Saturday, August 14, 2010

Chips and modules

Standard name

Memory clock
Cycle time[4]
I/O bus clock
Data rate
Module name

Peak transfer rate

DDR-200 100 10 100 200 2.5±0.2 PC-1600 1600
DDR-266 133 7.5 133 266 2.5±0.2 PC-2100 2133
DDR-333 166 6 166 333 2.5±0.2 PC-2700 2667
DDR-400C 200 5 200 400 2.6±0.1 PC-3200 3200 2.5-3-3

Note: All above listed are specified by JEDEC as JESD79F.[5] All RAM data rates in-between or above these listed specifications are not standardized by JEDEC—often they are simply manufacturer optimizations using higher-tolerance or overvolted chips.

The package sizes in which DDR SDRAM is manufactured are also standardized by JEDEC.

There is no architectural difference between DDR SDRAM designed for different clock frequencies, for example, PC-1600, designed to run at 100 MHz, and PC-2100, designed to run at 133 MHz. The number simply designates the data rate at which the chip is guaranteed to perform, hence DDR SDRAM is guaranteed to run at lower and can possibly run at higher clock rates than those for which it was made.[6] These practices are known as underclocking and overclocking respectively.

DDR SDRAM modules for desktop computers, commonly called DIMMs, have 184 pins (as opposed to 168 pins on SDRAM, or 240 pins on DDR2 SDRAM), and can be differentiated from SDRAM DIMMs by the number of notches (DDR SDRAM has one, SDRAM has two). DDR SDRAM for notebook computers, SO-DIMMs, have 200 pins, which is the same number of pins as DDR2 SO-DIMMs. These two specifications are notched very similarly and care must be taken during insertion if unsure of a correct match. DDR SDRAM operates at a voltage of 2.5 V, compared to 3.3 V for SDRAM. This can significantly reduce power consumption. Chips and modules with DDR-400/PC-3200 standard have a nominal voltage of 2.6 V.

Many new chipsets use these memory types in dual-channel configurations, which doubles or quadruples the effective bandwidth.

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