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 tSD 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 tHZOE after the OE is deasserted or tHZWE 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 tDDD 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 tACE after CE or tDOE 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 (INTR) is set. This flag is cleared when the right port reads that same location. Setting the left port’s interrupt flag (INTL) 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. INTR and INTL are push-pull outputs and do not require pull-up resistors to operate. BUSYL and BUSYR 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 tPS of each other the Busy logic determines which port has access. If tPS is violated, one port definitely gains permission to the location, but it is not guaranteed which one. BUSY will be asserted tBLA after an address match or tBLC 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 tSOP before attempting to read the semaphore. The semaphore value is available tSWRD + tDOE 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). A0–2represents 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/O0 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 tSPS 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.
