DDR2 is an evolutionary enhancement over DDR. This technology is currently the largest selling technology today and offers the best cost per bit. Most new system designs today would likely choose to use DDR2.
It takes advantage of the DDR improvements to transfer data on both edges of the clock. In addition, it further improves the speeds to provide standard transfer rates of PC2-3200 (5.0 ns clock), PC2-4200 (3.75 ns clock), PC2-5300 (3.0 ns clock), and PC2-6400 (2.5 ns clock). DDR2 chips save further power over the previous technologies because these chips use a standard voltage of 1.8 volts.
For low power applications, STEC has capabilities to provide low power 1.55V modules.
DDR2 doubles the bandwidth available to your system and runs twice as fast as regular DDR. DDR2 works very similar to DDR except that double the data bits are fetched internally to enable transferring the sequential data at twice the speed. Like DDR, the data is transferred on both the rising and falling edge of the clock cycle enabling DDR2 to literally double the data transfer rate of the original bus clock. For example; with DDR2 SDRAM, a memory running with a clock of 333 or 400MHz will yield an effective data rate of 667 or 800 mega-transfers per second (MTS), doubling the data rate.
DDR2 is a technology supported by a newer generation of DRAM controllers with DDR2 enabled logic. DDR2 DIMM modules are available in a variety of form factors including 240 pin full-sized DIMM, 200 pin So-DIMM, 240 pin Very Low Profile (VLP) DIMM, 244 pin Mini-DIMMs, and 240 pin FB-DIMMs. These form factors are not interchangeable with the preceding DDR or succeeding DDR3 technology form factors.
DIMMs have several different options for providing signal buffering on the DIMM to improve the performance and allow the system to support more DIMMs in a system. The list below describes the different types of buffering that can be on a DIMM.
The Very Low Profile (VLP) DIMM was created to save height for space constrained systems, including blade servers and ATCA (Advanced Telecommunications Architecture) applications. The DIMMS are also 133.5mm wide and have the same pinout as the DDR, DDR2, and DDR3 full-sized DIMMs. However, these modules will typically have a height between 18-19mm. This shortened height allows the designer to use a vertical socket and still fit the DIMM into his constrained chassis space.
The So-DIMM is a shortened DIMM form factor that was created originally for use in notebook computers. This module has a width of 67.6mm and these will typically use a horizontal socket when designed into the system. The original So-DIMM was defined without ECC for use in notebooks. But additional versions that include ECC have been created for embedded, telecomm, and other applications that require additional reliability. The So-DIMM can come in versions with pin-counts of 144 pins (SDRAM), 200 pins (DDR and DDR2), and 204 pins (DDR3).
The mini-DIMM is a relatively new form factor that was invented to provide the benefits of a shortened DIMM length but still provide additional pin-count to support new features in the DDR2 technology. Mini-DIMMs are currently available for only DDR2 in a 244-pin module and will always include ECC. These modules are 82mm long. It is likely that future extensions to the DDR3 standard will also provide this form factor for DDR3 technologies.
The VLP mini-DIMM is a new form factor that combines the shorter length of the mini-DIMM with the shorter height of the VLP DIMM. This form factor is a good solution for highly space constrained systems that only need a small to moderate amount of memory. The typical size of a VLP mini-DIMM is 82 mm long with a height between 18-19 mm.
A Registered DIMM has a chip called a Register chip on the DIMM module that will latch and buffer the address lines. Because of this Register chip, the DRAM controller will only see one load for the address lines instead of the 9 or 18 or more loads that would be present if the addresses were routed to every DRAM chips. The lighter loading will allow the DRAM controller to drive more DIMMs and at faster speeds. This type of DIMM module is commonly used in servers where there may be 8 or 16 or more DIMMs in a system. Registered DIMMs are only offered in versions with ECC.
An Unbuffered DIMM has no buffering on the DIMM. The DRAM controller will drive the address lines directly to each chip on the DIMM and the DIMM outputs will be go from the DRAM chip to the DRAM controller. This type of DIMM is commonly used on desktop and laptop PCs where the total amount of memory is limited and only one or two DIMMs are used in a system. Unbuffered DIMMs can be offered in versions without or without ECC.
The Fully Buffered DIMM is an architecture that was specifically developed for use specifically with Intel-based DDR2 server chipsets. It is similar to the LRDIMM because both the address and the data lines are buffered. However, the FBDIMM also implements a serialization protocol where the address/data/commands are transmitted in narrow channels of 14 northbound (towards the chipset) or 10 southbound (towards additional DIMMs) lanes using a differential signaling. The channels are clocked at a very fast rate of 12 times the memory clock rate. All the DIMMs in a channel are organized in a daisy chain fashion so very large systems with very large numbers of DIMMs can be configured. The FBDIMM is also only available in versions with ECC and is only available for use with DDR2 chip technology.
• Low Power Consumption & System Cost
• Operating Voltage Only of 1.8 for DDR2 Compared to 2.5 in DDR
• Speeds for DDR2 can reach speeds as high as DDR2-800
• 40% Lower Power Consumption Than DDR-400
• Lower Heat Generation Allows Systems To Run Cooler
• Higher Density: Up to 8GB Modules (Using 2Gb Chips)
Four standard operating frequencies for the DDR2 SDRAM Memory Module specification were developed to provide the use with the most suitable option: PC2-3200, PC2-4200, PC2-5300,
and PC2-6400.
PC2-3200 memory module configurations utilize DDR2-400 components to achieve the highest performance, with a peak bandwidth of 3.2GB/s when used in systems operating on 200MHz memory clock frequency.
PC2-4200 memory module configurations utilize DDR2-533 components to achieve the highest performance, with a peak bandwidth of 4.2GB/s when used in systems operating on 266MHz memory clock frequency.
PC2-5300 memory module configurations utilize DDR2-667 components to achieve the highest performance, with a peak bandwidth of 5.3GB/s when used in systems operating on 333MHz memory clock frequency.
PC2-6400 memory module configurations utilize DDR2-800 components to achieve the highest performance, with a peak bandwidth of 6.4GB/s when used in systems operating on 400MHz memory clock frequency.
