DDR2 SDRAM

DDR2 SDRAM is a double data rate synchronous dynamic random access memory interface. It supersedes the original DDR SDRAM specification and the two are not compatible. In addition to double pumping the data bus as in DDR SDRAM (transferring data on the rising and falling edges of the bus clock signal), DDR2 allows higher bus speed and requires lower power by running the internal clock at one quarter the speed of the data bus. The two factors combine to require a total of 4 data transfers per internal clock cycle.

With data being transferred 64 bits at a time, DDR2 SDRAM gives a transfer rate of (memory clock rate) × 2 (for bus clock multiplier) × 2 (for dual rate) × 64 (number of bits transferred) / 8 (number of bits/byte). Thus with a memory clock frequency of 100 MHz, DDR2 SDRAM gives a maximum transfer rate of 3200 MB/s.

Since the DDR2 internal clock runs at half the DDR internal clock rate, DDR2 memory operating at the same external data bus clock rate as DDR results in DDR2 being able to provide the same bandwidth but with higher latency. Consequently, DDR2 RAM possesses inferior performance. Alternatively, DDR2 memory operating at twice the external data bus clock rate as DDR may provide twice the bandwidth with the same latency. The best-rated DDR2 memory modules are at least twice as fast as the best-rated DDR memory modules.