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DDR2-SDRAM is a high-performance synchronous dynamic random-access memory interface. It is the successor to DDR-SDRAM. It offers new features, greater bandwidth and lesser power consumption than its predecessor. DDR2-SDRAM was succeeded by DDR3-SDRAM.
Compared to DDR, DDR2-SDRAM is complex in architecture. The memory cells in DDR2-SDRAM are activated in a manner that helps them to operate with an external bus. Similar to DDR, DDR2 also transfers data at twice the clock speed. But in case of DDR2, the bus is clocked at twice speed compared to DDR-SDRAM. A salient feature in DDR2-SDRAM is in the reduction of operating voltage, thereby leading to lower power consumption and higher operating speed. Data strobe in DDR2-SDRAM can be programmed to operate in a differential mode. All the above factors help DDR2-SDRAM to achieve four data transfers per internal clock cycle. The storage limit associated with a single memory module based on DDR2-SDRAM is 4GB. The bus frequency in DDR2-SDRAM is also boosted with the help of electrical interface improvements, off-chip drivers and prefetch drivers. Another improved feature in DDR2-SDRAM is in the increase in prefetch length, which is four bits compared to two bits for every bit in a word of DDR-SDRAM. DDR2-SDRAM is capable of transferring 64 bits of data twice per clock cycle.
Improvements achieved by DDR2-SDRAM unfortunately comes at a cost. DDR2-SDRAM is more expensive compared to its predecessor. Although DDR2-SDRAM can provide the same bandwidth as DDR-SDRAM, it is with higher latency. DD2-SDRAM is both forward and backward non-compatible.