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Rewiring Transistors for the Internet of Things

Minima Processor is based in Oulu, a small city located in central Finland, thousands of miles from the birthplace of the transistor, the fundamental building block of all electronics. The company has not raised heaps of venture capital, nor has it targeted markets like artificial intelligence, but it is the latest to acknowledge that transistors need a facelift.

The transistors packed into integrated circuits act like switches. They can be switched on so that current flows through them or switched off to stem the flow of current. The digital switch is flipped when the transistor’s supply voltage passes a certain threshold, and that input voltage is an important factor in the entire chip’s power consumption.

Minima and its competitors are targeting ways to lower the threshold voltage in any and every chip. It would a significant boon to Internet of Things applications like wireless sensors embedded in bridges or buried in farmland, which need chips that consume so little power that they last on a single battery for a decade while shooting data to the cloud.

Minima is targeting near-threshold voltage transistors that switch below the normal threshold. But it is not stooping to the same level as rivals Eta Compute and Ambiq Micro, which enable lower subthreshold voltage operation. Even though near-threshold transistors eat more energy, Minima said that they sidestep the shortcomings of subthreshold silicon.

Instead of selling actual chips, the company peddles hardware and software solutions to other companies that want to tweak their chips for near-threshold voltages. Minima claims that it can optimize any chip’s operating voltage, giving it the perfect balance of power and performance. That could allow processors to be powered by tiny sensors that turn light in an office building into energy or piezoelectric devices that do that same with movement.

The technology did not come out of nowhere. Over forty years ago, the first subthreshold voltage chips were sold for watches and pacemakers. Transistors were not the microscopic slivers of silicon carved into modern chips, so engineers could optimize them manually. But the technique became obsolete as transistor density doubled every 18 months.

These chips could get a second life after Moore’s Law ends. At the recent International Electron Devices conference, Advanced Micro Devices’ chief executive Lisa Su said that the average chip consumes 7% more power per year and that it now takes around 29 months to double the number of transistors that fit into a square inch of silicon.

“That is harder and more expensive now, and you might not get the power that you want, even though on the performance side you probably will,” said Lauri Koskinen, Minima’s co-founder and chief technology officer, in an August interview. “[We] are a classic case of not taking the straightforward route of advancing process nodes.”

Typically, near-threshold voltage chips switch with around 400 to 700 millivolts. On the other hand, subthreshold voltage designs operate between 200 and 400 millivolts, significantly lower than normal threshold voltages around 1,200 millivolts. Both lower power consumption, which is proportional to the square of the voltage.

Minima’s technology is called dynamic margining. The company inserts tiny bits of circuitry that monitor the voltage and timing of the transistors and order the chip to modify its power usage based on performance needs. This is also critical because transistors operating below the normal threshold voltage are sensitive to slight variations in input voltage.

These variations can be produced by the slightest manufacturing flaws, extreme temperature changes and other sources of noise, causing severe changes in the transistor’s output voltage. Minima’s timing and voltage monitors can detect these voltage palpitations and take transistors out of commission until they return to normal.

In addition to dynamic margining, the company sells middleware that acts like a car’s cruise control system on hilly highways. The dynamic voltage and frequency scaling technology presses the gas for intensive computing tasks and throttles down for simpler jobs, switching between bursts of performance and long stretches of inactivity. Together, the two technologies can lower any existing processor’s power to microwatts, Minima said.

Other companies are targeting lower voltages. PsiKick claims that its subthreshold voltage wireless microcontrollers consume less than one percent of the power of competing devices. The company has raised $35 million in several rounds of funding since it was founded by chip architects David Wentzloff and Benton Calhoun in 2012.

Wentzloff and Calhoun, former wards of Massachusetts Institute of Technology’s dean Anantha Chandrakasan, have tested a prototype chip in a wearable heart rate monitor that runs entirely on body heat. The company, based in Santa Clara, California, has yet to release its first product. It has swapped out its C.E.O. three times in the last two years.

Eta Compute is trying to compel chips to operate in the deep subthreshold voltage range. The company based in Westlake, California, claims to cut supply voltages down to 250 millivolts. Its delay insensitive asynchronous logic can be used to craft computer cores that act like neurons in the human brain, making them extremely efficient at artificial intelligence tasks. The company recently hired Intel’s chief scientist in charge of neuromorphic computing.

Eta Compute is staffed with some former employees of Ambiq Micro, whose $80 million of financing since 2010 and slate of corporate customers give it a leading position in the market for subthreshold voltage chips. Huawei uses Ambiq’s microcontrollers in its latest line of fitness trackers, which it claims can last 21 days on a single battery charge.

Ambiq retails the lowest power microcontrollers on the market, according to the Embedded Microprocessor Benchmark Consortium. It is on the second generation of its Apollo line of ARM Cortex-M4 chips, which can withstand variations that invariably afflict subthreshold voltage circuits. It uses Taiwan Semiconductor Manufacturing Corp. for production.

It is still unclear whether companies are willing to tolerate the idiosyncrasies of subthreshold voltage chips. With such small voltages, the differences between 1s and 0s shrink significantly. The transistors, therefore, are more susceptible to temperature conditions and manufacturing defects, which can mess with output voltages and cause functional faults.

ARM, the founding father almost all smartphone chips, is in the process of concocting its own near-threshold voltage chips. But it has deployed a different strategy for subthreshold silicon, playing the role of investor and partner vis-à-vis Ambiq and Minima. Eta Compute has declined to state its relationship with ARM, which has plugged its novel architecture.

Mike Muller, ARM’s chief technology officer, however, has thrown cold water on subthreshold voltage chips. In 2013, he argued that computer chips with kilohertz of clock frequency can hardly do anything useful. He also warned against the risks of outsourcing manufacturing to foundries that cannot guarantee that subthreshold voltage transistors will work at such low voltages.

Minima’s technology has been used to tape out a cryptographic accelerator chip, but it did not disclose the customer’s name. In two years, the company has raised $8.4 million to expand its engineering and sales teams, giving it more financial firepower to compete with rivals fighting below normal threshold voltages to chart a new course in transistors.

“Companies in the semiconductor space always fight each other in the same way to lower their power,” said Paul Washkewicz, vice president of sales and marketing for Eta Compute. “We created a different way,” he said in a March interview with Electronic Design.

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