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DDR RAM stands for Double Data Rate Random Access Memory. DDR2 is the next generation of DDR RAMs. DDR and DDR2 are both types of SDRAM. The key difference between DDR and DDR2 is that in DDR2 the bus is clocked at twice the speed of the memory cells, so four words of data can be transferred per memory cell cycle. Thus, without speeding up the memory cells themselves, DDR2 can effectively operate at twice the bus speed of DDR.
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. While DDR SDRAM has typical read latencies of between 2 and 3 bus cycles, DDR2 may have read latencies between 4 and 6 cycles. Thus, DDR2 memory must be operated at twice the bus speed to achieve the same latency in nanoseconds.
With DDR, excess signal noise was eliminated by resistors built into the motherboard, but DDR2 has the terminating resistors built into each memory chip. On-Die Termination for both memory and controller in DDR2 improves signaling and reduces system cost.
Another cost of the increased speed is the requirement that the chips are packaged in a more expensive and more difficult to assemble BGA package as compared to the TSSOP package of the previous memory generations such as DDR. This packaging change was necessary to maintain signal integrity at higher speeds.[citation needed]
Power savings are achieved primarily due to an improved manufacturing process through die shrinkage, resulting in a drop in voltage requirements (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 speed.
DDR2 was introduced in the second quarter of 2003 at two initial speeds: 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 normally tops out at speeds around 266 MHz (533 MHz effective). DDR2 started to become competitive with the older DDR standard by the end of 2004, as modules with lower latencies became available.
DDR2 DIMMs are not designed to be 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.
Faster DDR2 DIMMs though, are compatible with slower DDR2 DIMMs. The memory would just run at the slower speed. Using slower DDR2 memory in a system capable of higher speeds results in the bus running at the speed of the slowest memory in use.
