Review: AMD's DDR chipset and the new Athlon

Advantages offered by DDR-SDRAM

AMD committed themselves to DDR-SDRAM at an early point in time. In TeamDDR, AMD supports the DDR-SDRAM technology as the memory technology of the future in cooperation with VIA, ALi, NVidia, Infineon, Hyundai, and many other companies. The group's target is to make DDR-SDRAM a widespread technology available at a low price as quickly as possible. Even Intel has recognized the potential of DDR and are turning their back on the Rambus memory slowly but surely. Intel's first DDR chipsets are scheduled for mid-2001, though yet unconfirmed.

As compared to Rambus memory, DDR-SDRAM offers a major advantage: it is an evolutionary technology based on standard SDRAM and - to put it in simple terms - only uses the two clock pulse edges for data transmission. PC133-SDRAM runs on the memory bus at a clock speed of 133 MHz. This rate is maintained for the PC266 DDR-SDRAM. The use of both clock pulse edges may - wrongly - cause users to expect a clock speed of 266 MHz.

Although the timing of a DDR memory bus is more sensitive than with an SDR bus, the path layout of a mainboard does not have to be re-designed completely. Mainboards based on 4-layer technology will continue to be sufficient. This is a major advantage for the mainboard manufacturers since existing production lines only have to be adapted to a minor extent, and the plants will continue to be able to produce new boards at low cost. Thus, the integration of PC266 in existing mainboard layouts is relatively simple.

The same applies to memory modules: DDR-SDRAM uses DIMM boards with the same dimensions as with the SDRAM technology. However, the number of pins has risen from 168 to 184. The new pins are required for additional control signals. DDR-DIMMs, moreover, feature different notches in order to avoid the incorrect mounting in PC100/133 slots. The reference designs and specifications for DDR-SDRAM modules are available for free use from AMI and JEDEC.

The Rambus memory did not enjoy the advantage of an evolutionary development. And this is the key problem of Rambus. RDRAM works with a data capacity of only 16 bits and a very high frequency in order to achieve equivalent data transmission rates. This requires completely new mainboard and memory module designs. The propagation delay of signals at frequencies of 800 MHz (both clock pulse edges) is extremely critical. That is why only a few manufacturers come to grips with the production of RIMM boards since maximum precision is required to do so.