6.42

70V261S/L

High-Speed 16K x 16 Dual-Port Static RAM with Interrupt

Industrial and Commercial Temperature Ranges

15

Jun.04.21

D

3040 drw 18

0

D

Q

WRITE

D0

D

Q

WRITE

SEMAPHORE

REQUEST FLIP FLOP

SEMAPHORE

REQUEST FLIP FLOP

L PORT

R PORT

SEMAPHORE

READ

SEMAPHORE

READ

SystemswhichcanbestusetheIDT70V261containmultipleproces-

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

softwarecontrolledorsoftwareintensive.Thesesystemscanbenefitfrom

a performance increase offered by the IDT70V261's hardware sema-

phores, which provide a lockout mechanism without requiring complex

programming.

Software handshaking between processors offers the maximum in

system flexibility by permitting shared resources to be allocated in

varying configurations. The IDT70V261 does not use its semaphore

flags to control any resources through hardware, thus allowing the

system designer total flexibility in system architecture.

An advantage of using semaphores rather than the more common

methods of hardware arbitration is that wait states are never incurred

in either processor. This can prove to be a major advantage in very

high-speed systems.

How the Semaphore Flags Work

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

dent of the Dual-Port SRAM. 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 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 verifies its

success in setting the latch by reading it. If it was successful, it

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

successful in setting the latch, it determines that the right side

processor 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

relinquished the token, the left side should succeed in gaining control.

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 IDT70V261 in a

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

space is accessed by placing a LOW input on the SEM pin (which acts

as a chip select for the 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 through address pins A0 – A2. When

accessing the semaphores, none of the other address pins has any

effect.

When writing to a semaphore, only data pin D0is 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 V). That

semaphore can now only be modified by the side showing the zero.

When a one is written into the same location from the same side, 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

Figure 4. IDT70V261 Semaphore Logic

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

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.

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 Truth Table V). 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-

fully to that location and will assume control over the resource in

question. Meanwhile, if a processor on the right side attempts to write

a zero to the same semaphore flag it will fail, as will be verified by the

fact that a one will be read from that semaphore on the right side during

subsequent read. Had a sequence of READ/WRITE been used

instead, system contention problems could have occurred during the

gap between the read and write cycles.

It is important to note that a failed semaphore request must be

followed by either repeated reads or by writing a one into the same

location. The reason for this is easily understood by looking at the

simple logic diagram of the semaphore flag in Figure 4. Two sema-

phore request latches feed into a semaphore flag. Whichever latch is

first to present a zero to the semaphore flag will force its side of the

semaphore flag low and the other side HIGH. This condition will

continue until a one is written to the same semaphore request latch.

Should the other side’s semaphore request latch have been written to

a zero in the meantime, the semaphore flag will flip over to the other

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

second side’s flag will now stay LOW until its semaphore request latch

is written to a one. From this it is easy to understand that, if a