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IDT7006SL Datasheet(PDF) 18 Page - Integrated Device Technology

Part No. IDT7006SL
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Manufacturer  IDT [Integrated Device Technology]
Direct Link  http://www.idt.com
Logo IDT - Integrated Device Technology

IDT7006SL Datasheet(HTML) 18 Page - Integrated Device Technology

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High-Speed 16K x 8 Dual-Port Static RAM
Military, Industrial and Commercial Temperature Ranges
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-
How the Semaphore Flags Work
The semaphore logic is a set of eight latches which are independent
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
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
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
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.
The eight semaphore flags reside within the IDT7006 in a separate
memoryspacefromtheDual-PortRAM.This addressspaceisaccessed
by placing a LOWinput on the
semaphore flags) and using the other control pins (Address,
OE, and
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
a one is written into the same location from the same side, the flag will be
is pending) and then can be written to by both sides. The fact that the side
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
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
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
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
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
gap between the read and write cycles.
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
have been written to a zero in the meantime, the semaphore flag will flip
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
semaphore request latch.
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
semaphore logic is specially designed to resolve this problem. If simulta-
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
One caution that should be noted when using semaphores is that
semaphores alone do not guarantee that access to a resource is secure.
or misinterpreted, a software error can easily happen.
via the initialization program at power-up. Since any semaphore request
flag which contains a zero must be reset to a one, all semaphores on both
sides should have a one written into them at initialization from both sides
to assure that they will be free when needed.
was to be divided into two 8K x 8 blocks which were to be dedicated at any

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