CY7C024AV/025AV/026A
CY7C0241AV/0251AV/036A
Document #: 38-06052 Rev. *B Page 6 of 19
Read Operation
When reading the device, the user must assert both the OE
and CE pins. Data will be available t
ACE
after CE or t
DOE
after
OE is asserted. If the user wishes to access a semaphore flag,
then the SEM
pin must be asserted instead of the CE pin, and
OE
must also be asserted.
Interrupts
The upper two memory locations may be used for message
passing. The highest memory location (FFF for the
CY7C024AV/41AV, 1FFF for the CY7C025AV/51AV, 3FFF for
the CY7C026AV/36AV) is the mailbox for the right port and the
second-highest memory location (FFE for the CY7C024AV/
41AV, 1FFE for the CY7C025AV/51AV, 3FFE for the
CY7C026AV/36AV) is the mailbox for the left port. When one
port writes to the other port’s mailbox, an interrupt is generated
to the owner. The interrupt is reset when the owner reads the
contents of the mailbox. The message is user defined.
Each port can read the other port’s mailbox without resetting
the interrupt. The active state of the busy signal (to a port)
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 CY7C024AV/025AV/026AV and CY7C0241AV/0251AV/
036AV provide on-chip arbitration to resolve simultaneous
memory location access (contention). If both ports’ CE
s are
asserted and an address match occurs within t
PS
of each
other, the busy logic will determine which port has access. If
t
PS
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 t
BLA
after an address
match or t
BLC
after CE is taken LOW.
Master/Slave
A 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 (t
BLC
or t
BLA
),
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 CY7C024AV/025AV/026AV and CY7C0241AV/0251AV/
036AV 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 t
SOP
before attempting to read the semaphore.
The semaphore value will be 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 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). 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 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 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 will definitely be obtained by one side or the other,
but there is no guarantee which side will control the
semaphore.
