Sub-threshold designs begin to appear in low-power applications

Subthreshold designs are able to scrape by on fractional amounts of system power because they trade off processing speed for low power.(Learn more ultra-low power and subthreshold desing in this sidbar, "Subthreshold design holds low-power promise.")

Toumaz Technology, with its 6 x 6 mm-square Sensium wireless sensor transceiver chip has become one of the first companies to successfully exploit the power-miserly characteristics of subthreshold-biased IC transistors. The chip, developed for medical applications, combines a radio receiver/transmitter for the ISM (~900MHz) band, processor/memory, sensor interface circuitry, and Toumaz' proprietary communications software. If this sounds like 10 lbs of stuff in a 5 lb bag, it is – but the real constraint upon the system was that it had to do all of its sensing and transmitting on an ultra-low-power disposable battery. (More on the power source in a following article.)

The trick with subthreshold design is dealing with yield variations for ultra-low bias currents in the nano-amp or even pico-amp range, because your design is so much more susceptible to offset and threshold voltage variation.: If you want to operate 50 mV below the threshold voltage, and your threshold voltage shifts by 200mV, then you're going to be way off, with a lousy manufacturing yield.

I asked Dr. Alison Burdett, director of technology at Toumaz, about the challenges Toumaz encountered in applying low-threshold design techniques. "In the past, sub-threshold has been very much an academic tool because people can make a few circuits and trim all the bias currents, and say, "Look, I got one to work!" "

"We can't live with a yield of 10% — we can't trim everything by hand. So what we do is use the on-chip processor – which is there anyway to run the sensor interface and communication system - to run calibration routines. When the chip powers up, the processor will go off and apply test signals to the chip and measure the response and automatically tune them." The processor measures the affect of the process variation and adjusts the bias currents as necessary.

As SOCs for embedded systems proliferate, each one with a usually-underutilized processor, look for subthreshold designs to come to the forefront for systems operating on the equivalent of flea-power.