70T653MS12BCGI datasheet(19/24 Pages) IDT

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IDT70T653M

High-Speed 2.5V 512K x 36 Asynchronous Dual-Port Static RAM

Industrial and Commercial Temperature Ranges

verifiesitssuccessinsettingthelatchbyreadingit. Ifitwassuccessful,it

proceeds to assume control over the shared resource. If it was not

successfulinsettingthelatch,itdeterminesthattherightsideprocessor

has set the latch first, has the token and is using the shared resource.

The left processor can then either repeatedly request that

semaphore’s status or remove its request for that semaphore to

perform another task and occasionally attempt again to gain control of

the token via the set and test sequence. Once the right side has

relinquishedthetoken,theleftsideshouldsucceedingainingcontrol.

The semaphore flags are active LOW. A token is requested by

writing a zero into a semaphore latch and is released when the same

side writes a one to that latch.

The eight semaphore flags reside within the IDT70T653M in a

separate memory space from the Dual-Port RAM. This address space

is accessed by placing a low input on the SEM pin (which acts as a chip

selectforthesemaphoreflags)andusingtheothercontrolpins(Address,

CE0

, CE1,R/W and BEn) as they would be used in accessing a

standardStaticRAM.Eachoftheflagshasauniqueaddresswhichcan

beaccessedbyeithersidethroughaddresspinsA0–A2.Whenaccessing

the semaphores, none of the other address pins has any effect.

When writing to a semaphore, only data pin D0 is used. If a low level

is written into an unused semaphore location, that flag will be set to

a zero on that side and a one on the other side (see Truth Table IV).

Thatsemaphorecannowonlybemodifiedbythesideshowingthezero.

Whenaoneiswrittenintothesamelocationfromthesameside,the flag

will be set to a one for both sides (unless a semaphore request

from the other side is pending) and then can be written to by both sides.

The fact that the side which is able to write a zero into a semaphore

subsequently locks out writes from the other side is what makes

semaphoreflagsusefulininterprocessorcommunications.(Athorough

discussionontheuseofthisfeaturefollowsshortly.)Azerowrittenintothe

samelocationfromtheothersidewillbestoredinthesemaphorerequest

latch for that side until the semaphore is freed by the first side.

Whenasemaphoreflagisread,itsvalueisspreadintoalldatabitsso

thataflagthatisaonereadsasaoneinalldatabitsandaflagcontaining

a zero reads as all zeros for a semaphore read, the SEM, BEn, and OE

signals need to be active. (Please refer to Truth Table II). Furthermore,

the read value is latched into one side’s output register when that side's

semaphoreselect(SEM,BEn)andoutputenable(OE)signalsgoactive.

Thisservestodisallowthesemaphorefromchangingstateinthemiddle

of a read cycle due to a write cycle from the other side.

A sequence WRITE/READ must be used by the semaphore in

order to guarantee that no system level contention will occur. A

processor requests access to shared resources by attempting to write

a zero into a semaphore location. If the semaphore is already in use,

the semaphore request latch will contain a zero, yet the semaphore

flag will appear as one, a fact which the processor will verify by the

subsequent read (see Table IV). As an example, assume a processor

writes a zero to the left port at a free semaphore location. On a

subsequent read, the processor will verify that it has written success-

fullytothatlocationandwillassumecontrolovertheresourceinquestion.

Meanwhile,ifaprocessorontherightsideattemptstowriteazerotothe

samesemaphoreflagitwillfail,aswillbeverifiedbythe factthataonewill

be read from that semaphore on the right side during subsequent read.

HadasequenceofREAD/WRITEbeenusedinstead,systemcontention

problemscouldhaveoccurredduringthegapbetweenthereadandwrite

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.Iftheoppositesidesemaphorerequestlatchhas

beenwrittentozerointhemeantime,thesemaphoreflagwillflipoverto

the other side as soon as a one is written into the first request latch. The

oppositesideflagwillnowstayLOWuntilitssemaphorerequestlatchis

writtentoaone.Fromthisitiseasytounderstandthat,ifasemaphoreis

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

thetoken.Ifonesideisearlierthantheotherinmakingtherequest,thefirst

side to make the request will receive the token. If

both requests arrive 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.

As with any powerful programming technique, if semaphores

are misused or misinterpreted, a software error can easily happen.

Initializationofthesemaphoresisnotautomaticandmustbehandled

viatheinitializationprogramatpower-up.Sinceanysemaphorerequest

flagwhichcontainsazeromustberesettoaone,allsemaphoresonboth

sidesshouldhaveaonewrittenintothematinitializationfrombothsides

to assure that they will be free when needed.

Figure 4. IDT70T653M Semaphore Logic

5679 drw 21

WRITE

SEMAPHORE

REQUEST FLIP FLOP

SEMAPHORE

REQUEST FLIP FLOP

LPORT

RPORT

SEMAPHORE

READ

SEMAPHORE

READ

How the Semaphore Flags Work

The semaphore logic is a set of eight latches which are indepen-

dent of the Dual-Port RAM. These latches can be used to pass a flag,

or token, from one port to the other to indicate that a shared resource

is in use. The semaphores provide a hardware assist for a use

assignment method called “Token Passing Allocation.” In this method,

the state of a semaphore latch is used as a token indicating that a

shared resource is in use. If the left processor wants to use this

resource, it requests the token by setting the latch. This processor then

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