A Picowatt Processor

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Additionally, the researchers paid close attention to the energy loss that occurs while the chip is in sleep mode, or not collecting or processing data. Transistors in the newest computers are made using a 45-nanometer process in which features on a chip are 45 nanometers in size. While this allows for more transistors on a smaller chip, it also results in electrical leakage, due to the physics of the materials at this scale. Blaauw and his team opted for larger transistors made using a 180-nanometer process, from a previous generation of chips. These transistors are in a "sweet spot," says Blaauw. They are big enough to have minimal leakage and yet small enough for the researchers to fit a large number on a one-millimeter-square chip.

To further minimize leakage, the researchers added special transistors that completely shut off the power supply to the processing transistors when the chip is in standby mode. This is a common approach, says Blaauw, but his team took it to the extreme and dedicated much more of the chip than usual to these "power-gating" transistors. "If a normal [chip] designer would look at this, he'd say, 'You're out of your mind,'" Blaauw says. "But it gives us the power-savings trade-off we need." In sum, the researchers combined a number of already existing tricks and fine-tuned them to achieve the record-breaking low power consumption.

The Michigan team, which is also led by Dennis Sylvester, professor of electrical engineering and computer science, still must add a battery to the Phoenix, and it needs to develop a way for data to be offloaded from the chip for further analysis. Once this is done, the researchers can work on full integration within a biological system, which could take years.

Berkeley's Rabaey, who is writing a book on low-power processors, says that the work is significant. "What has impressed me is that they've driven this to quite extreme numbers," he says. "The energy consumption is extremely low. Nobody else has come even close to this." Rabaey notes that this processor is intended for specialty sensor applications and that it won't show up in a cell phone anytime soon. However, it's an important step toward building implantable medical sensors whose batteries can last for years.

The idea of this low voltage chip is not new, says Rabaey: it's been used successfully in the watch industry for decades. But within the past few years, academic and industry interest in such design has blossomed as engineers are exploring more varied and ubiquitous uses of sensors, devices that require energy-saving tricks in order to be practical.