Cypress CY7C138 - Manual (Page 13)

CY7C138, CY7C139
Document #: 38-06037 Rev. *D Page 13 of 17
Architecture
The CY7C138/9 consists of an array of 4K words of 8/9 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 simulta-
neous writes and 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 CY7C138/9 can function as a master (BUSY pins are
outputs) or as a slave (BUSY pins are inputs). The CY7C138/9
has an automatic power down feature controlled by CE. Each
port is provided with its own output enable control (OE), which
enables 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
controlled by either the OE pin (see Write Cycle No. 1 waveform)
or the R/W pin (see Write Cycle No. 2 waveform). Data can be
written to the device t
HZOE
after the OE is deasserted or t
HZWE
after the falling edge of R/W. Required inputs for non-contention
operations are summarized in Table 3 .
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 be met 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 CY7C138/9 wishes to access a
semaphore flag, then the SEM pin must be asserted instead of
the CE pin.
Interrupts
The interrupt flag (INT) permits communications between
ports.When the left port writes to location FFF, the right port’s
interrupt flag (INT
R
) is set. This flag is cleared when the right port
reads that same location. Setting the left port’s interrupt flag
(INT
L
) is accomplished when the right port writes to location FFE.
This flag is cleared when the left port reads location FFE. The
message at FFF or FFE is user-defined. See Table 4 for input
requirements for INT. INT
R
and INT
L
are push-pull outputs and
do not require pull-up resistors to operate. BUSY
L
and BUSY
R
in master mode are push-pull outputs and do not require pull-up
resistors to operate.
Busy
The CY7C138/9 provides on-chip arbitration to alleviate simulta-
neous 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
it is not guaranteed which one. 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 config-
uring 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
enables the device to interface to a master device with no
external components.Writing of slave devices must be delayed
until after the BUSY input has settled. Otherwise, the slave chip
may begin a write cycle during a contention situation.When
presented as a HIGH input, 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 CY7C138/9 provides 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 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 over 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 relin-
quished control of the semaphore (by writing a one), the left side
succeeds in gaining control of the a semaphore.If the left side no
longer requires the semaphore, a 1 is written to cancel its
request.
Semaphores are accessed by asserting SEM LOW. The SEM
pin functions as a chip enable 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 unused 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 is set to 1 for
both sides. However, if the right port had requested the
semaphore (written a zero) while the left port had control, the
right port immediately owns the semaphore after the left port
releases it. Table 5 shows sample semaphore operations.
When reading a semaphore, all eight or nine 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 is definitely
obtained by one side or the other, but there is no guarantee which
side controls the semaphore.
Initialization of the semaphore is not automatic and must be reset
during initialization program at power up. All semaphores on both
sides should have a 1 written into them at initialization from both
sides to assure that they are free when needed.
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