Cypress manual CY7C1546V18, CY7C1557V18 CY7C1548V18, CY7C1550V18, Functional Overview

CY7C1546V18, CY7C1557V18 CY7C1548V18, CY7C1550V18

Functional Overview

The CY7C1546V18, CY7C1557V18, CY7C1548V18, and CY7C1550V18 are synchronous pipelined Burst SRAMs equipped with a DDR interface.

Accesses are initiated on the rising edge of the positive input clock (K). All synchronous input and output timing is referenced from the rising edge of the input clocks (K and K).

All synchronous data inputs (D[x:0]) pass through input registers controlled by the rising edge of the input clocks (K and K). All synchronous data outputs (Q[x:0]) pass through output registers controlled by the rising edge of the input clocks (K and K).

All synchronous control (R/W, LD, NWS[x:0], BWS[x:0]) inputs pass through input registers controlled by the rising edge of the input clock (K).

CY7C1548V18 is described in the following sections. The same basic descriptions apply to CY7C1546V18, CY7C1557V18, and CY7C1550V18.

Read Operations

The CY7C1548V18 is organized internally as two arrays of 2M x

18.Accesses are completed in a burst of two sequential 18-bit data words. Read operations are initiated by asserting R/W HIGH and LD LOW at the rising edge of the positive input clock

(K). The address presented to the address inputs is stored in the read address register. Following the next two K clock rise, the corresponding 18-bit word of data from this address location is

driven onto the Q[17:0] using K as the output timing reference. On the subsequent rising edge of K, the next 18-bit data word is driven onto the Q[17:0]. The requested data is valid 0.45 ns from the rising edge of the input clock (K and K). To maintain the internal logic, each read access must be allowed to complete. Read accesses can be initiated on every rising edge of the positive input clock (K).

When read access is deselected, the CY7C1548V18 first completes the pending read transactions. Synchronous internal circuitry automatically tri-states the output following the next rising edge of the positive input clock (K). This enables a seamless transition between devices without the insertion of wait states in a depth expanded memory.

Write Operations

Write operations are initiated by asserting R/W LOW and LD LOW at the rising edge of the positive input clock (K). The address presented to address inputs is stored in the write address register. On the following K clock rise, the data presented to D[17:0] is latched and stored into the 18-bit write data register, provided BWS[1:0] are both asserted active. On the subsequent rising edge of the negative input clock (K), the infor- mation presented to D[17:0] is also stored into the write data register, provided BWS[1:0] are both asserted active. The 36 bits of data are then written into the memory array at the specified location. Write accesses can be initiated on every rising edge of

the positive input clock (K). Doing so pipelines the data flow such that 18 bits of data is transferred into the device on every rising edge of the input clocks (K and K).

When the write access is deselected, the device ignores all inputs after the pending write operations are completed.

Byte Write Operations

Byte write operations are supported by the CY7C1548V18. A write operation is initiated as described in the Write Operations section. The bytes that are written are determined by BWS0 and BWS1, which are sampled with each set of 18-bit data words. Asserting the appropriate Byte Write Select input during the data portion of a write latches the data being presented and writes it into the device. Deasserting the Byte Write Select input during the data portion of a write enables the data stored in the device for that byte to remain unaltered. This feature can be used to simplify read, modify, or write operations to a byte write operation.

Double Date Rate Operation

The CY7C1548V18 enables high-performance operation through high clock frequencies (achieved through pipelining) and DDR mode of operation. The CY7C1548V18 requires a minimum of two No Operation (NOP) cycle during transition from a read to a write cycle. At higher frequencies, some applications require third NOP cycle to avoid contention.

If a read occurs after a write cycle, address and data for the write are stored in registers. The write information is stored because the SRAM cannot perform the last word write to the array without conflicting with the read. The data stays in this register until the next write cycle occurs. On the first write cycle after the read(s), the stored data from the earlier write is written into the SRAM array. This is called a Posted write.

If a read is performed on the same address on which a write is performed in the previous cycle, the SRAM reads out the most current data. The SRAM does this by bypassing the memory array and reading the data from the registers.

Depth Expansion

Depth expansion requires replicating the LD control signal for each bank. All other control signals can be common between banks as appropriate.

Programmable Impedance

An external resistor, RQ, must be connected between the ZQ pin on the SRAM and VSS to allow the SRAM to adjust its output driver impedance. The value of RQ must be 5x the value of the intended line impedance driven by the SRAM. The allowable range of RQ to guarantee impedance matching with a tolerance of ±15% is between 175Ω and 350Ω, with VDDQ = 1.5V. The output impedance is adjusted every 1024 cycles upon power up to account for drifts in supply voltage and temperature.

Document Number: 001-06550 Rev. *E Page 8 of 28

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