Functional Overview
All synchronous inputs pass through input registers controlled by the rising edge of the clock. All data outputs pass through output registers controlled by the rising edge of the clock.
The CY7C1218H supports secondary cache in systems utilizing either a linear or interleaved burst sequence. The interleaved burst order supports Pentium and i486™ processors. The linear burst sequence is suited for processors that utilize a linear burst sequence. The burst order is user selectable, and is determined by sampling the MODE input. Accesses can be initiated with either the Processor Address Strobe (ADSP) or the Controller Address Strobe (ADSC). Address advancement through the burst sequence is controlled by the ADV input. A two-bit on-chip wraparound burst counter captures the first address in a burst sequence and automatically increments the address for the rest of the burst access.
Byte Write operations are qualified with the Byte Write Enable (BWE) and Byte Write Select (BW[A:D]) inputs. A Global Write Enable (GW) overrides all Byte Write inputs and writes data to all four bytes. All Writes are simplified with on-chip synchronous self-timed Write circuitry.
Three synchronous Chip Selects (CE1, CE2, CE3) and an asynchronous Output Enable (OE) provide for easy bank selection and output tri-state control. ADSP is ignored if CE1 is HIGH.
Single Read Accesses
This access is initiated when the following conditions are satisfied at clock rise: (1) ADSP or ADSC is asserted LOW,
(2)CE1, CE2, CE3 are all asserted active, and (3) the Write signals (GW, BWE) are all deasserted HIGH. ADSP is ignored if CE1 is HIGH. The address presented to the address inputs
(A) is stored into the address advancement logic and the address register while being presented to the memory array. The corresponding data is allowed to propagate to the input of the output registers. At the rising edge of the next clock the data is allowed to propagate through the output register and onto the data bus within tCO if OE is active LOW. The only exception occurs when the SRAM is emerging from a deselected state to a selected state, its outputs are always tri-stated during the first cycle of the access. After the first cycle of the access, the outputs are controlled by the OE signal. Consecutive single Read cycles are supported. Once the SRAM is deselected at clock rise by the chip select and either ADSP or ADSC signals, its output will tri-state immediately.
Single Write Accesses Initiated by ADSP
This access is initiated when both of the following conditions are satisfied at clock rise: (1) ADSP is asserted LOW, and
(2)CE1, CE2, CE3 are all asserted active. The address presented to A is loaded into the address register and the address advancement logic while being delivered to the
memory array. The Write signals (GW, BWE, and BW[A:D]) and ADV inputs are ignored during this first cycle.
ADSP-triggered Write accesses require two clock cycles to complete. If GW is asserted LOW on the second clock rise, the data presented to the DQ inputs is written into the corre- sponding address location in the memory array. If GW is HIGH, then the Write operation is controlled by BWE and BW[A:D]
signals. The CY7C1218H provides Byte Write capability that is described in the Write Cycle Descriptions table. Asserting the Byte Write Enable input (BWE) with the selected Byte Write (BW[A:D]) 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.
Because the CY7C1218H is a common I/O device, the Output Enable (OE) must be deasserted HIGH before presenting data to the DQ inputs. Doing so will tri-state the output drivers. As a safety precaution, DQ are automatically tri-stated whenever a Write cycle is detected, regardless of the state of OE.
Single Write Accesses Initiated by ADSC
ADSC Write accesses are initiated when the following condi- tions are satisfied: (1) ADSC is asserted LOW, (2) ADSP is deasserted HIGH, (3) CE1, CE2, CE3 are all asserted active, and (4) the appropriate combination of the Write inputs (GW, BWE, and BW[A:D]) are asserted active to conduct a Write to the desired byte(s). ADSC-triggered Write accesses require a single clock cycle to complete. The address presented to A is loaded into the address register and the address advancement logic while being delivered to the memory array. The ADV input is ignored during this cycle. If a global Write is conducted, the data presented to DQs is written into the corre- sponding address location in the memory core. If a Byte Write is conducted, only the selected bytes are written. 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.
Because the CY7C1218H is a common I/O device, the Output Enable (OE) must be deasserted HIGH before presenting data to the DQ inputs. Doing so will tri-state the output drivers. As a safety precaution, DQs are automatically tri-stated whenever a Write cycle is detected, regardless of the state of OE.
Burst Sequences
The CY7C1218H provides a two-bit wraparound counter, fed by A1, A0, that implements either an interleaved or linear burst sequence. The interleaved burst sequence is designed specif- ically to support Intel Pentium applications. The linear burst sequence is designed to support processors that follow a linear burst sequence. The burst sequence is user selectable through the MODE input.
Asserting ADV LOW at clock rise will automatically increment the burst counter to the next address in the burst sequence. Both Read and Write burst operations are supported.
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, CE3, ADSP, and ADSC must remain inactive for the duration of tZZREC after the ZZ input returns LOW.
