CY7C025AV-25AC Cypress Semiconductor Corp, CY7C025AV-25AC Datasheet - Page 15

• Manufacturer Part Number: CY7C025AV-25AC
• Description: IC,SRAM,8KX16,CMOS,QFP,100PIN,PLASTIC
• Manufacturer: Cypress Semiconductor Corp

Architecture
The CY7C024AV/025AV/026AV and CY7C0241AV/0251AV/
036AV consist of an array of 4K, 8K, and 16K words of 16 and
18 bits each of dual-port RAM cells, I/O and address lines, and
control signals (CE, OE, R/W). These control pins permit inde-
pendent 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 utilized for port-to-port communication. Two Semaphore
(SEM) control pins are used for allocating shared resources.
With the M/S pin, the devices can function as a master (BUSY
pins are outputs) or as a slave (BUSY pins are inputs). The
devices also 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
of R/W in order to guarantee a valid write. A write operation is
controlled by either the R/W pin (see Write Cycle No. 1 wave-
form) or the CE pin (see Write Cycle No. 2 waveform). 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; other-
wise the data read is not deterministic. Data will be valid on the
port t
Read Operation
When reading the device, the user must assert both the OE
and CE pins. Data will be available t
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 con-
nect 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’ CEs are
DDD
after the data is presented on the other port.
ACE
SD
before the rising edge
after CE or t
PRELIMINARY
DOE
after
15
asserted and an address match occurs within t
er, the busy logic will determine which port has access. If t
is violated, one port will definitely gain permission to the loca-
tion, but it is not predictable which port will get that permission.
BUSY will be asserted t
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
otherwise, the slave chip may begin a write cycle during a con-
tention situation. When tied HIGH, the M/S pin allows the de-
vice to be used as a master and, therefore, the BUSY line is
an output. BUSY can then be used to send the arbitration out-
come 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 deassert-
ed for t
semaphore value will be available t
edge of the semaphore write. If the left port was successful
(reads a zero), it assumes control of the shared resource, oth-
erwise (reads a one) it assumes the right port has control and
continues to poll the semaphore. When the right side has re-
linquished 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
semaphore address. OE and R/W are used in the same man-
ner 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
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 con-
trol by writing a one to the semaphore, the semaphore will be
set to one for both sides. However, if the right port had request-
ed the semaphore (written a zero) while the left port had con-
trol, the right port would immediately own the semaphore as
soon as the left port released it. Table 3 shows sample sema-
phore 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 ac-
cess the semaphore within t
will definitely be obtained by one side or the other, but there is
no guarantee which side will control the semaphore.
SOP
before attempting to read the semaphore. The
CY7C0241AV/0251AV/036AV
CY7C024AV/025AV/026AV
BLA
SPS
after an address match or t
of each other, the semaphore
SWRD
+ t
0
is used. If a zero is
0–2
DOE
PS
represents the
after the rising
BLC
of each oth-
or t
BLA
BLC
PS
),