Document Number: 38-06046 Rev. *J Page 8 of 24
Semaphore Operation
The CY7C026A provides eight semaphore latches, which are
separate from the dual-port memory locations. Semaphores are
used to reserve resources that are shared between the two ports.
The state of the semaphore indicates that a resource is in use.
For example, if the left port wants to request a given resource, it
sets a latch by writing a zero to a semaphore location. The left
port then verifies its success in setting the latch by reading it.
After writing to the semaphore, SEM
or OE must be deasserted
for t
SOP
before attempting to read the semaphore. The
semaphore value is available t
SWRD
+ t
DOE
after the rising edge
of the semaphore write. If the left port was successful (reads a
zero), it assumes control of the shared resource, otherwise
(reads a one) it assumes the right port has control and continues
to poll the semaphore. When the right side has relinquished
control of the semaphore (by writing a one), the left side
succeeds in gaining control of the semaphore. If the left side no
longer requires the semaphore, a one is written to cancel its
request.
Semaphores are accessed by asserting SEM
LOW. The SEM
pin functions as a chip select for the semaphore latches (CE
must remain HIGH during SEM LOW). A
0–2
represents the
semaphore address. OE
and R/W are used in the same manner
as a normal memory access. When writing or reading a
semaphore, the other address pins have no effect.
When writing to the semaphore, only I/O
0
is used. If a zero is
written to the left port of an available semaphore, a one appears
at the same semaphore address on the right port. That
semaphore can now only be modified by the side showing zero
(the left port in this case). If the left port now relinquishes control
by writing a one to the semaphore, the semaphore is set to one
for both sides. However, if the right port had requested the
semaphore (written a zero) while the left port had control, the
right port would immediately own the semaphore as soon as the
left port released it. Table 3 on page 8 shows sample semaphore
operations.
When reading a semaphore, all sixteen/eighteen data lines
output the semaphore value. The read value is latched in an
output register to prevent the semaphore from changing state
during a write from the other port. If both ports attempt to access
the semaphore within t
SPS
of each other, the semaphore is
definitely obtained by one side or the other, but there is no
guarantee which side controls the semaphore.
Table 3. Semaphore Operation Example
Function I/O
0
–I/O
15
Left I/O
0
–I/O
15
Right Status
No action 1 1 Semaphore free
Left port writes 0 to semaphore 0 1 Left port has semaphore token
Right port writes 0 to semaphore 0 1 No change. Right side has no write access to semaphore
Left port writes 1 to semaphore 1 0 Right port obtains semaphore token
Left port writes 0 to semaphore 1 0 No change. Left port has no write access to semaphore
Right port writes 1 to semaphore 0 1 Left port obtains semaphore token
Left port writes 1 to semaphore 1 1 Semaphore free
Right port writes 0 to semaphore 1 0 Right port has semaphore token
Right port writes 1 to semaphore 1 1 Semaphore free
Left port writes 0 to semaphore 0 1 Left port has semaphore token
Left port writes 1 to semaphore 1 1 Semaphore free
