“Double Data Rate, Two” or simply, DDR2, is an extension of a system’s standard DDR memory. Like DDR, DDR2 transfers data on both edges of the system clock, which allows the bandwidth of the memory to equal twice that of Single Data Rate memory, or SDRAM. DDR2’s architecture is intended to increase efficiency and performance over its predecessor, DDR.
- DDR’s standard voltage setting is 2.5 V, while DDR2’s voltage setting is 1.8 V.
- DDR2 memory is DDR2’s 4-bit data pre-fetch. DDR has two sets of data that are read and written to the memory core, while DDR2 allows for four sets of data to be processed.
- DDR’s write latency is one clock. This allows for data to be written to the memory one clock after the write command has been issued. With DDR2, the write latency becomes two clocks. The doubled clock cycle allows for twice the data to be written to the memory after the write command has been issued.
- DDR2’s new feature called On-Die Termination (ODT). ODT permits the user to terminate signals in the memory itself. The termination of DDR is done on the motherboard. ODT drastically reduces signal wave reflections from the termination network and promotes enhanced systems margins. ODT permits fewer capacitors and resistors on the motherboard, which helps reduce cost and makes memory subsystem layouts to be more efficient.
