6.42
IDT70V05S/L
High-Speed 3.3V 8K x 8 Dual-Port Static RAM Industrial and Commercial Temperature Ranges
20
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 read from,
or accessed, at the same time with the only possible conflict arising from
the simultaneous writing of, or a simultaneous READ/WRITE of, a non-
semaphore location. 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 and SEM pins
control 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 II where CE and SEM are both HIGH.
Systems which can best use the IDT70V05 contain multiple 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 IDT70V05'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 IDT70V05 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 IDT70V05 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 A
0 – A2. 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 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 semaphore flags useful in interprocessor
communications. (A thorough 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 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 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 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
semaphore 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.
