8.2.2.1.8 DDR2 SDR Memory Interface

The DDR2 SDRAM is a high-speed CMOS, dynamic random-access memory. It is internally configured as a multibank DRAM. The controller DDR-2 memory interface consists of four 32-Mb by 16-bit wide, DDR-2 interfaces with double-data-rate signaling, operating at 133.33 MHz (nominal). A bidirectional data strobe (DQS, DQS) is transmitted externally, along with data, for use in data capture at the receiver. DQS is a strobe transmitted by the DDR2 SDRAM during READ commands and by the memory controller during WRITE commands. DQS is edge-aligned with data for READ commands and center-aligned with data for WRITE commands.

The DDR2 SDRAM 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 DDR2 SDRAM 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 ACTIVATE command, which is then followed by a READ or WRITE command. The address bits registered coincident with the ACTIVATE 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.