Екінші буын DDR SDRAM жады: DDR2 технологиясы

Second generation of double data rate synchronous dynamic random access memory
In addition to double pumping the data bus as in DDR SDRAM (transferring data on the rising and falling edges of the bus clock signal), DDR2 allows higher bus speed and requires lower power by running the internal clock at half the speed of the data bus. The two factors combine to produce a total of four data transfers per internal clock cycle. Since the DDR2 internal clock runs at half the DDR external clock rate, DDR2 memory operating at the same external data bus clock rate as DDR results in DDR2 being able to provide the same bandwidth but with better latency. Alternatively, DDR2 memory operating at twice the external data bus clock rate as DDR may provide twice the bandwidth with the same latency. The best rated DDR2 memory modules are at least twice as fast as the best rated DDR memory modules. The maximum capacity on commercially available DDR2 DIMMs is 8GB, but chipset support and availability for those DIMMs is sparse and more common 2GB per DIMM are used.

DDR2 SDRAM was first produced by Samsung in 2001. In 2003, the JEDEC standards organization presented Samsung with its Technical Recognition Award for the company's efforts in developing and standardizing DDR2. DDR2 was officially introduced in the second quarter of 2003 at two initial clock rates: 200 MHz (referred to as PC2 3200) and 266 MHz (PC2 4200). Both performed worse than the original DDR specification due to higher latency, which made total access times longer. However, the original DDR technology tops out at a clock rate around 200 MHz (400 MT/s). Higher performance DDR chips exist, but JEDEC has stated that they will not be standardized. These chips are mostly standard DDR chips that have been tested and rated to be capable of operation at higher clock rates by the manufacturer. Such chips draw significantly more power than slower clocked chips, but usually offered little or no improvement in real world performance. DDR2 started to become competitive against the older DDR standard by the end of 2004, as modules with lower latencies became available.

The key difference between DDR2 and DDR SDRAM is the increase in prefetch length. In DDR SDRAM, the prefetch length was two bits for every bit in a word; whereas it is four bits in DDR2 SDRAM. During an access, four bits were read or written to or from a four bit deep prefetch queue. This queue received or transmitted its data over the data bus in two data bus clock cycles (each clock cycle transferred two bits of data). Increasing the prefetch length allowed DDR2 SDRAM to double the rate at which data could be transferred over the data bus without a corresponding doubling in the rate at which the DRAM array could be accessed. DDR2 SDRAM was designed with such a scheme to avoid an excessive increase in power consumption. 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 four bits deep, whereas it is two bits deep for DDR. While DDR SDRAM has typical read latencies of between two and three bus cycles, DDR2 may have read latencies between three and nine cycles, although the typical range is between four and six. Thus, DDR2 memory must be operated at twice the data rate to achieve the same latency. Another cost of the increased bandwidth is the requirement that the chips are packaged in a more expensive and difficult to assemble BGA package as compared to the TSSOP package of the previous memory generations such as DDR SDRAM and SDR SDRAM. This packaging change was necessary to maintain signal integrity at higher bus speeds. Power savings are achieved primarily due to an improved manufacturing process through die shrinkage, resulting in a drop in operating voltage (1.8 V compared to DDR's 2.5 V). The lower memory clock frequency may also enable power reductions in applications that do not require the highest available data rates. According to JEDEC the maximum recommended voltage is 1.9 volts and should be considered the absolute maximum when memory stability is an issue (such as in servers or other mission critical devices). In addition, JEDEC states that memory modules must withstand up to 2.3 volts before incurring permanent damage (although they may not actually function correctly at that level).

DDR2 DIMMs are not backward compatible with DDR DIMMs. The notch on DDR2 DIMMs is in a different position from DDR DIMMs, and the pin density is higher than DDR DIMMs in desktops. DDR2 is a 240 pin module, DDR is a 184 pin module. Notebooks have 200 pin SO DIMMs for DDR and DDR2; however, the notch on DDR2 modules is in a slightly different position than on DDR modules. Higher speed DDR2 DIMMs can be mixed with lower speed DDR2 DIMMs, although the memory controller will operate all DIMMs at same speed as the lowest speed DIMM present.

GDDR2, a form of GDDR SDRAM, was developed by Samsung and introduced in July 2002. The first commercial product to claim using the "DDR2" technology was the Nvidia GeForce FX 5800 graphics card. However, this GDDR2 memory used on graphics cards is not DDR2 per se, but rather an early midpoint between DDR and DDR2 technologies. Using "DDR2" to refer to GDDR2 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 based on DDR2 SDRAM, though with several additions suited for graphics cards. GDDR3 was commonly used in graphics cards and some tablet PCs. However, further confusion has been added to the mix with the appearance of budget and mid range graphics cards which claim to use "GDDR2". These cards actually use standard DDR2 chips designed for use as main system memory although operating with higher latencies to achieve higher clockrates. These chips cannot achieve the clock rates of GDDR3 but are inexpensive and fast enough to be used as memory on mid range cards.