Double Data Rate SDRAM: Fast Performance at an Economical Price

Double Data Rate SDRAM: Fast

TECHNOLOGY BRIEF (cont.) Performance at an Economical Price

Doc Number TC020603TB

Reduced Power Requirements

DDR SDRAM has another significant improvement. The voltage supply for DDR SDRAM uses

only 2.5 V, instead of 3.3 V. The lower voltage and the lower capacities inside the memory chips

lead to a significantly reduced power consumption, which makes DDR SDRAM a good choice for

density-optimized servers.

DDR SDRAM DIMMs

DDR SDRAM DIMMs come with 184 pins instead of the 168 pins used by standard SDRAM

DIMMs. Figure 6 shows a photo of an unbuffered DDR SDRAM DIMM. Note that the DDR

DIMM has one notch instead of the two notches found in SDRAM DIMMs. DDR SDRAM

DIMMs cannot be used in servers that support standard SDRAM DIMMs.

Figure 6. Unbuffered DDR SDRAM DIMM

DDR SDRAM Performance

Initially, DDR SDRAM that operates at 100 MHz was named PC200 and DDR SDRAM that

operates at 133-MHz was named PC266. The numbering for PC200 and PC266 referred to the

effective clock of the data transfer. Then, Rambus used PC600 and PC800 to describe their 300-

MHz and 400-MHz RDRAM modules. PC600 and PC800 may sound faster than PC200 and

PC266, but they are not. Therefore, the memory industry developed a numbering system based on

the actual peak data transfer rate in MB/s; for example, PC1600 and PC2100. Thus, PC200 is the

same as PC1600 (64 bits * 2 * 100 MHz = 1600 MB/s) and PC266 is the same as PC 2100 (64 bits

* 2 * 133 MHz = 2133 MB/s).

Table 1 summarizes the naming conventions of various types of DDR SDRAM and their theoretical

bandwidths.

Table 1. Naming Conventions and Theoretical Bandwidths of DDR SDRAM

Component Naming

Convention

Module Naming

Convention

Type Bus Speed Peak Bandwidth

DDR200 PC1600 DDR-I 100MHz 1.6 GB/s

DDR266 PC2100 DDR-I 133 MHz 2.1 GB/s

DDR333 PC2700 DDR-I 166 MHz 2.7 GB/s

DDR400 PC3200 DDR-II 200 MHz 3.2 GB/s

DDR533 PC4300 DDR-II 266 MHz 4.3 GB/s

DDR667 PC5300 DDR-II 333 MHz 5.3 GB/s

DDR800 PC6400 DDR-II 400 MHz 6.4 GB/s