Tải bản đầy đủ - 0 (trang)
Chapter 21. PC Mods: Overclocking and Cooling

Chapter 21. PC Mods: Overclocking and Cooling

Tải bản đầy đủ - 0trang

theback.Bringingmoderninterfacestothefrontpanel;

adaptingnewerdrivesfordifferentformfactors;andadding

windows,vents,andfansareallmodificationsthatcanbemade

toachassisorcasetoimproveorupdateittohandlethelatest

hardware.

Finally,youcanperformpurelycosmeticmodifications,suchas

custompaintingorinternallighting,whichcanmakeyour

systemstandoutfromtherest.Icoverthefunctional

modificationsinthischapter,withsoundengineeringprinciples

appliedsuchthatyoucaneithermodifyanexistingsystem

correctlyorpurchasenewcomponentswiththedesiredfeatures

alreadyintegrated.







Overclocking

Oneofthemostpopularperformance-enhancingmodifications

ofalltimeisoverclocking,whichcanbedefinedasrunningall

orpartofthesystemfasterthanitwasoriginallyratedor

intendedtorun.Overclockingisusuallyappliedtothe

processor,butitcanalsobeappliedtoothercomponentsinthe

system,includingmemory,videocards,busspeeds,andmore.

OverclockingPCsdatesallthewaybacktotheoriginal4.77MHz

IBMPCand6MHzATsystemsoftheearly1980s.Infact,IBM

madeoverclockingtheATeasybecausethequartzcrystalthat

controlledthespeedoftheprocessorwassocketed.Youcould

obtainafasterreplacementforaboutadollarandeasilyplugit

in.Thefirstseveraleditionsofthisbookcoveredhowto

performthismodificationindetail,resultinginasystemthat

wasupto1.5timesfasterthanitstartedout.Modernsystems

allowoverclockingwithoutreplacinganypartsbyvirtueof

simpleandeasy-to-changeBIOSSetupoptions.



QuartzCrystals

Tounderstandoverclocking,youneedtoknowhowcomputer

systemspeedsarecontrolled.Themaincomponentcontrolling

speedinacomputerisaquartzcrystal.Quartzissilicondioxide

(SiO2)incrystallineform.Oxygenandsiliconarethemost

commonelementsonearth(sandandrockaremostlysilicon

dioxide),andcomputerchipsaremademainlyfromsilicon.

Quartzisahard,transparentmaterialwithadensityof2649

kg/m3(1.531oz/in3)andameltingpointof1750°C(3182°F).

Quartzisbrittlebutwithalittlebitofelasticity,whichis

perhapsitsmostusefulattribute.

Incrystallineform,quartzcanbeusedtogenerateregularand



consistentsignalpulsestoregulateelectroniccircuits,similarto

thewayametronomecanbeusedtoregulatemusic.Quartz

crystalsareusedbecausetheyarepiezoelectric,whichis

definedashavingapropertythatgeneratesavoltagewhen

subjectedtomechanicalstressandthatalsogenerates

mechanicalstresswhensubjectedtoavoltage.Piezoelectricity

wasfirstdiscoveredbyPierreandJacquesCuriein1880,andit

istheessentialfeatureofquartzthatmakesitusefulin

electroniccircuits.

Thekeytopiezoelectricityisthatwhenyoutwist,bend,deform,

orsimplyapplypressureorstresstoaquartzcrystal,asmall

voltageisgenerated.Thispropertyiscommonlyusedinsensors

ortransducerstodetectpressureorsoundwaves.Forexample,

theknocksensorsusedinvirtuallyallmodernautomobile

enginesarebasedonquartzcrystalsthataresensitivetothe

vibrationsproducedintheengineblockwhendetonation

(uncontrolledburningoffuel)occurs.Becausedetonationcan

rapidlydamageanengine,theenginemanagementcomputer

usesinputfromoneormoreknocksensorstoretardthe

ignitiontiming,thuspreventingenginedamage.Some

microphonesandphonographpickupdevicesalsousequartz

crystalsforconvertingsoundwavesorthemotionoftheneedle

inthegrooveintoavoltage.

Piezoelectricityworkstwoways,meaningthatifavoltageis

generatedwhenyoubendacrystal,likewiseifyouapply

voltagetoacrystalitbends(contracts,expands,ortwists)ina

similarfashion.Althoughthecrystalismostlybrittleinnature,

itisstillsomewhatelastic,suchthatanydeformationtendsto

snapbackandthenoccuragain,resonatingatanatural

frequencyaslongasthevoltageispresent.Muchlikeatuning

forkorthepipeinanorgan,thenaturalresonantfrequency

dependsonthesizeandshapeofthecrystal.Ingeneral,the

smallerandthinneritis,thefasteritvibrates.

Theactualmovementisexceedinglysmall,ontheorderof68

nanometers(billionthsofameter)percentimeter,whichina



normalcrystalisonlyafewatomsinlength.Althoughthe

movementisverysmall,itisalsoquiterapid,whichmeans

tremendousforcescanbegenerated.Forexample,thesurface

accelerationofa50MHzcrystalcanexceedfivemilliontimes

theforceofgravity.

Crystalresonatorsaremadefromslabsofquartzsawedfrom

rawquartzcrystalstock.Althoughthestockcancomefrom

naturalquartz,mostcrystalsaremadefromsyntheticallygrown

quartz.Therawstockslabsarecutintosquares,rounded,and

groundintoflatdiscscalledblanks.Thethinnerthedisc,the

highertheresonantfrequency;however,therearelimitsasto

howthinthediscscanbemadebeforetheybreak.Theupper

limitforfundamentalmoderesonatorsisapproximately50MHz.

Atthatfrequency,thediscsarepaperthinandaregenerallytoo

fragiletowithstandfurthergrinding.Still,higher-frequency

crystalscanbeachievedbyusingharmonicsofthefundamental

frequency,resultingincrystalsofupto200MHzormore.Even

higherfrequenciescanbeachievedbyusingfrequency

synthesizercircuits,whichuseabasecrystal-generated

frequencyfedtoacircuitthatthengeneratesmultiplesof

frequencythatcanextendwellintothegigahertzorterahertz

range.Infact,crystal-basedfrequencysynthesizercircuitsare

usedtogeneratethehighoperatingspeedsofmodernPCs.

Thecrystalpackages,aswellastheshapeoftheactualquartz

crystalsinsidethepackages,canvary.Thepackagesareusually

ametalcanthatiseithercylindricaloroblonginshape,but

theycanalsohaveothershapesorbeconstructedofplasticor

othermaterials(seeFigure21.1).



Figure21.1.Crystalpackagesofvaryingshapes.



Thesliverofquartzinsidethepackageisnormallydiscshaped,

butitisshapedlikeatuningforkinsomeexamples.Figure

21.2showsacylindricalcrystalpackagewiththecover

removed,exposingthetuningforkshapedsliverofquartz

inside.



Figure21.2.Crystalinteriorshowingthequartz

tuningfork.



Mostcrystalsuseadisc-shapedsliverofquartzasaresonator.

Thedisciscontainedinahermeticallysealedevacuated

enclosure.

Figure21.3showstheinteriorviewofatypicalcrystalwitha

disc-shapedresonatorinside.



Figure21.3.Crystalinteriorshowingdisc-shaped



quartzresonator.



Thequartzdiscinsidehaselectrodesoneachside,allowinga

voltagetobeappliedtothedisc.Thedetailsareshownin

Figure21.4.



Figure21.4.Disc-shapedquartzresonatordetails.



WalterG.Cadywasthefirsttouseaquartzcrystaltocontrol

anelectronicoscillatorcircuitin1921.Hepublishedhisresults



in1922,whichledtothedevelopmentofthefirstcrystalcontrolledclockbyWarrenA.Marrisonin1927.Today,all

moderncomputershavemultipleinternaloscillatorsandclocks,

someforcontrollingbusandprocessorspeedsandatleastone

forastandardtimeofdayclock.



OverclockingHistory

Overclockinghasbeenaroundsincethebeginningof

computing.Aslongastherehavebeencomputers,therehave

beenpeopletryingtomakethemrunfaster.Ibegan

overclockingPCsbackintheearly1980s,usingavarietyof

devicesonthemarket.Theeasiestsystemtooverclockwasthe

originalIBMAT,whichusedasocketedcrystaltorunthe

processor.TwoversionsoftheXTwereavailableonethatuseda

12MHzcrystalandonethatuseda16MHzcrystal.Thetimer

chipdividedthecrystalspeedby2,resultinginprocessor

speedsof6MHzand8MHz.

Apopulartrickatthetimewastopopthe12MHzor16MHz

crystalsoutoftheirsocketsandreplacethemwith18MHzor

20MHzcrystals,whichspedupthesystemsto9MHzor10MHz.

Backin1984,Iwasabletopurchasecrystalsforaslittleas$1

eachfromRadioShack,andtheycouldbechangedinseconds.

Inmy6MHzsystem,Iwasabletousean18MHzcrystaltoget

thesystemtorunat9MHz.Thatwasa50%increaseinspeed

forabout$1!Ihadtrieda20MHzcrystal(10MHzspeed),but

thesystemwouldnotbootupatthatspeed,soIhadtodrop

backtothe18MHz(9MHzspeed)crystalinstead.

Takingthisfurther,somecompaniesreleasedvariablefrequency

oscillators,whichbasicallyamountedtoavariablespeedcrystal

youcouldusetoreplacethefixedstockcrystal.Acontrolpanel

wasthenmountedonthebackofthesystem,whichhadan

adjustmentknobyoucouldusetochangethesystemspeed.

ThemostsophisticatedofthesewastheXCELX,whichallowed



youtoadjusttheoriginalIBMATfrom6.5MHzto12.7MHz.

Figure21.5showstheXCELX,whichdatedfrom1985.



Figure21.5.TheXCELXfrequencyadjuster,a

"throttle"thatcouldbeusedtospeeduptheIBM

AT.



Theinstructionstoldyoutoturnupthethrottleuntilthesystem

crashedandthenbackdownanotchortwo.Howfastagiven

systemwouldrundependedonallthespecificcomponents

containedwithin.



Tip

OneprobleminthevintageIBMATandXT-286

systemswasthattheclockspeedwaschecked

duringthePowerOnSelfTest(POST).Thiscaused

problemsifyoureplacedthesocketedclockcrystal



inthesesystemsforacheapandeasyspeedup.In

IBMATandXT-286systems,thePOSTcheckedthe

systemrefreshrate(clockspeed)toensurethatit

was6MHzor8MHz,dependingonwhichsystemyou

had.Amarginallyfasterorslowerratecausedthe

testtofailandresultedinaPOSTerrorofonelong

andoneshortbeep,followedbyahalt(HLT)

instruction.

Toeliminatethetestandenableafaster-thannormalclockrate,youhadtopatchtheinstruction

attheproperlocationfroma73h(JAEJumpifAbove

orEqual)toanEBh(JMPJumpunconditionally).

Whenthisinstructionwaschanged,thetestnever

fellintotheerrorroutine,nomatterhowfastthe

rate.TheJAEinstructionoccurredatF000:05BCin

IBMATsystemswiththe06/10/85or11/15/85ROM

BIOSversionsandatF000:05C0inXT-286systems.

NotethattheoriginalIBMATsystemwiththe

01/10/84BIOSdidnothavethistest.Bycreatinga

newsetofROMchipswiththisinstructionchanged

byanEPROMprogrammer,youcouldeliminatethis

speedcheckandenablefasterclockrates.



Fewpeopleareinterestedinoverclockingsystemsthatare

morethan20yearsold,butitisinterestingtoseethat

hotroddingPCshasbeenaroundforaslongasPCsthemselves

haveexisted!



ModernPCClocks

AmodernPChasatleasttwocrystalsonthemotherboard;the



maincrystalisusedtocontrolthespeedofthemotherboard

andmotherboardcircuitry,andtheotherisusedtocontrolthe

realtimeclock(RTC).Themaincrystalisalways14.31818MHz

(itmightbeabbreviatedas14.318orjust14.3),andtheRTC

crystalisalways32.768KHz.



Why14.31818MHz?

Theoriginal1981vintageIBMPCranat4.77MHz,aspeedderivedbytakinga

14.31818MHzcrystalandusingadividercircuittodividethefrequencyby3to

get4.77MHz.Thesame14.31818MHzcrystalwasalsodividedby4toget

3.58MHz,whichistheexactfrequencyneededfortheNTSCcolorvideo

modulationsignalrequiredtobecompatiblewithcolorTV.Anothercircuitdivided

thecrystalfrequencyby12toget1.193182MHz,whichwasusedbyan8253

programmablethree-channel16-bitintervaltimer/counterchip.Eachchannel

couldbeusedtotakeaninputclocksignalandproduceanoutputsignalby

dividingbyanarbitrary16-bitnumber.Channel0wasusedtomakethetimeof

dayclockticks.ItwasprogrammedbytheBIOStocallINT08hevery65,536

ticks,whichwasabout18.2timespersecond(oraboutevery55milliseconds).

ThesoftwareroutineslinkedtoINT08hcausedthetimeofdayclocktobe

updatedandcouldalsochaintoanyotheractivitiesthatneededtobedone

periodically.Channel1wasusedtotelltheDMAtorefreshthedynamicRAM

every72cycles(about15microseconds),andchannel2wasusedtomakean

audiosignalforthespeaker(beeps)differenttonescouldbemadebychanging

thedivisor.

Asatestamenttocomputerevolution,allmodernPCsarestillcontrolledbya

14.318MHzcrystal!Thecrystal,inconjunctionwithafrequencytiminggenerator

chip,isusedtoderivevirtuallyallthefrequenciesusedonamodern

motherboardbytheCPUbus,PCIbus,AGPbus,memorybus,andUSB.



PCsdon'trunat14.318MHz,sohowcanacrystalofthatspeed

beusedtocontroltheirspeed?Andwhathappenswhenyou

installadifferentprocessor?Howdoesthesystemadjustthe

busandotherspeedstoaccommodatethenewchip?The

answeristhataspecialchipcalledafrequencytiminggenerator

(FTG)orfrequencysynthesizerisusedinconjunctionwiththe

crystaltoderivetheactualspeedsofthesystem.Figure21.6

showsaportionofamotherboardwithanFTGchipwitha

14.318MHzcrystalbelowit.



Figure21.6.Frequencysynthesizerchipwitha

14.318MHzcrystal.



Tài liệu bạn tìm kiếm đã sẵn sàng tải về

Chapter 21. PC Mods: Overclocking and Cooling

Tải bản đầy đủ ngay(0 tr)

×