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| Home > Support > Technical Resources > FB DIMM Overview |
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Increased Capacity  Higher Performance Improved Scalability
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| The migration of server platforms to 64-bit architectures is shifting the performance bottleneck back to the memory subsystem. Consequently, the requirements for system memory bandwidth and capacity are increasing dramatically. Because of this trend, traditional DDR and DDR2 registered memory modules no longer offer adequate bandwidth or capacity limits for today's mainstream servers. The DDR2 Fully Buffered DIMM (FB-DIMM) design represents a solution to the capacity limitations while offering other significant advantages such as increased performance and scalability. |
Figure 1: FB-DIMM with Heat Spreader |
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While DRAM bit density and data rates for server memory continue to increase, conventional memory module architecture will have impedance (a measure of resistance to electrical current) interruptions that affect signal integrity. This impedance limits the number of DIMMs per channel as data rates increase, resulting in a growing gap between demand for memory capacity and the maximum density possible (as defined by the number of DIMMs per channel). |
Figure 2: Server Capacity Demand Chart |
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Point-to-Point Connection with AMB |
As a newly defined JEDEC standard, the FB-DIMM is based on an innovative architecture that is supported only by a new memory controller, and hence a new chipset on the motherboard. The FB-DIMM itself employs an Advanced Memory Buffer (AMB) chip that buffers the DRAM data pins from the motherboard's memory bus. High speed, point-to-point, serial connections help reduce the motherboard's routing area, (see Figure 3).
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Figure 3: High Speed Point-To-Point Southbound and Northbound Signals |
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Manufactured by leading semiconductor companies, the AMB chip is essential for high-bandwidth, large memory capacity workstations and server applications. The AMB chip collects and distributes data from or to a DIMM. The data is buffered internally on the chip and forwarded to the next DIMM or AMB chip. This unique channel structure allows system designers to use a large number of DIMMs within a single system without adversely affecting speed.
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Significantly Greater Memory Capacity! |
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| Module Density | IC's Employed | 16 FB-DIMM Motherboard | 48 FB-DIMM Motherboard |
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| 1GB | 18 pcs 512Mbit | 16GB | 48GB |
| 2GB | 18 pcs 1Gbit, or 36 pcs 512Mbit | 32GB | 96GB |
| 4GB | 18 pcs 2Gbit, or 36 pcs 1Gbit | 64GB | 192GB |
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Figure 4: FB-DIMM without Heat Spreader |
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Actually, the new FB-DIMM architecture requires one-third fewer pins than previous technology DDR2 registered DIMMs and also delivers enhanced data transfer rates and lower latency. Equally important, the FB-DIMM architecture increases existing memory bandwidth per channel and will accommodate future generations of DRAM technology. |
A robust cyclic redundancy check (CRC) design protects both commands and data signals to boost reliability. Also, transient bit-error detection and retry and "bit-lane fail-over correction" features enable the server board to shut down a bad data path immediately. Optional bit widths and CRC coverage will be applicable to a wide range of server applications. Developed through collaboration with Intel, the FB-DIMM is equipped with a heat spreader that is required to improve heat dissipation.
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FB-DIMM Standard Features:
240-pin DDR2 FB DIMM with ECC to detect and report channel errors to the host memory controller High-speed differential Point-to-point serial, dual-simplex bit lanes: 10-pair southbound (to FBDIMM) and 14-pair northbound (from FBDIMM) 3.2 Gb/s for DDR2-533 and 4.0 Gb/s for DDR2-667 Northbound and southbound single lane fail over and channel error detection transmitter de-emphasis to reduce Inter Signal Interference (ISI) SMBus interface to AMB for configuration register access; supporting-band and out-of-band command access Test features supported include:
- Integrated thermal sensor and status indicator
- Supports MBIST, IBIST and Virtual Host mode
- transparent mode and direct access mode for DRAM testing
Over-temperature detection and alert CAS Latency (CL): DDR2-667 CL=5, DDR2-533 CL=4 Burst Length: 4, 8 Sequential or Interleave Operating Voltage: 1.8V SDRAM; 1.5V AMB Intel Certified Heat Spreader RoHS Compliant product
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