Serial links move data between microprocessors and other electronic devices. As vital as they are, serial links are idle 50-70% of the time. Idle, however, does not mean off.
Current technology demands that serial links stay powered up, consuming microprocessor power to the tune of 20%. And, when everything is added up, that could amount to 7% of a data-center‘s power budget.
A breakthrough by researchers at the University of Illinois
A research team from the Department of Electrical and Computer Engineering at the University of Illinois headed by Associate Professor Pavan Kumar Hanumolu and PhD student and lead author Tejasvi Anand have developed a transceiver that converts the “always on” serial link into a burst-mode communication link that is able to power-off when idle and quickly power-on when required.
“Ideally, a burst mode link must be turned on/off in zero time, must consume zero power in the off-state and must incur zero energy overhead while making on/off transitions,” from the abstract of the team’s research paper. “However, these requirements are difficult to meet in practice and as a consequence, the efficacy of burst mode communication in achieving energy proportional operation is reduced.”
Difficult but doable
Hanumolu explains the research sponsored by Semiconductor Research Corporation (which is part of North Carolina’s Research Triangle Park) was to reduce serial-link power consumption by eliminating the link’s power component.
“Consuming zero power” may or may not be not possible, but the research team will do well to improve over earlier burst-mode technology with power-on/off times of a couple hundred nanoseconds for memory interfaces and a few microseconds in the case of Energy Efficient Ethernet. “The long power-on lock time limits the opportunities of the serial link to save power,” explains the press release about the breakthrough. “The Illinois research reduced the power-on lock time to less than 20 nanoseconds, and this resulted in a 44 times reduction in power.”
This press release from the Department of Electrical and Computer Engineering at the University of Illinois states, “The team estimates that data centers in North America can save $870 million annually using this approach, with the yearly serial-link power savings at data centers worldwide by 2020 equaling Japan’s yearly electricity consumption.”
The transceiver’s specifications
To get an idea of what the researchers accomplished, you need to disassemble the title of their paper: 3.7 A 7Gb/s rapid on/off embedded-clock serial-link transceiver with 20ns power-on time, 740μW off-state power for energy-proportional links in 65nm CMOS.
The device consists of a 7 Gb per second transceiver that requires 20 nanoseconds to go from off state to full on state. Hanumolu and Anand almost got to zero power consumption when the link was off with a 740μW off-state power for energy-proportional links in 65nm CMOS. The mention of 65nm CMOS is a reference to the Complementary Metal-Oxide-Semiconductor (CMOS) fabrication process where the printed line widths are 65 nanometers.
“The reduction of serial link power consumption not only benefits processors and mobile platforms,” adds Hanumolu, “but it also reaps continued benefits from Moore’s Law by increasing the computational capacity and off-chip bandwidth for a few more years without adding pressure on the power envelop of the processor.”
The transceiver technology developed by Hanumolu and Anand is amenable to memory and networking applications and from a manufacturing standpoint can apply to any technology node. If I understand correctly, this research should improve mobile-device power usage as well.