In 2004 the computer industry has moved to a next-generation memory architecture called DDR2; that will allow the industry to continue implementing faster and higher-density main memory solutions.
The transition from DDR to DDR2 is more evolutionary than revolutionary. DDR2 architecture is the same DRAM-based technology the industry has used for years, but with some enhancements.


DDR2 Enhancements
Higher Speeds — DDR2 will provide higher memory transfer rates. Current industry specifications call for transfer rates of 400, 533, and 667 megatransfers per second (MT/s).
Differential Strobes — This improvement provides greater noise immunity, improved timing margins, and less dependence on voltage reference (Vref) circuit accuracy.

On-Die Termination (ODT) — ODT is probably the most well- known DDR2 feature. DDR2 provides for ODT of data, strobe, and datamask signals on the DRAM, rather than on the motherboard. Two termination values are provided:


75
150
ODT improves signal integrity. It also reduces system component costs, because system designers do not have to terminate the signal on the motherboard.

Off-Chip Driver (OCD) Calibration — Perhaps the most misunderstood new feature is OCD calibration. This feature provides a method for ensuring symmetry between the rising and falling edges of the signal. The result is improved timing margins. OCD calibration can also reduce the impact of variations in vendor silicon processes on the memory interface.

Reduced Voltage Rail — Like previous semiconductor processes, the voltage technology for DDR2 has been scaled down from 2.5 V to 1.8 V. This can result in a power savings, but the trade-off is reduced noise margin.


Implementation Challenges
As the old saying goes, "You can't get something for nothing. Along with increased memory performance, there are a few new design challenges.
Routing Density — In general, DDR2 routing calls for wider (that is, lower-impedance) traces, as well as greater spacing between adjacently routed signals. This consumes valuable space on shrinking board form factors.

Electrical Loading — As the operating speed of the memory interface continues to increase, electrical loading (that is, the number of capacitive loads) will become a limiting factor.


Memory Ranks
Memory ranks may be a new term you may not have seen used in describing system memory. Single-Rank or Dual-Rank is describing whether a memory module is dual sided or single sided.
A single-ranked memory module will have the memory chips or devices mounted to the module on one side. While a dual-ranked memory module will have the memory chips or devices mounted on both sides of the memory module.