IDT7027S/L
High-Speed 32K x 16 Dual-Port Static RAM
Military, Industrial and Commercial Temperature Ranges
performanothertaskandoccasionallyattemptagaintogaincontrolofthe side during subsequent read. Had a sequence of READ/WRITE been
tokenviathesetandtestsequence.Oncetherightsidehasrelinquished usedinstead,systemcontentionproblemscouldhaveoccurredduringthe
thetoken,theleftsideshouldsucceedingainingcontrol.
gap between the read and write cycles.
Thesemaphoreflagsareactivelow.Atokenisrequestedbywriting
It isimportanttonotethatafailedsemaphorerequestmustbefollowed
azerointoasemaphorelatchandisreleasedwhenthesamesidewrites byeitherrepeatedreadsorbywritingaoneintothesamelocation.The
aonetothatlatch. reasonforthisiseasilyunderstoodbylookingatthesimplelogicdiagram
The eightsemaphore flags reside withinthe IDT7027ina separate ofthesemaphoreflaginFigure4.Twosemaphorerequestlatchesfeed
memoryspacefromtheDual-PortSRAM.Thisaddressspaceisaccessed into a semaphore flag. Whichever latch is first to present a zero to the
byplacingalowinputontheSEMpin(whichactsasachipselectforthe
semaphore flags) and using the other control pins (Address, OE, and
L PORT
R PORT
R/W)as theywouldbeusedinaccessingastandardStaticRAM.Each
oftheflagshasauniqueaddresswhichcanbeaccessedbyeitherside
throughaddresspinsA0–A2.Whenaccessingthesemaphores,noneof
theotheraddresspinshasanyeffect.
SEMAPHORE
SEMAPHORE
REQUEST FLIP FLOP
D0
REQUEST FLIP FLOP
D0
D
D
Q
Q
WRITE
WRITE
Whenwritingtoasemaphore,onlydatapinD0isused.IfaLOWlevel
iswrittenintoanunusedsemaphorelocation,thatflagwillbesettoazero
on that side and a one on the other side (see Truth Table VI). That
semaphorecannowonlybemodifiedbythesideshowingthezero.When
aoneiswrittenintothesamelocationfromthesameside,theflagwillbe
settoaoneforbothsides(unlessasemaphorerequestfromtheotherside
ispending)andthencanbewrittentobybothsides.Thefactthattheside
whichisabletowriteazerointoasemaphoresubsequentlylocksoutwrites
fromtheothersideiswhatmakessemaphoreflagsusefulininterprocessor
communications.(Athoroughdiscussionontheuseofthisfeaturefollows
shortly.)Azerowrittenintothesamelocationfromtheothersidewillbe
storedinthesemaphorerequestlatchforthatsideuntilthesemaphoreis
freedbythefirstside.
Whenasemaphoreflagisread,itsvalueisspreadintoalldatabitsso
thataflagthatisaonereadsasaoneinalldatabitsandaflagcontaining
azeroreadsasallzeros.Thereadvalueislatchedintooneside’soutput
registerwhenthatside'ssemaphoreselect(SEM)andoutputenable(OE)
signalsgoactive.Thisservestodisallowthesemaphorefromchanging
stateinthemiddleofareadcycleduetoawritecyclefromtheotherside.
Becauseofthislatch,arepeatedreadofasemaphoreinatestloopmust
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,
afactwhichtheprocessorwillverifybythesubsequentread(seeTruth
TableVI).Asanexample,assumeaprocessorwritesazerototheleftport
atafreesemaphorelocation.Onasubsequentread,theprocessorwill
verifythatithaswrittensuccessfullytothatlocationandwillassumecontrol
overtheresourceinquestion.Meanwhile,ifaprocessorontherightside
attempts towriteazerotothesamesemaphoreflagitwillfail,as willbe
verifiedbythefactthataonewillbereadfromthatsemaphoreontheright
SEMAPHORE
READ
SEMAPHORE
READ
.
3199 drw 18
Figure 4. IDT7027 Semaphore Logic
semaphoreflagwillforceitssideofthesemaphoreflagLOWandtheother
sideHIGH.Thisconditionwillcontinueuntilaoneiswrittentothesame
semaphorerequestlatch.Shouldtheotherside’ssemaphorerequestlatch
havebeenwrittentoazerointhemeantime,thesemaphoreflagwillflip
overtotheothersideassoonasaoneiswrittenintothefirstside’srequest
latch.Thesecondside’sflagwillnowstayLOWuntilitssemaphorerequest
latchiswrittentoaone.Fromthisitiseasytounderstandthat,ifasemaphore
is requestedandthe processorwhichrequesteditnolongerneeds the
resource, the entire system can hang up until a one is written into that
semaphorerequestlatch.
The criticalcase ofsemaphore timingis whenbothsides requesta
single token by attempting to write a zero into it at the same time. The
semaphorelogicisspeciallydesignedtoresolvethisproblem.Ifsimulta-
neousrequestsaremade,thelogicguaranteesthatonlyonesidereceives
thetoken.Ifonesideis earlierthantheotherinmakingtherequest,the
first side to make the request will receive the token. If both requests
arriveatthesametime,theassignmentwillbearbitrarilymadetooneport
or the other.
One caution that should be noted when using semaphores is that
semaphoresalonedonotguaranteethataccesstoaresourceissecure.
Aswithanypowerfulprogrammingtechnique,ifsemaphoresaremisused
ormisinterpreted, a software errorcaneasilyhappen.
Initializationofthesemaphoresisnotautomaticandmustbehandled
viatheinitializationprogramatpower-up.Sinceanysemaphorerequest
flagwhichcontainsazeromustberesettoaone,allsemaphoresonboth
sidesshouldhaveaonewrittenintothematinitializationfrombothsides
to assure that they will be free when needed.
18
6.42
IDT [ INTEGRATED DEVICE TECHNOLOGY ]
