Double Data Rate (DDR) is an advancement over SDRAM technology that increases memory bandwidth and performance while maintaining competitive pricing. DDR is the evolutionary technology that succeeded the previous standard Synchronous DRAM technology, which is now called SDR (Single Data Rate).
DDR achieves its performance by transferring data on both the rising edge and the falling edge of the clock. Standard speeds for DDR include PC-2100 (7.5 ns clock), PC-2700 (6.0 ns clock), and PC-3200 (5.0 ns clock). Chips with DDR technology also save power because they run at 2.5 or 2.6 volts rather than the higher voltages used by the earlier technologies.
DDR doubles the bandwidth available to your system and runs twice as fast as regular SDRAM. DDR SDRAM works very similar to SDR except that data transactions are performed at both the rising and falling edge of the clock cycle enabling DDR to literally double the data transfer rate of the original bus clock. For example; with DDR SDRAM, a system running with a clock of 100 or 133MHz will yield an effective data rate of 200 or 266 mega-transfers per second (MTS), doubling the data rate.
DDR is a technology supported by a newer generation of DRAM controllers with DDR enabled logic. DDR DIMM modules are available in a variety of form factors including 184 pin full-sized DIMM, 200 pin So-DIMM, and 184 pin Very Low Profile (VLP) DIMM. These form factors are not interchangeable with the preceding SDRAM or succeeding DDR2 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.
• Scalability of Speeds to Reach up to 400 Megatransfers/second
• Lower System Cost
• Lower Heat Generation, Systems Run Cooler
• Available Form Factors: Full-sized DIMM, VLP DIMM & So-DIMM
• Higher Density: Up to 4GB Modules (Using 1Gb chips)
Three standard operating frequencies for the DDR SDRAM Memory Module specification were developed to provide the use with the most suitable option: PC2100, PC2700, and PC3200.
PC2100 memory module configurations utilize DDR-266 components to achieve the highest performance, with a peak bandwidth of 2.1GB/s when used in systems operating on 133MHz clock frequency.
PC2700 memory module configurations utilize DDR-333 components to achieve the highest performance, with a peak bandwidth of 2.7GB/s when used in systems operating on 167MHz clock frequency.
PC3200 memory module configurations utilize DDR-400 components to achieve the highest performance, with a peak bandwidth of 3.2GB/s when used in systems operating on 200MHz clock frequency.
