參數(shù)資料
型號(hào)： IDT70V06S55G
廠商： INTEGRATED DEVICE TECHNOLOGY INC
元件分類(lèi)： DRAM
英文描述： 100V N-Channel PowerTrench MOSFET; Package: TO-252(DPAK); No of Pins: 2; Container: Tape & Reel
中文描述： 16K X 8 DUAL-PORT SRAM, 55 ns, CPGA68
封裝： CERAMIC, PGA-68
文件頁(yè)數(shù)： 20/22頁(yè)
文件大小： 171K
代理商： IDT70V06S55G
20
IDT70V06S/L
High-Speed 16K x 8 Dual-Port Static RAM Industrial and Commercial Temperature Ranges
to control any resources through hardware, thus allowing the system
designer total flexibility in systemarchitecture.
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.
;&.&-
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, fromone 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
verifies its success in setting the latch by reading it. If it was successful,
it assumes control over the shared resource. If it was not successful
in setting the latch, it determnes 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 performanother 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 IDT70V06 in a
separate memory space fromthe 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
A
0
–
A
2
. When accessing the semaphores, none of the other address
pins has any effect.
When writing to a semaphore, only data pin D
0
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 V). That
semaphore can now only be modified by the side showing the zero.
When a one is written into the same location fromthe same side, the
flag will be set to a one for both sides (unless a semaphore request
fromthe 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 fromthe other side is what makes
semaphore flags useful in interprocessor communications. (A thor-
ough discussion on the use of this feature follows shortly.) A zero
written into the same location fromthe 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 register when that side's semaphore select (SEM) and
output enable (
OE
) signals go active. This serves to disallow
the semaphore fromchanging state in the mddle of a read cycle
due to a write cycle fromthe 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 systemlevel 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 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 fromthat semaphore on the right side during
subsequent read. Had a sequence of READ/WRITE been used
instead, systemcontention 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 diagramof 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. Fromthis it is easy to understand that, if a
semaphore is requested and the processor which requested it no
longer needs the resource, the entire systemcan hang up until a one
is written into that semaphore request latch.
The critical case of semaphore timng 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
simultaneous requests are made, the logic guarantees that only one
side receives the token. If one side is earlier than the other in making
the request, the first 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
semaphores alone do not guarantee that access to a resource is
secure. As with any powerful programmng technique, if semaphores
are msused or msinterpreted, a software error can easily happen.
Initialization of the semaphores is not automatic and must be
handled via the initialization programat power-up. Since any sema-
phore request flag which contains a zero must be reset to a one, all
semaphores on both sides should have a one written into themat
initialization fromboth sides to assure that they will be free when
needed.
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