May 17, 2007

DDR3 vs. DDR2

About three years ago DDR2 memory first appeared on the desktop PC scene. It would be impossible to say it burst on the scene since it was introduced with the unimpressive Intel NetBurst processors. In that market DDR2 was more like a trickle since it was mainly a curiosity for a processor that was running a distant second place to the leading AMD Athlon chips, which were still powered by DDR memory.

DDR2 finally became the universal standard last May/June when AMD switched to DDR2 on their new AM2 platform and Intel introduced Core 2 Duo, the new CPU performance leader. Core 2 Duo resided on socket 775, which also was fed by DDR2. While it sometimes seems like centuries ago, it is worth remembering that Intel Core 2 Duo regained the CPU performance crown less than a year ago, and the two years prior to that all the fastest systems used AMD Athlon 64/X2/FX processors.

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To provide compatibility and interchangeability for computer memory, the structure and form factor are controlled by a standards organization known as JEDEC. JEDEC specifies voltages, speeds, timings, communication protocols, bank addressing, and many other factors in the design and development of memory DIMMs. Taking a closer look at publications at www.jedec.org can provide insight into what DDR3 brings to the market and where it might go. Comparing DDR2 and DDR3 several interesting points stand out.

Official JEDEC Specifications

DDR2 DDR3
Rated Speed 400-800 Mbps 800-1600 Mbps
Vdd/Vddq 1.8V +/- 0.1V 1.5V +/- 0.075V
Internal Banks 4 8
Termination Limited All DQ signals
Topology Conventional T Fly-by
Driver Control OCD Calibration Self Calibration with ZQ
Thermal Sensor No Yes (Optional)

Please keep in mind that JEDEC specs are official. They are a starting point for enthusiast memory companies. However, since there was never a JEDEC standard for memory faster than DDR-400 then DDR memory running at faster speeds is really overclocked DDR-400. Similarly DDR2 memory faster than DDR2-800 is actually overclocked DDR2-800 since there is currently no official JEDEC spec for DDR2-1066.


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