What is DDR2?
“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.
What are some of the major differences between DDR and DDR2?
One of the main differences between DDR and DDR2 memory is the required voltage at which the memory runs. DDR's standard voltage setting is 2.5 V, while DDR2's voltage setting is 1.8 V. Although it may not be a phenomenal difference, the lower voltage specification allows more space for operation at higher frequencies.
A second difference between DDR and 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. DDR2's internal data bus is doubled, which increases the die size. Although the die size is increased, the memory core internally operates at half the frequency. This allows DDR2 to run at the same frequency of DDR, but at twice the speed. For instance, DDR2 memory at 400MHz runs at the same frequency as DDR memory running at 200MHz.
A third difference is the write latency of DDR2. 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.
A fourth difference is 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.
Why use DDR2?
There are several reasons why DDR2 should be considered. First, the required voltage is lower and reduces the system power demanded by the memory. Second, the increased pre-fetch produces a reduced core speed dependency for better yields. Third, DDR2 allows migration of higher bus speeds. Fourth, its write latency provides an improvement in command bus frequency. Finally, DDR2 provides additional features that improve the overall performance and effectiveness of the memory.
So why isn't everyone using DDR2?
DDR2's performance gains are not evident until you get to DDR2-533. In fact, it is not until you get to DDR2-667 where you begin to see measurable performance gains. This is due to the increase in latency that offsets any speed advantage until 667MHz. For the sake of comparison, DDR-400 outperforms DDR2-400 and runs comparable to DDR2-533.
DDR2 also costs significantly more than DDR. Since the 110nm fabrication process is far from standard, the cost of the DDR2 DIMMS is much higher and quantities are much more limited.
The true benefit of DDR2 is in its ability to achieve faster speeds with its lower voltage requirement. Eventually, DDR2 will make its presence felt in the marketplace, but it will be some time before the performance gains are truly realized.
To learn more about DDR2 and its specifications, AMD recommends contacting memory vendors who have developed or are developing DDR2 memory.
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