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7005S20PFGB Datasheet(PDF) 18 Page - Integrated Device Technology

Part # 7005S20PFGB
Description  HIGH-SPEED 8K x 8 DUAL-PORT STATIC RAM
Download  21 Pages
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Manufacturer  IDT [Integrated Device Technology]
Direct Link  http://www.idt.com
Logo IDT - Integrated Device Technology

7005S20PFGB Datasheet(HTML) 18 Page - Integrated Device Technology

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6.42
IDT7005S/L
High-Speed 8K x 8 Dual-Port Static RAM
Military, Industrial and Commercial Temperature Ranges
18
cause either signal (SEM or OE) to go inactive or the output will
never change.
AsequenceWRITE/READmustbeusedbythesemaphoreinorder
to guarantee that no system level contention will occur. A processor
requestsaccesstosharedresourcesbyattemptingtowriteazerointoa
semaphorelocation.Ifthesemaphoreisalreadyinuse,thesemaphore
requestlatchwillcontainazero,yetthesemaphoreflagwillappearasone,
a fact which the processor will verify by the subsequent read (see Truth
TableV).Asanexample,assumeaprocessorwritesazerototheleftport
at a free semaphore location. On a subsequent read, the processor will
verifythatithaswrittensuccessfullytothatlocationandwillassumecontrol
overtheresourceinquestion.Meanwhile,ifaprocessorontherightside
attempts to write a zero to the same semaphore flag it will fail, as will be
verifiedbythefactthataonewillbereadfromthatsemaphoreontheright
side during subsequent read. Had a sequence of READ/WRITE been
usedinstead,systemcontentionproblemscouldhaveoccurredduringthe
gap between the read and write cycles.
Itisimportanttonotethatafailedsemaphorerequestmustbefollowed
by either repeated reads or by writing a one into the same location. The
reasonforthisiseasilyunderstoodbylookingatthesimplelogicdiagram
ofthesemaphoreflaginFigure4.Twosemaphorerequestlatchesfeed
into a semaphore flag. Whichever latch is first to present a zero to the
semaphoreflagwillforceitssideofthesemaphoreflagLOWandtheother
side HIGH. This condition will continue until a one is written to the same
semaphorerequestlatch.Shouldtheotherside’ssemaphorerequestlatch
havebeenwrittentoazerointhemeantime,thesemaphoreflagwillflip
overtotheothersideassoonasaoneiswrittenintothefirstside’srequest
latch.Thesecondside’sflagwillnowstayLOWuntilitssemaphorerequest
latchiswrittentoaone.Fromthisitiseasytounderstandthat,ifasemaphore
is requested and the processor which requested it no longer needs the
resource, the entire system can hang up until a one is written into that
semaphorerequestlatch.
The critical case of semaphore timing is when both sides request a
single token by attempting to write a zero into it at the same time. The
semaphorelogicisspeciallydesignedtoresolvethisproblem.Ifsimulta-
neousrequestsaremade,thelogicguaranteesthatonlyonesidereceives
the token. If one side is earlier than the other in making the request, the
firstsidetomaketherequestwillreceivethetoken.Ifbothrequestsarrive
at the same time, the assignment will be arbitrarily made to one port or
the other.
One caution that should be noted when using semaphores is that
semaphoresalonedonotguaranteethataccesstoaresourceissecure.
Aswithanypowerfulprogrammingtechnique,ifsemaphoresaremisused
or misinterpreted, a software error can easily happen.
Initializationofthesemaphoresisnotautomaticandmustbehandled
viatheinitializationprogramatpower-up.Sinceanysemaphorerequest
flagwhichcontainsazeromustberesettoaone,allsemaphoresonboth
sidesshouldhaveaonewrittenintothematinitializationfrombothsides
to assure that they will be free when needed.
UsingSemaphores—SomeExamples
Perhapsthesimplestapplicationofsemaphoresistheirapplicationas
resourcemarkersfortheIDT7005’sDual-PortRAM.Saythe8Kx8RAM
wastobedividedintotwo4Kx8blockswhichweretobededicatedatany
onetimetoservicingeithertheleftorrightport.Semaphore0couldbeused
toindicatethesidewhichwouldcontrolthelowersectionofmemory,and
configurations.TheIDT7005doesnotuseitssemaphoreflagstocontrol
anyresourcesthroughhardware,thusallowingthesystemdesignertotal
flexibilityinsystemarchitecture.
An advantage of using semaphores rather than the more common
methodsofhardwarearbitrationisthatwaitstatesareneverincurredin
either processor. This can prove to be a major advantage in very
high-speedsystems.
How the Semaphore Flags Work
Thesemaphorelogicisasetofeightlatcheswhichareindependent
oftheDual-PortRAM.Theselatchescanbeusedtopassaflag,ortoken,
fromoneporttotheothertoindicatethatasharedresourceisinuse.The
semaphores provide a hardware assist for a use assignment method
called“TokenPassingAllocation.”Inthismethod,thestateofasemaphore
latchisusedasatokenindicatingthatsharedresourceisinuse.Iftheleft
processorwantstousethisresource,itrequeststhetokenbysettingthe
latch.Thisprocessorthenverifiesitssuccessinsettingthelatchbyreading
it. If it was successful, it proceeds to assume control over the shared
resource.Ifitwasnotsuccessfulinsettingthelatch,itdeterminesthatthe
rightsideprocessorhassetthelatchfirst, hasthetokenandisusingthe
sharedresource.Theleftprocessorcantheneitherrepeatedlyrequest
thatsemaphore’sstatusorremoveitsrequestforthatsemaphoretoperform
anothertaskandoccasionallyattemptagaintogaincontrolofthetokenvia
thesetandtestsequence.Oncetherightsidehasrelinquishedthetoken,
theleftsideshouldsucceedingainingcontrol.
ThesemaphoreflagsareactiveLOW.Atokenisrequestedbywriting
azerointoasemaphorelatchandisreleasedwhenthesamesidewrites
a one to that latch.
The eight semaphore flags reside within the IDT7005 in a separate
memoryspacefromtheDual-PortRAM.Thisaddressspaceisaccessed
byplacingaLOWinputontheSEMpin(whichactsasachipselectforthe
semaphore flags) and using the other control pins (Address, OE, and
R/W) as they would be used in accessing a standard static RAM. Each
of the flags has a unique address which can be accessed by either side
throughaddresspinsA0–A2.Whenaccessingthesemaphores,noneof
the other address pins has any effect.
Whenwritingtoasemaphore,onlydatapinD0isused.IfaLOWlevel
iswrittenintoanunusedsemaphorelocation,thatflagwillbesettoazero
on that side and a one on the other side (see Truth Table V). That
semaphorecannowonlybemodifiedbythesideshowingthezero.When
aoneiswrittenintothesamelocationfromthesameside,theflagwillbe
settoaoneforbothsides(unlessasemaphorerequestfromtheotherside
ispending)andthencanbewrittentobybothsides. Thefactthattheside
whichisabletowriteazerointoasemaphoresubsequentlylocksoutwrites
fromtheothersideiswhatmakessemaphoreflagsusefulininterprocessor
communications.(Athoroughdiscussionontheuseofthisfeaturefollows
shortly.) A zero written into the same location from the other side will be
storedinthesemaphorerequestlatchforthatsideuntilthesemaphoreis
freed by the first side.
Whenasemaphoreflagisread,itsvalueisspreadintoalldatabitsso
thataflagthatisaonereadsasaoneinalldatabitsandaflagcontaining
azeroreadsasallzeros.Thereadvalueislatchedintooneside’soutput
registerwhenthatside'ssemaphoreselect(SEM)andoutputenable(OE)
signalsgoactive.Thisservestodisallowthesemaphorefromchanging
stateinthemiddleofareadcycleduetoawritecyclefromtheotherside.
Becauseofthislatch,arepeatedreadofasemaphoreinatestloopmust


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