DDR2 SDRAM
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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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Overview
The advantage of DDR2 over DDR SDRAM is the ability for much higher clock speeds, due to design improvements. With a clock frequency of 100 MHz, "SDR-SDRAM" transfers data on every rising edge of the clock pulse, thus achieving an effective 100 MHz data transfer rate. Unlike SDR, 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. DDR2's clock frequency is further 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's 2 bits wide for DDR and 8 bits wide for DDR3.
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 — DDR2 can use a real clock frequency 1/2 that of SDRAM whilst maintaining the same bandwidth.
Spec standards
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
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 bandwidth
- 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. Because DDR2 is 64-bit wide memory, there are eight bits in a byte, and 64 divided by eight is eight.
Backwards compatibility
DDR2 is NOT backwards compatible with DDR. The notch in DDR2 is in a different position than DDR.[1]
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 the "DDR2" memory used on graphics cards (officially referred to as GDDR2) is not DDR2 per se but an early midpoint of DDR and DDR2 technologies. In particular, the (very important) 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 format further, into GDDR3, which is more true to the DDR2 specifications, though with several graphics card - suited additions.
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.
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-600 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.
Currently, at least Intel supports DDR2 in their 9xx chipsets. AMD also has plans to add DDR2 support into their AMD64 processors (all of which have on-die memory controllers) in spring 2006 with the introduction of Socket AM2.
DDR2 SDRAM DIMMs have 240 pins (as opposed to 184 pins on DDR DIMMs, and 168 pins on SDRAM DIMMs).
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.
DDR-2's as of yet unfinalized evolution is DDR3.