DDR2 SDRAM
DDR-II SDRAM (Double Data Rate Two Synchronous Dynamic Random Access Memory) is a computer memory technology that, as of 2005, is becoming the mainstream standard for personal computer memory. DDR-II is part of the SDRAM family of random access memory technologies.
The advantage of DDR-II over DDR-I is its ability to work at higher effective clock speeds.
DDR-II represents a further doubling of the "effective" clock rate over DDR, which itself doubled the effective clock rate of SDR. With a clock frequency of 100Mhz, SDR will transfer data on every rising edge of the clock, thus achieving an effective 100Mhz transfer rate. Like DDR, DDR-II will transfer data on every rising and falling edge of the clock (double pumped), achieving an effective rate of 200MHz. Other speed increases are achieved through an increased number of buffers, an improved prefetch, reduced electrical requirements, improved packaging and on-die termination. However, latency is increased.
Power savings are achieved primarily due to the improved process technology used to manufacture chips, but also the lower clock frequency used. A DDR-II product can use a clock frequency of 1/4 that of SDR, whilst maintaining the same bandwidth. A lower clock frequency is both easier to route across a circuit board, and results in lower power usage, particularly when the data bus is latent.
Chip specification
- PC3200: DDR2-SDRAM memory chips specified to run with 400 MHz
- PC4200: DDR2-SDRAM memory chips specified to run with 533 MHz
History
The first commercial product to use the technology was the nVidia GeForce FX 5800 series of graphics cards. The 5900 series reverted to DDR, as did the 5950, but nVidia's old mainstream card, the 5700 Ultra, used DDR-II clocked at 900MHz (compared to 800MHz on the 5800, and 1GHz on the 5800 Ultra). ATI Technologies's Radeon 9800 Pro with 256MB memory (not the 128MB version) also used DDR-II, but this was because DDR-II requires fewer pins than DDR. The Radeon 9800 Pro 256MB only runs its memory at 20MHz faster than the 128MB version, primarily to counter the performance loss caused by higher latency and the increased number of chips. It's thought that the DDR-II used on the 9800 Pro 256MB is actually memory that was supposed to be used on the GeForce FX 5800 series, but ended up being unused after nVidia decided to halt production of the 5800 line. The newest ATI chip, the 9800XT reverted to DDR, and ATI later began to use GDDR-3 memory on their Radeon X800 line.
It's important to note that the DDR-II memory used on graphics cards (sometimes referred to as GDDR-2) is actually something of a melding of DDR and DDR-II technologies, and had trouble with heat overproduction owing to the fact that DDR voltages were still being used. ATI has since created GDDR-3 memory, which is more true to the DDR-II specifications (though has a few additional features that help suit it to graphics cards), and has largely replaced DDR-II in graphics cards.
Future
DDR-II was introduced at two initial speeds - 400MHz (referred to as PC2-3200) and 533MHz (PC2-4300). Both perform worse than their DDR equivalents owing to the fact that total access times are (in the worst case) double that of their equivalents, but DDR is unlikely to be introduced at 533MHz ("DDR533" RAM exists, but JEDEC have stated that such a standard will never be approved by them, and these modules use significantly more power than slower modules). Intel supports DDR-II in their 9xx chipsets, and AMD may integrate DDR-II support into their Athlon 64/Opteron processors at some point in 2005 or 2006.
DDR-II is expected to have little competition in the main memory sector. There are three alternatives. The first is Rambus eXtreme Data Rate (XDR) DRAM. This is expected to achieve very high clockspeeds, but Rambus has been virtually disowned by the chipset makers, and it is considered more likely that XDR will be used in set-top appliances. Next is Kentron Quad Band Memory (QBM), which uses DDR modules with effectively two channels routed to the module. This was briefly supported by VIA, but they have dropped support for the technology, and there are doubts about Kentron’s commercial viability. The final alternative is Quad Data Rate (QDR) RAM, which is considered to be the natural successor to DDR technologies (DDR-II uses some QDR transfer methods, though is still based on DDR technology). However, QDR is not currently considered to be even a remotely viable product owing to high production costs and poor current speeds achievable by such modules (most barely achieve 66MHz, 266MHz effective), and may not be viable until later in the decade.
Finally, the yet-to-be-finalised evolution of DDR-II is... DDR-III.