CY7C008/009
CY7C018/019
Document #: 38-06041 Rev. *A Page 15 of 19
Architecture
The CY7C008/009 and CY7C018/019 consist of an array of
64K and 128K words of 8 and 9 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 utilized for port-to-port communication. Two sema-
phore (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
SD
before the rising edge
of R/W
in order to guarantee a valid write. A write operation is con-
trolled by either the R/W
pin (see Write Cycle No. 1 waveform) 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
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 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 (FFFF for the
CY7C008/18, 1FFFF for the CY7C009/19) is the mailbox for
the right port and the second-highest memory location (FFFE
for the CY7C008/18, 1FFFE for the CY7C009/19) is the mail-
box 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
is summarized in Table 2.
Busy
The CY7C008/009 and CY7C018/019 provide on-chip arbitra-
tion to resolve simultaneous memory location access (conten-
tion). 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 configur-
ing 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 al-
lows 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 CY7C008/009 and CY7C018/019 provide eight sema-
phore 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 at-
tempting to read the semaphore. The semaphore value will be avail-
able 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 re-
quest.
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. Howev-
er, 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 sam-
ple semaphore operations.
When reading a semaphore, all data lines output the sema-
phore 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 sema-
phore 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.
