By Ed Sperling

Ratcheting down the voltage in an SoC design seems like the simplest way to reduce power consumption, but it doesn’t always work out that way. In fact, reducing voltage can have some rather strange and unexpected effects at all levels of chip design, including testing and debugging.

The problem is that not all parts of the chip work the same way without a minimum amount of voltage flowing through it. Gates don’t shut, current leaks in strange places and power budgets fall apart.

Jan Rabaey, who heads the University of California’s Berkeley Wireless Research Center, said one of the biggest surprises is that reducing voltage can actually increase current leakage.

“Everyone will have to reduce the voltage,” he said. “There is no other way. But one of the major challenges is that for a device to work, the gate must work. If you decrease the threshold voltage, you get more leakage.”

So far, this is relatively new ground for most chip engineers because the main focus has been on performance, not reducing the power of the chip. But he said that will change at future nodes as power budgets become the key competitive differentiator for devices rather than performance.

“There are other challenges, too,” he said. “If you decrease the supply voltage, it is more susceptible to noise.”

The research under way at universities is being mirrored in the corporate world, as well. Rob Aitken, R&D Fellow at ARM, said his company has been working on dropping the operating voltage, but with each step a new problem crops up that has to be solved.

“Similar to what was proposed for synthesis 20 years ago, a lot of these low-power techniques operate by taking slack in the design and stealing it to save power,” said Aitken. “So you lower the voltage, which causes it to run a little slower. That causes paths that were near critical before to become critical. Then you start adding multi-threshold devices to improve the leakage, and the paths start wandering up toward critical again. When you subsequently put things in different power domains, you have a low-voltage domain and a high-voltage domain, and that makes a significant step toward critical.”

Adding multiple power domains only makes the problem worse sometimes.

“When you start doing multiple power domains you can also get weird leakage paths and weird analog behavior where things you think are shut off really aren’t,” he said. “You get strange leakage currents that flow back and forth. When you try to debug this stuff, it helps a lot if you have scan wrappers. Power gating is the other thing that matters a lot in debugging. It’s easy to find out if a switch is turned on. You need to manage the inrush current. So when you have a domain that’s off, if you just flip the switch and it all goes on, there’s a good chance that any state retention you were thinking of has been corrupted by the power bouncing around on the network.”

Still, most researchers are optimistic that these problems can be solved, and often with huge power savings. But exactly what devices will look like in the future, and how many workarounds are necessary to make them work, remains to be seen.

For additional information on this subject, check out Bhanu Kapoor’s blog on verifying low-voltage designs.

By Ed Sperling

In portable devices, low-power design has always been about stretching out the amount of time between battery charges or replacement. But the focus of current research throws that approach to the wind.

The new goal is to get rid of batteries altogether and generate power using a variety of different mechanisms ranging from differences in temperature, the motion of waves, static electricity and even pH imbalances.. Known as energy scavenging—or by the more marketing-friendly term of energy harvesting—this technique is the next frontier of low-power engineering. And as you might expect, the possibilities are mind-boggling.

Imagine, for example, the possibility of a pacemaker with no battery. Currently, most pacemakers require an operation to remove a large battery once every decade or so, which requires a medical operation to open up the chest and replace the battery. While there has been work in using rechargeable batteries, with wires or electrodes extending through the chest, there is a better alternative. Work is under way to use the heart’s own pumping action to power the pacemaker. The strategy is that only enough current has to be stored for an activity cycle, not for perpetual use.

Use only what you need is a constant theme in energy harvesting. Voltree Power, based in Canton, Mass., is in the final stages of prototyping a sensor network that can be shimmed into trees and connected as a grid to monitor whether trees are healthy, dry, or on fire—something that can provide an early warning to firefighters before a blaze can get started or burn out of control. The power for this network is derived from pH differences between the soil and the tree.

This is more than science fiction. It’s big business, and many of the largest electronics companies on the planet are working feverishly to get a foothold in this portion of the embedded market. “These are all ultra-low designs,” said Adrian Valenzuela, product marketing engineer in Texas Instruments’ Advanced Embedded Group. “We’re talking microwatts in standby and 100 to 200 microamps per MIP.”

For TI, the question is just how small can a device be shrunk and still harvest enough and store power for the duty or activity cycle. Size is relative to the amount of energy that can be stored, and devices that harvest energy generally store at least some of it, although not as much as a traditional battery provides. That opens the door for thin-film batteries, which can be printed on anything from silicon to a piece of foil or plastic, or a capacitor that releases its energy all at once instead of steadily like a battery. But by shrinking the amount of power needed for one or two cycles of a microcontroller or embedded processor, which may be all that’s necessary to trip a signal, even the battery can be shrunk.

Other sources of energy are being tested, as well, for different applications, but the general goal is the same—using less power requires less storage. In some cases, the power can even be generated as you need it. “The Holy Grail of the medical community is to harvest energy from the skin so you can monitor vital signs,” said Valenzuela. “It’s wireless and battery-less.”

Vibration in roads can be used to power sensors in roads and bridges that monitor everything from strain on materials to traffic patterns. In-ground meters can be powered by solar cells. And the military is looking at the motion of the body to power electronics needed by a soldier, something that TI believes will be able to cut as much as 30 pounds from a typical soldier’s gear.

Peter Harrop, chairman of IDTechEx, a Cambridge, England-based company, said the ultimate goal is to make small devices self-sufficient and self-healing—something that will require a level of low-power intelligence in devices. “Right now, about 90 percent of the applications that are possible we can’t do because the batteries can’t be replaced,” Harrop said. “The good news is that when you use energy harvesting, we can make these things last 20 years or more. The typical battery lasts two to three years.”

That can be extremely costly, as the Port Authority of New York and New Jersey discovered with its EZPass electronic toll booth devices. The batteries in the RFID sensors failed and 1.5 million had to be replaced earlier this decade at a cost of $24 million.

“In energy harvesting, the use of solar cells is well-established,” Harrop said. “There’s a lot of work being done in other areas, though. One is a mechanism located in the heel of your shoe, although you probably will never get beyond airport security with that. Another is a device that ties around your knee. And there are devices being tested that use the heat differential between your car exhaust and the outside air, which can generate a lot of electricity.”

Harrop said MIT is researching ways to generate electricity from car shock absorbers, as well. “The key is that we need better ways of turning ambient energy into electricity, which is what we’re starting to see with microbiofuel cells, which are generating electricity from dirt. That’s being studied at Harvard. There also will be body area networks, which may be star networks without wires. That’s being developed by the Holst Centre in the Netherlands.”

Also under study is piezoelectricity, which is generated when mechanical pressure is applied to materials such as lead zirconate titanate crystals. EnOcean, based in Oberhaching, Germany, can use pressure from your fingers to generate piezoelectricity to turn light switches on and off.

Much work also is being done by the University of California at Berkeley in the form of low-power wireless sensors, with the ultimate hope that someday the chemicals in the body will power in-body networks and transmitters. (See the related video.)

History of energy scavenging

For all the interesting work being done in energy scavenging, it is hardly a new concept. Man has been scavenging energy in one form or another since the dawn of time.

However, it is only in the past couple of centuries that devices have actually been engineered to take advantage of energy scavenging. One of the most famous examples is Foucault’s Pendulum, first demonstrated in 1851 and located in the Pantheon in Paris, which swung in accordance with the rotation of the Earth. And while it was a great scientific breakthrough at the time, the buzz surrounding Foucault’s swinging ball had nothing to do with energy scavenging. It was proof of the Earth’s rotation, something that was already well understood at the time but hard to prove to the average person.

Portable energy scavenging took hold in a number of auto-wind watches, whose mechanism was first patented in 1863 by Adrien Philippe, the founder of the venerable watchmaker, Patek Philippe. That slipping mechanism has been applied to quartz watches more recently, but the real winner in many devices is small solar cells. They began finding their way into handheld calculators in the 1980s and in watches in the 1990s.

Jan Rabaey, head of UC Berkeley’s Wireless Research Center and the author of a new book entitled, “Low Power Design Essentials,” discusses how to generate small amounts of power and what can be done to reduce the amount of power needed for semiconductor design.