CY7C024E, CY7C0241E
CY7C025E, CY7C0251E
Document Number: 001-62932 Rev. *A Page 6 of 22
Architecture
The CY7C024E/CY7C0241E and CY7C025E/CY7C0251E
consist of an array of 4 K words of 16/18 bits each and 8 K words
of 16/18 bits each of dual-port RAM cells, I/O and address lines,
and control signals (CE
, OE, R/W). These control pins permit
independent access for reads or writes to any location in
memory. To handle simultaneous writes/reads to the same
location, a BUSY
pin is provided on each port. Two interrupt
(INT
) pins can be used for port-to-port communication. Two
semaphore (SEM
) control pins are used for allocating shared
resources. With the M/S
pin, the CY7C024E/CY7C0241E and
CY7C025E/CY7C0251E can function as a master (BUSY
pins
are outputs) or as a slave (BUSY
pins are inputs). The
CY7C024E/CY7C0241E and CY7C025E/CY7C0251E have an
automatic power-down feature controlled by CE
. Each port is
provided with its own output enable control (OE
), which allows
data to be read from the device.
Functional Description
Write Operation
Data must be set up for a duration of t
SD
before the rising edge
of R/W
to guarantee a valid write. A write operation is controlled
by either the R/W
pin (see Figure 7) or the CE pin (see Figure 8).
Required inputs for non-contention operations are summarized
in Table 1.
If a location is being written to by one port and the opposite port
attempts to read that location, a port-to-port flowthrough delay
must occur before the data is read on the output; otherwise the
data read is not deterministic. Data is valid on the port t
DDD
after
the data is presented on the other port.
Read Operation
When reading the device, the user must assert both the OE and
CE
pins. Data is available t
ACE
after CE or t
DOE
after OE is
asserted. If the user of the CY7C024E/CY7C0241E and
CY7C025E/CY7C0251E 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
CY7C024E/CY7C0241E, 1FFF for the CY7C025E/CY7C0251E)
is the mailbox for the right port and the second-highest memory
location (FFE for the CY7C024E/CY7C0241E, 1FFE for the
CY7C025E/CY7C0251E) 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 your 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 on page 8.
Busy
The CY7C024E/CY7C0241E and CY7C025E/CY7C0251E
provide on-chip arbitration to resolve simultaneous memory
location access (contention). If both ports’ CEs are asserted and
an address match occurs within t
PS
of each other, the busy logic
determines which port has access. If t
PS
is violated, one port
definitely gains permission to the location, but which one is not
predictable. BUSY
is asserted t
BLA
after an address match or
t
BLC
after CE is taken LOW.
Master/Slave
A M/S pin is provided 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 allows
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 CY7C024E/CY7C0241E and CY7C025E/CY7C0251E
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
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). 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.
