HP ProLiant DL380p DDR3 memory technology - Page 4

mode whenever the system CPU enters a power down state (C6 for Intel processors). When this occurs, DDR3 DIMMs are placed in a still lower power state and will perform their own refresh cycles. Low voltage DDR3 memory DDR3 memory is also available in low-voltage DIMMs, which can operate at 1.35V instead of the 1.5V for standard DDR3 DIMMs. Low-voltage DDR3 DIMMs consume 10 to 15% less power than standard DDR3 DIMMs. A dual-rank 8 GB low-voltage DDR3 DIMM running at 1333 MT/s consumes about 8.5 watts compared to the 10 watts consumed by a standard voltage DIMM. At lower voltage, electrical loading and signal integrity are greater challenges; therefore, low-voltage DDR3 DIMMs are currently available in only single- and dual-rank configurations from HP. At 1.35V, low-voltage DDR3 DIMMs cannot operate at full speed when the number of DIMMs installed per channel increases. Tables 2 and 3 summarize low-voltage DDR3 DIMM operation in ProLiant servers. Low-voltage DDR3 memory is not currently supported in the Intel-based 4P ProLiant G7 servers. Table 2. Operation of low-voltage DDR3 DIMMs in AMD-based ProLiant G7 servers Single Rank Dual Rank 1 DIMM per channel 1333 MT/s @ 1.35V 1333 MT/s @ 1.35V 2 DIMMs per channel 1333 MT/s @ 1.35V 1333 MT/s @ 1.35V 3 DIMMs per channel 1066 MT/s @ 1.35V 800 MT/s @ 1.35V Table 3. Operation of low-voltage DDR3 DIMMs in Intel-based 2P ProLiant G6 servers Single Rank Dual Rank 1 DIMM per channel 1333 MT/s @ 1.35V 1333 MT/s @ 1.35V 2 DIMMs per channel 1333 MT/s @ 1.35V 1066 MT/s @ 13.5V (UDIMMs) 1066 MT/s @ 1.35V 3 DIMMs per channel 800 MT/s @ 1.35V 800 MT/s @ 1.35V As Tables 2 and 3 illustrate, low-voltage DDR3 DIMMs do reduce power consumption, but they operate at lower data rates in larger configurations. In some cases, these speeds are lower than the data rates at which standard voltage DIMMs operate. Core DDR3 Technologies DDR3 memory specifies data transfer rates that are up to twice that of DDR2 memory. Achieving these rates required significant engineering work to improve electrical signal integrity and new technologies to address the increasingly small timing tolerances. DDR3 has also added features to improve overall reliability and manageability. Fly-by topology Fly-by topology is one of the key technological innovations that allow DDR3 DIMMs to achieve twice the speed of DDR2. In general, it refers to how address and command lines are routed to the DRAMs on the DIMM module, and the timing adjustments required on the memory controller to compensate for this. Fly-by topology is best understood in comparison to the T-Branch topology used in DDR2. Figure 2 shows how one of the address/command signals is routed to the DRAMs in an unbuffered DDR2 DIMM compared to a DDR3 DIMM. The symmetrical T-branch topology of DDR2 is designed to 4