DDR2 SDRAM
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DDR2 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. DDR2 is part of the SDRAM family of random access memory technologies.
The advantage of DDR2 over DDR-I is its ability to work at higher effective clock speeds.
DDR2 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 100 MHz, SDR will transfer data on every rising edge of the clock, thus achieving an effective 100 MHz transfer rate. Like DDR, DDR2 will transfer data on every rising and falling edge of the clock (double pumped), achieving an effective rate of 200 MHz. However, DDR2's bandwidth is further boosted by an increased number of buffers, improved prefetch, reduced electrical requirements, improved packaging and on-die termination. However, latency is dramatically increased as a trade-off.
Power savings are achieved primarily due to the improved process technology used to manufacture chips, but also because of the slight drop in operating voltage, and the lower clock frequency used (compared to a DDR equivalent). A DDR2 product can use a clock frequency of 1/4 that of SDR, whilst maintaining approximately 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.
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1 The GDDR2 offshoot |
Chip specification
- PC2 3200 (DDR2-400): clocked at 100MHz, data moves at 400 MHz (bandwith 3.2 GB/s)
- PC2 4200 (DDR2-533): clocked at 133MHz, data moves at 533 MHz (bandwith 4.2 GB/s)
The following new DDR2 SDRAM specifications are currently rare, highly expensive and are only used on top-end Intel (925X or above) chipset models.
- DDR2-600
- DDR2-733
- DDR2-800
- DDR2-933
- DDR2-1066
DDR2-SDRAM are made with the following memory amount:
- 64 MB (rare and obsolete)
- 128 MB (becoming rare)
- 256 MB (lower mainstream)
- 512 MB (upper mainstream)
- 1024 MB (top end and expensive)
The GDDR2 offshoot
The first commercial product to claim using the "DDR2" technology was the NVIDIA GeForce FX 5800 series of graphics cards. However, it is important to note that the "DDR2" memory used on graphics cards (officially referred to as GDDR2) is not exactly the same as DDR2, but rather something of an early melding of the DDR and DDR2 technologies. In particular, the ability to transfer 4 bits of data per pin per clock cycle is missing. It had severe trouble with overheating, because nominal DDR voltages were still being used. ATI has since created the improved GDDR3 memory, which is more true to the DDR2 specifications (though with several differing features, making it better suited for graphics card use) and has largely replaced GDDR2 in graphics cards.
After DDR2's introduction with the GeForce FX 5800 series the 5900 series reverted to DDR, as did the 5950, but nVidia's old mainstream card, the 5700 Ultra, used DDR2 clocked at 900 MHz (compared to 800 MHz on the regular 5800 or 1GHz on the 5800 Ultra).
ATI Technologies's Radeon 9800 Pro with 256 MB memory (not the 128 MB version) also used DDR2, but this was because it required fewer pins than DDR. The Radeon 9800 Pro 256 MB only runs its memory at 20 MHz faster than the 128 MB version, primarily to counter the performance hit caused by higher latency and the increased number of chips. It is speculated that the DDR2 used on ATI's 9800 Pro 256 MB was actually supposed to be used on the GeForce FX 5800 series, but ended up unused after NVIDIA decided to halt the 5800 line's production. The 9800XT that followed reverted to DDR, and later on ATI began to use GDDR3 memory on their Radeon X800 line.
Integration
DDR2 was introduced at two initial speeds - 400 MHz (referred to as PC2-3200) and 533 MHz (PC2-4300). Both perform worse than their DDR equivalents owing to the fact that total access times are (in the worst case) twice as large. However, DDR won't officially be introduced at 533 MHz. "DDR533" RAM exists, but JEDEC has stated that such a standard won't be approved by them. These modules use significantly more power than slower modules.
Currently, at least Intel supports DDR2 in their 9xx chipsets. AMD also has plans to add DDR2 support into their AMD64 processors (that all have memory controllers integrated into them) at some point in 2005 or 2006.
Alternatives
Generally, DDR2 is expected to have little competition in main computer memory sector. However, there are three alternatives.
The first is Rambus XDR DRAM (eXtreme Data Rate DRAM). This technology can achieve very high clock speeds, but Rambus has been virtually disowned by IBM PC compatible chipset makers, and it is considered more likely that XDR will find use in set-top appliances and the like. Sony has selected XDR for use in PlayStation 3.
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 SDRAM (QDR), which is considered the natural successor to DDR technologies (DDR2 uses some QDR transfer methods, though is still very much based on DDR technology). However, QDR is not currently considered to be even a remotely viable product due to high production costs and poor speeds currently achieved by such modules - most barely achieve 66 MHz (266 MHz effective), and the technology may not be viable until late in the decade.
Finally, the yet-to-be-finalised evolution of DDR2 is... DDR-III.
External links
- Overview of DDR-II technology (http://www.lostcircuits.com/memory/ddrii/)
- Overview of Rambus XDR (http://www.gamepc.com/labs/view_content.asp?id=xdrpreview&page=1)
- Overview of QBM (http://www.xbitlabs.com/articles/memory/display/qbm.html)pt:DDR-II