IDT70V07 Integrated Device Technology, IDT70V07 Datasheet - Page 16

• Manufacturer Part Number: IDT70V07
• Description: 32k X 8 3.3v Dual-port
• Manufacturer: Integrated Device Technology

I where CE and SEM are both HIGH.
processors or controllers and are typically very high-speed systems
which are software controlled or software intensive. These systems
can benefit from a performance increase offered by the IDT70V07's
hardware semaphores, which provide a lockout mechanism without
requiring complex programming.
system flexibility by permitting shared resources to be allocated in
varying configurations. The IDT70V07 does not use its semaphore
flags to control any resources through hardware, thus allowing the
system designer total flexibility in system architecture.
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
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.
writing a zero into a semaphore latch and is released when the same
side writes a one to that latch.
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 A
accessing the semaphores, none of the other address pins has any
effect.
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
IDT70V07S/L
High-Speed 32K x 8 Dual-Port Static RAM
Systems which can best use the IDT70V07 contain multiple
Software handshaking between processors offers the maximum in
An advantage of using semaphores rather than the more common
The semaphore logic is a set of eight latches which are indepen-
The semaphore flags are active LOW. A token is requested by
The eight semaphore flags reside within the IDT70V07 in a
When writing to a semaphore, only data pin D
0
is used. If a LOW
0
– A
2
. When
16
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 from the other side will be stored in the
SEMAPHORE
semaphore request latch for that side until the semaphore is freed by
the first side.
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 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.
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.
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
WRITE
L PORT
When a semaphore flag is read, its value is spread into all data bits
A sequence WRITE/READ must be used by the semaphore in
It is important to note that a failed semaphore request must be
REQUEST FLIP FLOP
D
0
READ
SEMAPHORE
D
Industrial and Commercial Temperature Ranges
Figure 4. IDT70V07 Semaphore Logic
Q
REQUEST FLIP FLOP
SEMAPHORE
Q
D
SEMAPHORE
READ
R PORT
D
WRITE
2943 drw 20
0
,