CY7C138AV/144AV/006AV
CY7C139AV/145AV/016AV
CY7C007AV/017AV
Document #: 38-06051 Rev. *C
Page 6 of 20
prevents the port from setting the interrupt to the winning port.
Also, an active busy to a port prevents that port from reading
its own mailbox and, thus, resetting the interrupt to it. If an
application does not require message passing, do not connect
the interrupt pin to the processor’s interrupt request input pin.
The operation of the interrupts and their interaction with Busy
are summarized in Table 2.
Busy
The CY7C138AV/144AV/006AV/007AV and CY7C139AV/
145AV/016AV/017AV provide on-chip arbitration to resolve
simultaneous memory location access (contention). If both
ports’ CEs are asserted and an address match occurs within
tPS of each other, the busy logic will determine which port has
access. If tPS is violated, one port will definitely gain
permission to the location, but it is not predictable which port
will get that permission. BUSY will be asserted tBLA after an
address match or tBLC after CE is taken LOW.
Master/Slave
An M/S pin is provided in order to expand the word width by
configuring the device as either a master or a slave. The BUSY
output of the master is connected to the BUSY input of the
slave. This will allow the device to interface to a master device
with no external components. Writing to slave devices must be
delayed until after the BUSY input has settled (tBLC or tBLA),
otherwise, the slave chip may begin a write cycle during a
contention situation. When tied HIGH, the M/S pin allows the
device to be used as a master and, therefore, the BUSY line
is an output. BUSY can then be used to send the arbitration
outcome to a slave.
Semaphore Operation
The CY7C138AV/144AV/006AV/007AV and CY7C139AV/
145AV/016AV/017AV provide 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 tSOP before attempting to
read the semaphore. The semaphore value will be available
tSWRD + tDOE 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 will succeed 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). A0–2 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/O0 is used. If a zero is
written to the left port of an available semaphore, a one will
appear 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 will
be 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 shows
sample semaphore operations.
When reading a semaphore, all 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 tSPS of each other, the semaphore will
definitely be obtained by one side or the other, but there is no
guarantee which side will control the semaphore.
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