Cypress CY7C1231H manual Functional Overview, Sleep Mode

CY7C1231H

Functional Overview Single Write Accesses

The CY7C1231H is a synchronous flow-through burst SRAM designed specifically 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. Maximum access delay from the clock rise (tCDV) is 6.5 ns (133-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[A:B] 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

A read 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 array and control logic. The control logic determines that a read access is in progress and allows the requested data to propagate to the output buffers. The data is available within 6.5 ns (133-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. On the subsequent clock, another operation (Read/Write/Deselect) can be initiated. When the SRAM is deselected at clock rise by one of the chip enable signals, its output will be tri-stated immediately.

Burst Read Accesses

The CY7C1231H has 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 incremented sufficiently. A HIGH input on ADV/LD will increment the internal burst counter regardless of the state of Chip Enable 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.

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 bus is loaded into the Address Register. The write signals are latched into the Control Logic block. 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 DQs and DQP[A:B].

On the next clock rise the data presented to DQs and DQP[A:B] (or a subset for Byte Write operations, see Truth Table for details) inputs is latched into the device and the write is complete. Additional accesses (Read/Write/Deselect) can be initiated on this cycle.

The data written during the Write operation is controlled by BW[A:B] signals. The CY7C1231H provides Byte Write capability that is described in the Truth Table. Asserting the Write Enable input (WE) with the selected Byte Write Select 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 opera- tions.

Because the CY7C1231H is a common I/O device, 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 DQs and DQP[A:B] inputs. Doing so will tri-state the output drivers. As a safety precaution, DQs and DQP[A:B].are automatically tri-stated during the data portion of a write cycle, regardless of the state of OE.

Burst Write Accesses

The CY7C1231H 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[A:B] inputs must be driven in each cycle of the burst write, in order to write the correct bytes of data.

Sleep Mode

The ZZ input pin is an asynchronous input. Asserting ZZ places the SRAM in a power conservation “sleep” mode. Two clock cycles are required to enter into or exit from this “sleep” mode. While in this mode, data integrity is guaranteed. Accesses pending when entering the “sleep” mode are not considered valid nor is the completion of the operation guaranteed. The device must be deselected prior to entering the “sleep” mode. CE1, CE2, and CE3, must remain inactive for the duration of tZZREC after the ZZ input returns LOW.

Document #: 001-00207 Rev. *B Page 4 of 12

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