HYB25D256T

The 256Mb DDR SDRAM HYB25D256T is a high-speed CMOS, dynamic random-access memory containing 268,435,456 bits. It is internally configured as a quad-bank DRAM.


The 256Mb DDR SDRAM HYB25D256T uses a double-data-rate architecture to achieve high-speed operation. The double data rate architecture is essentially a 2n prefetch architecture with an interface designed to transfer two data words per clock cycle at the I/O pins. A single read or write access for the 256Mb DDR SDRAM effectively consists of a sinnal DRAM ore and two corresponding n-bit wide, onehalf-clock-cycle data transfers at the I/O pins.


A bidirectional data strobe (DQS) is transmitted externally, along with data, for use in data capture at the receiver. DQS is a strobe transmitted by the DDR SDRAM during Reads and by the memory controller during Writes. DQS is edge-aligned with data for Reads and center-aligned with data for Writes.


The 256Mb DDR SDRAM HYB25D256T operates from a differential clock (CK and CK; the crossing of CK going HIGH and CK going LOW is referred to as the positive edge of CK). Commands (address and control signals) are registered at every positive edge of CK. Input data is registered on both edges of DQS, and output data is referenced to both edges of DQS, as well as to both edges of CK.


Read and write accesses to the DDR SDRAM HYB25D256T are burst oriented; accesses start at a selected location and continue for a programmed number of locations in a programmed sequence. Accesses begin with the registration of an Active command, which is then followed by a Read or Write command. The address bits registered coincident with the Active command are used to select the bank and row to be accessed. The address bits registered coincident with the Read or Write command are used to select the bank and the starting column location for the burst access.


The DDR SDRAM HYB25D256T provides for programmable Read or Write burst lengths of 2, 4 or 8 locations. An Auto Precharge function may be enabled to provide a self-timed row precharge that is initiated at the end of the burst access.

An auto refresh mode is provided along with a power-saving power-down mode. All inputs are compatible with the JEDEC Standard for SSTL_2. All outputs are SSTL_2, Class II compatible.