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参数资料
| 型号： | 70V34S20PFGI |
| 厂商： | INTEGRATED DEVICE TECHNOLOGY INC |
| 元件分类： | SRAM |
| 英文描述： | 4K X 18 DUAL-PORT SRAM, 20 ns, PQFP100 |
| 封装： | 14 X 14 MM, 1.40 MM HEIGHT, GREEN, PLASTIC, TQFP-100 |
| 文件页数： | 16/25页 |
| 文件大小： | 211K |
| 代理商： | 70V34S20PFGI |
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IDT70V35/34S/L (IDT70V25/24S/L)
High-Speed 3.3V 8/4K x 18 (8/4K x 16) Dual-Port Static RAM
Industrial and Commercial Temperature Ranges
software. As an example, the semaphore can be used by one processor
to inhibit the other from accessing a portion of the Dual-Port SRAM or any
other shared resource.
The Dual-Port SRAM features a fast access time, and both ports are
completely independent of each other. This means that the activity on the
left port in no way slows the access time of the right port. Both ports are
identical in function to standard CMOS Static RAM and can be accessed
atthesametimewiththeonlypossibleconflictarisingfromthesimultaneous
writingof,orasimultaneousREAD/WRITEof,anon-semaphorelocation.
Semaphores are protected against such ambiguous situations and may
be used by the system program to avoid any conflicts in the non-
semaphore portion of the Dual-Port SRAM. These devices have an
automatic power-down feature controlled by
CE, the Dual-Port SRAM
enable, and SEM, the semaphore enable. The
CE andSEMpinscontrol
on-chip power down circuitry that permits the respective port to go into
standby mode when not selected. This is the condition which is shown in
Truth Table I where
CE and SEM are both HIGH.
SystemswhichcanbestusetheIDT70V35/34(IDT70V25/24)contain
multiple processors or controllers and are typically very high-speed
systems which are software controlled or software intensive. These
systemscanbenefit fromaperformanceincreaseofferedbytheIDT70V35/
34 (IDT70V25/24)'s hardware semaphores, which provide a lockout
mechanism without requiring complex programming.
Software handshaking between processors offers the maximum in
systemflexibilitybypermittingsharedresourcestobeallocatedinvarying
configurations. The IDT70V35/34 (IDT70V25/24) does not use its sema-
phore 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-
speedsystems.
How the Semaphore Flags Work
The semaphore logic is a set of eight latches which are independent
oftheDual-PortSRAM.Theselatchescanbeusedtopassaflag,ortoken,
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“TokenPassingAllocation.”Inthismethod,thestateofasemaphore
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.Thisprocessorthenverifiesitssuccessinsettingthelatchbyreading
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
thatsemaphore’sstatusorremoveitsrequestforthatsemaphoretoperform
anothertaskandoccasionallyattemptagaintogaincontrolofthetokenvia
thesetandtestsequence.Oncetherightsidehasrelinquishedthetoken,
the left side should succeed in gaining control.
The semaphore flags are activeLOW. 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.
TheeightsemaphoreflagsresidewithintheIDT70V35/34(IDT70V25/
24) in a separate memory space from the Dual-Port SRAM. This address
space is accessed by placing aLOWinput on the
SEMpin(whichactsas
a chip select for the semaphore flags) and using the other control pins
(Address,
OE, and R/W)astheywouldbeusedinaccessingastandard
staticRAM.Eachoftheflagshasauniqueaddresswhichcanbeaccessed
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 D0 is used. If aLOW 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
semaphorecannowonlybemodifiedbythesideshowingthezero.When
a one is written into the same location from the same side, the flag will be
settoaoneforbothsides(unlessasemaphorerequestfromtheotherside
is pending) and then can be written to by both sides. The fact that the side
whichisabletowriteazerointoasemaphoresubsequentlylocksoutwrites
fromtheothersideiswhatmakessemaphoreflagsusefulininterprocessor
communications.(Athoroughdiscussionontheuseofthisfeaturefollows
shortly.) A zero written into the same location from the other side will be
stored in the semaphore request latch for that side until the semaphore is
freed by the first side.
When a semaphore flag is read, its value is spread into all data bits so
that a flag that is a one reads as a one in all data bits and a flag containing
a zero reads as all zeros. The read value is latched into one side’s output
registerwhenthatside'ssemaphoreselect(
SEM)andoutputenable(OE)
signals go active. This serves to disallow the semaphore from changing
state in the middle of a read cycle due to a write cycle from the other side.
Because of this latch, a repeated read of a semaphore in a test loop must
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
requestlatchwillcontainazero,yetthesemaphoreflagwillappearasone,
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
verifythatithaswrittensuccessfullytothatlocationandwillassumecontrol
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
usedinstead,systemcontentionproblemscouldhaveoccurredduringthe
gap between the read and write cycles.
Itisimportanttonotethatafailedsemaphorerequestmustbefollowed
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 semaphore request latches feed
into a semaphore flag. Whichever latch is first to present a zero to the
semaphoreflagwillforceitssideofthesemaphoreflagLOW andtheother
side HIGH. This condition will continue until a one is written to the same
semaphorerequestlatch.Shouldtheotherside’ssemaphorerequestlatch
have been written to a zero in the meantime, the semaphore flag will flip
overtotheothersideassoonasaoneiswrittenintothefirstside’srequest
latch.Thesecond side’sflagwillnowstay LOWuntilitssemaphorerequest
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
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