DDR2 SDRAM
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- "DDR2" redirects here. For the video game, see Dance Dance Revolution 2ndMIX.
DDR2 SDRAM or double-data-rate two synchronous dynamic random access memory is a computer memory technology. It is a part of the SDRAM family of random access memory technologies, which is one of many DRAM implementations.
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[edit] Overview
The advantage of DDR2 over its predecessor DDR SDRAM is the ability for much higher clock speeds, due to design improvements. With a clock frequency of 100 MHz, single-data-rate SDRAM transfers data on every rising edge of the clock pulse, thus achieving an effective 100 MHz data transfer rate. However, both DDR and DDR2 are double pumped; they transfer data on the rising and falling edges of the clock, at points of 0.0 V and 2.5 V (1.8 V for DDR2), achieving an effective rate of 200 MHz (and a theoretical bandwidth of 1.6 GB/s) with the same clock frequency. The difference of DDR2 to DDR is a doubled bus frequency for the same physical clock rate, thus doubling the effective data rate another time.
DDR2's bus frequency is boosted by electrical interface improvements, on-die termination, prefetch buffers and off-chip drivers. However, latency is greatly increased as a trade-off. The DDR2 prefetch buffer is 4 bits wide, whereas it is 2 bits wide for DDR and 8 bits wide for DDR3.
Although the effective clock speeds of DDR2 are higher than for DDR, the total performance was no greater in the early implementations, primarily due to the high latencies of the first DDR2 modules. DDR2 started to be effective by the end of 2004, as modules with lower latencies became available.[1]
Power savings are achieved primarily due to an improved manufacturing process, resulting in a drop in operating voltage (1.8 V compared to DDR's 2.5 V). The lower memory clock frequency could also help a board designer save power; DDR2 can use a real clock frequency 1/2 that of SDRAM whilst maintaining the same bandwidth.
[edit] Specification standards
[edit] Chips
- 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
[edit] Sticks/Modules
- PC2-3200: DDR2-SDRAM memory stick specified to run at 200 MHz using DDR2-400 chips, 3.200 GB/s bandwidth
- PC2-4200: DDR2-SDRAM memory stick specified to run at 266 MHz using DDR2-533 chips, 4.267 GB/s bandwidth
- PC2-5300: DDR2-SDRAM memory stick specified to run at 333 MHz using DDR2-667 chips, 5.333 GB/s bandwidth1
- PC2-6400: DDR2-SDRAM memory stick specified to run at 400 MHz using DDR2-800 chips, 6.400 GB/s bandwidth
Note: DDR2-xxx (or DDR-xxx) denotes effective clockspeed, whereas PC2-xxxx (or PC-xxxx) denotes theoretical bandwidth (though it is often rounded up or down). Bandwidth is calculated by taking effective clockspeed and multiplying by eight. This is because DDR2 can transfer 64 bits of data each clock cycle, and since a byte is comprised of 8 bits, this equates to 8 bytes of data per clock cycle.
1Some manufacturers label their DDR2-667 sticks as PC2-5400 instead of PC2-5300. At least one manufacturer has reported this reflects successful testing at a faster-than standard speed. [2]
[edit] Backwards compatibility
DDR2 DIMMs are not backwards compatible with DDR DIMMs. The notch on DDR2 DIMMs is in a different position than DDR DIMMs, and the pin density is slightly higher than DDR DIMMs. DDR2 is a 240-pin module, DDR is a 184-pin module.
[edit] The GDDR offshoot
The first commercial product to claim using the "DDR2" technology was the NVIDIA GeForce FX 5800 graphics card. However, it is important to note that this GDDR-2 memory used on graphics cards such as the GeForce FX 5800 is not DDR2 per se but rather an early midpoint of DDR and DDR2 technologies. Using "DDR2" to refer to GDDR-2 is a colloquial misnomer. In particular, the performance-enhancing doubling of the I/O clock rate is missing. It had severe overheating issues due to the nominal DDR voltages. ATI has since designed the GDDR technology further, into GDDR3, which is more true to the DDR2 specifications, though with several additions suited for graphics cards.
After GDDR2's introduction with the FX 5800 series, the 5900 and 5950 series reverted to DDR, but NVIDIA's old mainstream card, the 5700 Ultra, used GDDR2 clocked at 450 MHz (compared to 400 MHz on the regular 5800 or 500 MHz on the 5800 Ultra).
ATI Technologies's Radeon 9800 Pro with 256 MiB memory (not the 128 MiB version) also used GDDR2, but this was because it required fewer pins than DDR. The Radeon 9800 Pro 256 MiB only runs its memory at 20 MHz faster than the 128 MiB version, and primarily to counter the performance hit caused by higher latency and the increased number of chips. It is speculated that the GDDR2 used on ATI's 9800 Pro 256 MiB 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.
GDDR3 is now commonly used in most NVIDIA- or ATI-based video cards. However, further confusion has been added to the mix with the appearance of budget and mid-range graphics cards which claim to use "DDR2". These cards actually use standard DDR2 chips designed for use as main system memory. These chips cannot achieve the speeds that GDDR3 can but are fast and cheap enough to be used as memory on mid-range cards.
[edit] Integration
DDR2 was introduced at two initial speeds: 200 MHz (referred to as PC2-3200) and 266 MHz (PC2-4200). Both perform worse than their DDR equivalents since heightened latency makes total access times twice as long in the worst case scenario. However, DDR will not officially be introduced at any speeds above 266 MHz (533 MHz effective). DDR-533, and even DDR-1200 SDRAM exists, but JEDEC has stated that they will not be standardized. These modules are mostly manufacturer optimizations of highest-yielding chips, drawing significantly more power than slower-clocked modules, and usually do not offer much, if any, higher real-world performance.
Both Intel and AMD support DDR2.
DDR2 SDRAM DIMMs have 240 pins (as opposed to 184 pins on DDR DIMMs, and 168 pins on SDRAM DIMMs).
[edit] 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 third 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.
[edit] See also
[edit] External links
- Overview of DDR-II technology
- Overview of Rambus XDR
- Overview of QBM
- DDR2 vs DDR
- Corsair DDR2 Primerde:DDR2#DDR2-SDRAM
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