Electronic Design

Buffering Extends Capacity

Three types of dual-in-line memory modules (DIMMs) are sold in the aftermarket or included as original equipment. The lowest-cost modules, known as unbuffered, contain only the memory chips and a small serial presence detect EEPROM for identification. These DIMMs are used in most PCs and commodity systems, where bus loading is limited to just two or three modules. The modules are unbuffered, so memory-subsystem performance (access time) will deteriorate as more modules plug into the memory bus.

For servers and other memory-hungry systems, registered DIMMs can extend the memory capacity and maintain performance. They reduce bus loading and provide better timing margins by adding registers and a phase-locked loop between the memory chips and the bus. This works well for banks of up to about four DIMMs (8 Gbytes using 1-Gbit DRAMs and two ranks per DIMM). Registers latch the command/address input signals on the rising edge of the clock. The signals then are sent to the DDR2 DRAMs on the following rising edge of the clock. This delays data access by one clock cycle but improves overall performance.

However, many servers are pushing memory needs well beyond the 8-Gbyte limit. To satisfy those needs, the fully buffered (FB) DIMM architecture allows up to 192 Gbytes per controller-or 48 DIMMs, each DIMM built with two ranks of 1-Gbit DRAMs (x4 organization). This results in a 243 boost in storage capacity, a fourfold increase in data bandwidth (40 Gbytes/s versus 10 Gbytes/s with registered DIMMs), and a lower pin count (420 versus 480), simplifying board design.

At the heart of the FB DIMM lies an advanced memory buffer (AMB) chip. It contains the timing circuits, serialization and deserialization logic, and additional circuits to pass signals through to the next DIMM (see the figure). Infineon, NEC, Texas Instruments, and a few other companies are currently developing and sampling AMB chips to support DIMM manufacturers. A few DIMM vendors are expected to start volume production of FB DIMMs in the third quarter of this year.

For more about FB DIMMs, go to www.memforum.org.

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