DDR2 SDRAM, an abbreviation for double data rate two synchronous dynamic random access memory, is a type of a random access memory (RAM) commonly used in personal computers and various digital electronics today. This offspring from the SDRAM (synchronous dynamic random access memory), a part of DRAM (dynamic random access memory), is like an evolution from the DDR SDRAM; it can operate the external data bus twice as fast as its predecessor. This was possible by abandoning the original clock rate of the DDR, and operating the memory cells at half the rate. If the DDR2 was clocked at the same rate, the performance would be worse.
The DDR2 was first introduced in 2003 at 200 MHz (PC2-3200) and 266 MHz (PC2-4200), but it failed to outperform the original DDR due to a latency problem. Nonetheless, a combination of the original DDR technology being at a speed of about 266 MHz (but 533 MHz effective), the JEDEC (Joint Electron Device Engineering Council) Solid State Technology Association not standardizing the faster DDR chips, and a release of newer DDR2 modules with lower latencies, the DDR2 eventually began the better solution. By the end of 2004, the DDR2 really began to compete against the standard DDR.
Behind the DDR2 Technology
As DDR2 is a type of an SDRAM, it stores its memory in memory cells. These cells are then activated at according times to correctly operate with an external data bus. Although the DDR2 shares many similarities with the original DDR, as in the case with data transfer happening with the rise and fall at the edge of the clock, it is different in that the DDR2's bus is clocked at twice the speed. This basically allows the DDR2 to be twice as more efficient than the original DDR.
The key behind the high bus frequency with DDR2s (although the latency is augmented as a trade off) is the coalition of electrical interface improvements, prefetch buffers, and various off-chip drivers. This is because DDR's prefetch buffer is twice of the DDR, which is 2bits deep. Also, its read latencies are between four and six cycles; again this is twice that of the DDR. Thus, twice as much bus speed is necessary to achieve an equal latency.
Also, the DDR2 is packaged relatively more expensively and more complicatedly than the DDR SDRAM or the SDR SDRAM, in exchange for a more blazing speed.
Backward compatibility with DDR2 DIMMs are quite limited. In fact, they are not designed to be compatible with DDRM DIMMs at all. This is because firstly, the notch positions in the DDR2 and the DDR are different along with the pin density slightly higher on the DDR DIMMs. Although not much of a "backward compatibility," but faster DDR2 DIMMs can be used with slower DDR2 DIMMs. However, in such a situation, the faster one would run at the speed of the slower DDR2.
Standard DDR2 SDRAM DIMM's
DDR2 SDRAM is normally packaged in DIMM modules.
|DIMM Module||Chip Type||Clock Speed||Data Rate||Transfer Rate|