOI'/S, suohas thoseusedio storoa BIOS,mustb6 6Esed in iho r sntlBty b€loE newdatacan b€ wdtt6n.Flashmemorysto6s datain an aray ot c6llswith a uansistorat 6achlnt6€6ctionof a rcw and column.t erasesandwit€s dala ln chrfks, instsadof ono byt6at a iime, so it doss nol sufferJpm the samelimitationiypica or EEPBOI,IS. FlashmemorysiorBsdaia in an arrayof c€ ls consistingoi two transistoF-a iloating gai€ and a conirol gat€-sgparated by athin layorofoxld6insulation, Whens!fncientvo[ageis appliedto the contro]gat€, excit€delectronsare pushedihroughthe oxids ayd(in a pbcessknownas iunneling") andaccumulate insideth6floatirs gate.Whenthishappens, the ceilchang€s its stored b i valueirom 0 to 1. Undernormal conditions, the elecirons trappedon the othersideoJtheioatinggatswon'tdis-

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€r.Forth€s6f6asons,NOAfla6hmamoryis oil€nd6ploy6din handhslddsvlcEB, slch as Slngl€'lev6l c6ll(SLC)d€vicosslo€ one cellphon€s, and n 6mb6dded sysl€ms. bli ot daiain 6achcell.Muliil€vel call(l/LC) Toshlba d€v€op€dNANDtlashr.smory d6vc6s6af sio€ rnof6thanon€bit of data laier,but t lacksih€ random-accsssnarur6 in sachc€ | by accumulatlngdltf6r6nil6v€ls oi NOAm€moryThatprcdudes it ffom us6 or oloovloal chargEin5idolh6 tloatlnggat€. as a cplacerilent tor syst€mROI\,4. On rho A hvo-bltc6l, torinstancs, candEtinguish otherhand,NANDboastsmuchiast€rsras€ beiw€€nlow distnotvotiag€s. andwdtsllm€s,andlt pbvid€sgr€al€r sloraged€nsityand a loweroosi psr blt oi IT'S LOGICAL sto€96. NANDcannol b6 partition€dand ^4emory-chlp designors us€onsottwo data musl b6 f€adflorn il ono s€gm€ntat mgthodsof d6tamapplng, NANDor NOR a I mo.Th€s€charactor sticsrond€fNAND {thos6r€ms dBscibethoiyp€ of logicgat6s simil lo olhsltyp€sof E€coidarysionge, deploy€d), io int€rconn6ct mBmory cells. suchas had dives andopucaldiscs. You'll N€th€rapproachis univorsally supghor rindNANDmomorylnCompactFlash(CD, raiher,eachls bst suiledior a partlcular M6morySiiok,l',{ultllvl€dlaoard (Ml,lc),al applicalon. NORflashmomoryc6llsaf€ (SD),andxD-Pciure tormsoi SeoureDlgilal conn6ct€d in parallol, so €achIndividualcell Cad modia.Ih€t€chnology hasalsob€6n canbe readandprcgEmmedindividually. tappedforuseinsolld-statg had d/ves. NANDcellsareconnect€d in s€desand must be r6adfrom and wdnento in s6ri€s, FLASHFILESYSTEMS Theupshotls ihat datacanbe readtbm Mosl removableflashmediahasan 6mb€dNOFflashin muchlhs sam€waythai ii! ded mlcrocontro 16rThisenables SD, f€adrrcm Gndom-accessmgmory(MlV). CompaslFlash, andUSBthumbdrivs io be Thanks10this attribute,most microproces- romatied usingth6 familiarFATfile system solscan useNORfash m€moryd execuie- thatlvllcrcsoft deveopedfor MS,DOS. in-place(XlP)memoryInoiherwords,NOB Flashmemoryd€vicosthatdor t have tlash memorycan storeand sx€cutesoflan embeddsd m crccontrcl eriypicailyuse wa€ programswitholr wfiting the instrucFTL(FlashTEnslationLay€r),a Jilesysiem tionsintoFAMf rst,NoF nashmemorycan that makesaflashmemorydevicslook alsobepaditioned, so an applicaiion can likea FAT(Fll€Allocaiion Table)diskto tho runin onepartition whiledatais simultane- operatingsyst€mbut that also pe'Joms ously read,writtenio, or erasedlrom anoth- wearlev€ring. JFFS2(Joumaling Fl6h Fil6

Systemvers.2) was developedror using Iash devlceswth Lnuxiit supportsboth {iPEED ANN UURJ\AILN'ff prodTheetailpice ofNANDliash-memory ucts sD, cF,xD,andso on is deierrnined anosPeeo largelyby two iactorsrcapacrty Thefrst concopts easyto lndersianditlie secondis rnoreconiusngthaniishouldbe m€6ureperlor b6causemanufactureF mancein d ifsferl ways. Patrioltr,l€moryfor instance,Etes lhe card(pad speedof lts 2GBCompactFlash slreetprce$2r)as numberPSF2G5oCE be ng "50x-"Readthe specsheot,howevei andyou'lLlind lhat ths 50x relerson y lo the readsp6€d;no Eting s q venfof w.l6 speed. Assumins1x s 150KB/s(withKB/sderined thedatatfansas 1,024byt6spersecond, fer speedoi the f rst compact-dsc drvd), Patriot'scardis capabl€of transfedngdatato its hostdeviceat a rat€of 7.32[,lB/s. onlheotherhand,l stssepa' SanDisk, rale€ad andwrilesp€edsiof ils Ut|a ll card(slrealpr oe$36)or 2GBCompactFlash 1olvB/sand 9[48/s,Bpecllv€ly But since 1ssp€cchartme*u€s 1tr48asonsrnilion bj,,ies,you'llneedto do a I ttle mathto obtan (usis Patdoisteminol atruscomparison. ogy,SanDsk'sdsv c€ doLivec€ad spo€dsol 9.53148/s andwit€ spe€dsot 8.58M8/s.) NoF andNANDlash m€morybothhavo a I n te numb€roi €ras€-witecycl€s-anywh6€ batw€€n10,000andon€mllllon-but lhis imitaion s tyPcallyolfsetby wear lev€Llng nstruciionsn iimwareihal couni the numberof writscyclesfor 6achcel rernaplhe bocksso thai anddynamically evenly erasufas andMl€s aredishibutsd NANDllashmemoryd€vceshav€ l\,4any 6r lhat p€riorms an embedded microconirol a rolt ne knownas bad blockmanagement ih€ mcrocontror6r lla wrte op€raiioniails, can€map th€ datalo aspafesectorol memoryLnfaci,mostNANDdsvicesare knownto havebad blockswhoniheyeave thefactory,butsinc€lhosebad b ocksare known,ihey'remappedsotheydonigei andhold wrttento. Ths helpsincreaseyield downcostsbecausenoleverycomponent needstobe perlectas longasthedevice delivefsat leastth€ amouniof storageits clalmslor it. manlfacturer w6af Thanksiobad blockmanagement, ol anyrnovng leveling, andtheabs€nce paf6,moslf ashmemorymediashouldo!tast any devicathat us6 ii. Thal'snot io say howeveri anyonelsing t to stoe it s inialllbLe, conlentwoud d gtalpholosor othervaluable be w€ | advisedio makebackupcopies.m8

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