As a TI Fellow and director of TI’s Kilby Research Labs, Ajith Amerasekera’s job is to predict the future and plot a roadmap to it. His keynote at day two of the low-power electronics show (ISLPED) in Austin—“Ultra Low Power Electronics in the Next Decade”—did both. [Spoiler alert:] There are some major bridges to be crossed and the arrival end point is far from guaranteed.
Just as they have for the last several years, portable devices will continue to drive growth in the electronics industry. Far from just handsets, the mobile internet—also encompassing “the internet of things”—represents a huge expansion of the semiconductor application space to include a wide range of wireless home entertainment, automotive safety and autonomous industrial, military and medical devices. The mobile internet promises to be 10-100x larger in unit volume than the desktop internet ever was.
Amerasekera distinguishes between two types of portable electronics: performance “hub” devices such as computers, multi-media devices, wireless hubs and PDAs which have 1W to 5W needs today; and distributed, largely autonomous systems with micro and nano watt needs. A typical autonomous system—for example, wireless strain gauges in bridges and aircraft wings—has a life expectancy of up to 10 years. Assuming such a device is powered by today’s typical 100 mAh cell phone battery, the average power available from the battery is less than 1 µW. That isn’t possible with today’s technologies.
The problem is that battery technology has been scaling at about 2x every 10 years compared to semiconductor technology, which scales 2x every 18 months. The gap between what portable electronic devices demand and what batteries can deliver will continue to grow. Don’t expect much improvement from the battery camp any time soon. “The energy density of lithium-ion batteries is so high that they’re really like small hand grenades,” said Amerasekera. There isn’t much left on the atomic scale that has a higher energy density and isn’t radioactive.
How Do You Manage?
Lacking more capable batteries, silicon performance advances require power management. A lot of very effective techniques have been developed over the last several years. At 65 nm leakage power was reduced 300x vs. what it had been at 90 nm through a combination of SDRAM retention, logic power gating, channel length reduction, logic retention, process/temperature AVS and dynamic voltage and frequency scaling (DVFS). At 45 nm new techniques were devised—including adaptive body bias (ABB) and Retention ‘Til Access (RTA)—that resulted in 1000x reduction in active power. Still, Amerasekera—like Jan Rabaey in his keynote yesterday—is concerned that we’ve run out of tricks at the component level that will scale.
According to Amerasekera, future advances in ultra-low-power electronics will come at the system level. He gives the example of running an FFT in software, which requires 28 uW. Running it in hardware requires only 1.6 uW, an 18x improvement. Dropping the core voltage yields a further 1.8x power savings, for a total improvement of 28x. The SoC running the FFT now draws < 1uA.
3D chip techniques have finally evolved to the point where they can help optimize bandwidth, power and area. Currently 3D means package on package (POP) or stacked die. FinFET technology now enables more dense dies, and and die-to-die interconnects—vias connecting disparate digital, analog and RF layers—are becoming…viable.
Renewable energy—wind, solar, hydro and heating systems—has tremendous potential, though they’re all faced with economic as well as technical challenges. Energy harvesting also has a lot of potential, but efficiencies of such systems are quite low, as is the amount of energy they can deliver. Still, there’s a place for them going forward.
Divide and Conquer
Basically, the mobile internet will need a variety of energy sources:
- Batteries for general functionality
- Storage caps for high current functions
- Energy scavenging for extended battery life
- Wireless power sources for connection to the grid
The mobile internet will also require intelligent energy management and control, including
- Highly efficient on-chip power processing
- Control of energy sources and delivery
- Management of power demand and access
- Unreliable energy sources (aka wind, solar, etc.)
The challenge for the next decade will be coming up with another 2-3 orders of magnitude of power reduction to meet the demands of an increasingly wireless world.
Engineers always enjoy working on interesting problems, and this one should stay interesting for years to come.