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, which is itself one of many DRAM implementations.
The advantage of DDR2 over DDR SDRAM is its ability to run at much higher clock speeds due to an improved electrical interface. With a clock frequency of 100 MHz, SDR DRAM will transfer data on every rising edge of the clock pulse, thus achieving an effective 100 MHz transfer rate. Unlike SDR, DDR2 will transfer data on every rising and falling edge of the clock (double pumped), achieving an effective rate of 200 MHz with the same clock frequency. DDR2's clock frequency is further boosted by an improved electrical interface running twice as fast as the memory clock, on-die termination, prefetch buffers and off chip drivers. However, latency is increased as a trade-off, especially in the first DDR2 RAM chips that hit the market.
Power savings are achieved primarily due to the improved process technology used to manufacture chips, resulting in a slight drop in required operating voltage (1.8V compared to 2.5V used by DDR). The lower memory clock frequency used (compared to a DDR equivalent) also helps, as a DDR2 product can use a real clock frequency 1/4 that of SDRAM 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.
- DDR2-400: DDR-SDRAM memory chips specified to run at 100 MHz, I/O clock at 200 MHz
- DDR2-533: DDR-SDRAM memory chips specified to run at 133 MHz, I/O clock at 266 MHz
- DDR2-667: DDR-SDRAM memory chips specified to run at 166 MHz, I/O clock at 333 MHz
- DDR2-800: DDR-SDRAM memory chips specified to run at 200 MHz, I/O clock at 400 MHz
- DDR2-1066: DDR-SDRAM memory chips specified to run at 266 MHz, I/O clock at 533 MHz
- PC2-3200: DDR2-SDRAM memory stick specified to run at 200 MHz using DDR2-400 chips, 3.200 GByte/s bandwidth per channel.
- PC2-4200: DDR2-SDRAM memory stick specified to run at 266 MHz using DDR2-533 chips, 4.267 GByte/s bandwidth per channel
- PC2-5400: DDR2-SDRAM memory stick specified to run at 333 MHz using DDR2-667 chips, 5.333 GByte/s bandwidth per channel
- PC2-6400: DDR2-SDRAM memory stick specified to run at 400 MHz using DDR2-800 chips, 6.400 GByte/s bandwidth per channel
- PC2-8500: DDR2-SDRAM memory stick specified to run at 533 MHz using DDR2-1066 chips, 8.533 GByte/s bandwidth per channel
The GDDR2 offshootEdit
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 (very important) doubling of the I/O clock rate 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 (G)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 450 MHz (compared to 400 MHz on the regular 5800 or 500 MHz on the 5800 Ultra). Sounds somewhat confusing, but the technology used doesn't really matter to the end-user as long as acceptable speeds and temperatures are achieved.
ATI Technologies's Radeon 9800 Pro with 256 MByte memory (not the 128 MByte version) also used (G)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 (G)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.
DDR2 was introduced at two initial speeds - 200 MHz (referred to as PC2-3200) and 266 MHz (PC2-4300). Both perform worse than their DDR equivalents since total access times are (in the worst case) twice as large. However, DDR won't officially be introduced at any speeds above 200 MHz. "DDR-533" RAM exists, but JEDEC has stated that such a standard won't be approved by them. These modules are mostly manufacturer optimizations and 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) in early 2006. AMD seems to wait for at least DDR2-667 chips to be available for an affordable price to reduce the Latency penalty for their systems.
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... DDR3.