6.42

IDT7005S/L

High-Speed 8K x 8 Dual-Port Static RAM

Military, Industrial and Commercial Temperature Ranges

17

Busy Logic

BusyLogicprovidesahardwareindicationthatbothportsoftheRAM

haveaccessedthesamelocationatthesametime.Italsoallowsoneofthe

twoaccessestoproceedandsignalstheothersidethattheRAMis“busy”.

TheBUSYpincanthenbeusedtostalltheaccessuntiltheoperationon

theothersideiscompleted.Ifawriteoperationhasbeenattemptedfrom

thesidethatreceivesaBUSYindication,thewritesignalisgatedinternally

to prevent the write from proceeding.

TheuseofBUSYlogicisnotrequiredordesirableforallapplications.

InsomecasesitmaybeusefultologicallyORtheBUSYoutputstogether

and use any BUSY indication as an interrupt source to flag the event of

anillegalorillogicaloperation.IfthewriteinhibitfunctionofBUSYlogicis

notdesirable,theBUSYlogiccanbedisabledbyplacingthepartinslave

modewiththeM/Spin.OnceinslavemodetheBUSYpinoperatessolely

asawriteinhibitinputpin.Normaloperationcanbeprogrammedbytying

the BUSY pins HIGH. If desired, unintended write operations can be

prevented to a port by tying the BUSY pin for that port LOW.

TheBUSYoutputsontheIDT7005RAMinmastermode,arepush-

pull type outputs and do not require pull up resistors to operate. If these

RAMs are being expanded in depth, then the BUSY indication for the

resulting array requires the use of an external AND gate.

Width Expansion with Busy Logic

Master/Slave Arrays

WhenexpandinganIDT7005RAMarrayinwidthwhileusing BUSY

logic, one master part is used to decide which side of the RAM array will

receive a BUSYindication, and to output that indication. Any number of

slavestobeaddressedinthesameaddressrangeasthemaster,usethe

BUSYsignalasawriteinhibitsignal.ThusontheIDT7005RAMtheBUSY

Figure 3.

Iftwoormoremasterpartswereusedwhenexpandinginwidth,asplit

decisioncouldresultwithonemasterindicatingBUSYononesideofthe

arrayandanothermasterindicatingBUSYononeothersideofthearray.

Thiswouldinhibitthewriteoperationsfromoneportforpartofawordand

inhibitthewriteoperationsfromtheotherportfortheotherpartoftheword.

The BUSY arbitration on a master is based on the chip enable and

Figure 3. Busy and chip enable routing for both width and depth expansion with IDT7005 RAMs.

address signals only. It ignores whether an access is a read or write.

In a master/slave array, both address and chip enable must be valid

long enough for a BUSY flag to be output from the master before the

actualwritepulsecanbeinitiatedwiththeR/Wsignal.Failuretoobserve

thistimingcanresultinaglitchedinternalwriteinhibitsignalandcorrupted

data in the slave.

Semaphores

TheIDT7005isanextremelyfastDual-Port8Kx8CMOSStaticRAM

withanadditional8addresslocationsdedicatedtobinarysemaphoreflags.

TheseflagsalloweitherprocessorontheleftorrightsideoftheDual-Port

RAMtoclaimaprivilegeovertheotherprocessorforfunctionsdefinedby

thesystemdesigner’ssoftware.Asanexample,thesemaphorecanbe

usedbyoneprocessortoinhibittheotherfromaccessingaportionofthe

Dual-Port RAM or any other shared resource.

The Dual-Port RAM features a fast access time, and both ports are

completelyindependentofeachother.Thismeansthattheactivityonthe

left port in no way slows the access time of the right port. Both ports are

identicalinfunctiontostandardCMOSStaticRAMandcanbereadfrom,

orwrittento,atthesametimewiththeonlypossibleconflictarisingfromthe

simultaneous writing of, or a simultaneous READ/WRITE of, a non-

semaphorelocation.Semaphoresareprotectedagainstsuchambiguous

situationsandmaybeusedbythesystemprogramtoavoidanyconflicts

inthenon-semaphoreportionoftheDual-PortRAM.Thesedeviceshave

anautomaticpower-downfeaturecontrolledbyCE,theDual-PortRAM

enable,andSEM,thesemaphoreenable.TheCEandSEMpinscontrol

on-chip power down circuitry that permits the respective port to go into

standbymodewhennotselected. Thisistheconditionwhichisshownin

Truth Table I where CE and SEM are both HIGH.

SystemswhichcanbestusetheIDT7005containmultipleprocessors

or controllers and are typically very high-speed systems which are

softwarecontrolledorsoftwareintensive.Thesesystemscanbenefitfrom

aperformanceincreaseofferedbytheIDT7005'shardwaresemaphores,

whichprovidealockoutmechanismwithoutrequiringcomplexprogram-

ming.

Software handshaking between processors offers the maximum in

systemflexibilitybypermittingsharedresourcestobeallocatedinvarying

2738 drw 19

MASTER

Dual Port

RAM

BUSY (R)

CE

MASTER

Dual Port

RAM

BUSY (R)

CE

SLAVE

Dual Port

RAM

BUSY (R)

CE

SLAVE

Dual Port

RAM

BUSY (R)

CE

BUSY (L)

BUSY (R)

BUSY (L)

BUSY (L)

BUSY (L)

BUSY (L)

,