Functional Overview
The CY7C1470V25/CY7C1472V25/CY7C1474V25 are synchronous-pipelined Burst NoBL SRAMs designed specifi- cally to eliminate wait states during Write/Read transitions. All synchronous inputs pass through input registers controlled by the rising edge of the clock. The clock signal is qualified with the Clock Enable input signal (CEN). If CEN is HIGH, the clock signal is not recognized and all internal states are maintained. All synchronous operations are qualified with CEN. All data outputs pass through output registers controlled by the rising edge of the clock. Maximum access delay from the clock rise (tCO) is 3.0 ns (250-MHz device).
Accesses can be initiated by asserting all three Chip Enables (CE1, CE2, CE3) active at the rising edge of the clock. If Clock Enable (CEN) is active LOW and ADV/LD is asserted LOW, the address presented to the device will be latched. The access can either be a Read or Write operation, depending on the status of the Write Enable (WE). BW[x] can be used to conduct Byte Write operations.
Write operations are qualified by the Write Enable (WE). All writes are simplified with on-chip synchronous self-timed write circuitry.
Three synchronous Chip Enables (CE1, CE2, CE3) and an asynchronous Output Enable (OE) simplify depth expansion. All operations (Reads, Writes, and Deselects) are pipelined. ADV/LD should be driven LOW once the device has been deselected in order to load a new address for the next operation.
Single Read Accesses
ARead access is initiated when the following conditions are satisfied at clock rise: (1) CEN is asserted LOW, (2) CE1, CE2, and CE3 are ALL asserted active, (3) the Write Enable input signal WE is deasserted HIGH, and (4) ADV/LD is asserted LOW. The address presented to the address inputs is latched into the Address Register and presented to the memory core and control logic. The control logic determines that a Read access is in progress and allows the requested data to propagate to the input of the output register. At the rising edge of the next clock the requested data is allowed to propagate through the output register and onto the data bus within 2.6 ns (250-MHz device) provided OE is active LOW. After the first clock of the Read access the output buffers are controlled by OE and the internal control logic. OE must be driven LOW in order for the device to drive out the requested data. During the second clock, a subsequent operation (Read/Write/Deselect) can be initiated. Deselecting the device is also pipelined. Therefore, when the SRAM is deselected at clock rise by one of the chip enable signals, its output will tri-state following the next clock rise.
Burst Read Accesses
The CY7C1470V25/CY7C1472V25/CY7C1474V25 have an on-chip burst counter that allows the user the ability to supply a single address and conduct up to four Reads without reasserting the address inputs. ADV/LD must be driven LOW in order to load a new address into the SRAM, as described in the Single Read Access section above. The sequence of the burst counter is determined by the MODE input signal. A LOW input on MODE selects a linear burst mode, a HIGH selects an interleaved burst sequence. Both burst counters use A0 and A1 in the burst sequence, and will wrap-around when incre- mented sufficiently. A HIGH input on ADV/LD will increment
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the internal burst counter regardless of the state of chip enables inputs or WE. WE is latched at the beginning of a burst cycle. Therefore, the type of access (Read or Write) is maintained throughout the burst sequence.
Single Write Accesses
Write accesses are initiated when the following conditions are satisfied at clock rise: (1) CEN is asserted LOW, (2) CE1, CE2, and CE3 are ALL asserted active, and (3) the Write signal WE is asserted LOW. The address presented to the address inputs is loaded into the Address Register. The Write signals are latched into the Control Logic block.
On the subsequent clock rise the data lines are automatically tri-stated regardless of the state of the OE input signal. This allows the external logic to present the data on DQ and DQP
(DQa,b,c,d,e,f,g,h/DQPa,b,c,d,e,f,g,h for CY7C1474V25, DQa,b,c,d/DQPa,b,c,d for CY7C1470V25 and DQa,b/DQPa,b for CY7C1472V25). In addition, the address for the subsequent access (Read/Write/Deselect) is latched into the Address Register (provided the appropriate control signals are asserted).
On the next clock rise the data presented to DQ and DQP
(DQa,b,c,d,e,f,g,h/DQPa,b,c,d,e,f,g,h for CY7C1474V25, DQa,b,c,d/DQPa,b,c,d for CY7C1470V25 & DQa,b/DQPa,b for CY7C1472V25) (or a subset for Byte Write operations, see Write Cycle Description table for details) inputs is latched into the device and the Write is complete.
The data written during the Write operation is controlled by BW (BWa,b,c,d,e,f,g,h for CY7C1474V25, BWa,b,c,d for CY7C1470V25 and BWa,b for CY7C1472V25) signals. The CY7C1470V25/CY7C1472V25/CY7C1474V25 provides Byte Write capability that is described in the Write Cycle Description table. Asserting the Write Enable input (WE) with the selected Byte Write Select (BW) input will selectively write to only the desired bytes. Bytes not selected during a Byte Write operation will remain unaltered. A synchronous self-timed write mechanism has been provided to simplify the Write operations. Byte Write capability has been included in order to greatly simplify Read/Modify/Write sequences, which can be reduced to simple Byte Write operations.
Because the CY7C1470V25/CY7C1472V25/CY7C1474V25 are common I/O devices, data should not be driven into the device while the outputs are active. The Output Enable (OE) can be deasserted HIGH before presenting data to the DQ and DQP (DQa,b,c,d,e,f,g,h/DQPa,b,c,d,e,f,g,h for CY7C1474V25, DQa,b,c,d/DQPa,b,c,d for CY7C1470V25 and DQa,b/DQPa,b for CY7C1472V25) inputs. Doing so will tri-state the output drivers. As a safety precaution, DQ and DQP
(DQa,b,c,d,e,f,g,h/DQPa,b,c,d,e,f,g,h for CY7C1474V25, DQa,b,c,d/DQPa,b,c,d for CY7C1470V25 and DQa,b/DQPa,b for CY7C1472V25) are automatically tri-stated during the data portion of a Write cycle, regardless of the state of OE.
Burst Write Accesses
The CY7C1470V25/CY7C1472V25/CY7C1474V25 has an on-chip burst counter that allows the user the ability to supply a single address and conduct up to four Write operations without reasserting the address inputs. ADV/LD must be driven LOW in order to load the initial address, as described in the Single Write Access section above. When ADV/LD is driven HIGH on the subsequent clock rise, the Chip Enables (CE1, CE2, and CE3) and WE inputs are ignored and the burst counter is incremented. The correct BW (BWa,b,c,d,e,f,g,h for
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