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Semiconductors and Components

Freescale to Foundries: Help Us With Threshold-Voltage MCUs

27 February 2014

Geoff Lees, general manager of the microcontroller business unit at Freescale Semiconductor Inc. (Austin, Texas), wants the foundries and IP licensors to do more to help with low-power microcontrollers enabled by running them at low voltages.

In the extreme cases this is known as near-threshold and even sub-threshold operation. The transistor threshold voltage is a knee in the I-V curve where the transistor turns on and significant current flows and is in the region of 0.3 to 0.6V.

Traditionally transistors have been operated at much higher voltages because this is a linear part of the I-V curve. However, as power consumption is proportional to the square of the voltage there has been an ever-present incentive to reduce voltage and get closer to the non-linear operational region. Although transistors can be characterized at these near- and sub-threshold voltages and consume much less energy there, switching times are longer making performance slower.

Speaking at the Embedded World exhibition in Nuremberg, Germany, Lees said that Freescale engineers are looking at using forward well-biasing as a means of maintaining yield of suitably performing circuits while operating at low voltages. He also said that dynamic well-biasing on a circuit-by-circuit basis within a design may be the way to go. It has been shown by engineers from IBM in learned presentations that reverse bias can be used to reduce standby power, and forward bias used to improve the speed or to reduce active power of circuits.

But in an interview Lees also indicated that foundries and IP providers are not yet sufficiently focused on low voltage operation. "The foundries don't characterize libraries over a wide enough voltage range. We want characterized logic and memories into the sub-threshold region," he said.

"It's not only libraries and the process nodes. It's things like POP models," Lees continued. POP is the term used by processor IP licensor ARM Holdings plc (Cambridge, England) for processor optimization packages, which help licensees find the highest performance implementations of ARM Cortex processors. "POP models tend to be focused on performance," Lees said.

Although Freescale is a broad licensee of ARM cores at the high-end, the Cortex-A series, it is also a licensee of Cortex-M series cores optimized for microcontroller applications and which when deployed at the "leaf nodes" of the Internet of Things will come under pressure to reduce power consumption.

The Internet of Things (IoT) is a gift to Freescale because it maps so directly to the markets that the company already plays in, according to Steve Wainwright, the company's general manager of the Europe, Middle-east and Africa region for Freescale. And flashing up numbers from Cisco that estimate there will be 50 billion devices connected to the Internet in 2020 does paint a rosy picture as the microcontroller vendor plays in automotive, industrial, networking, medical and consumer applications.

However, there is the possibility that such growth could be limited by energy consumption – unless chip vendors such as Freescale can reduce power consumption by one, two or more orders of magnitude. Hence the interest in sub- to near-threshold operation for applications that do not need multimegahertz clock frequency performance but can operate more energy efficiently at kilohertz or low megahertz frequencies.

KLO3 microcontroller

Freescale already claims to have the smallest microcontroller and launched the KL03 at Embedded World.

The Kinetis KL03 MCU, launched at the exhibition, comes in a 1.6mm by 2.0mm package and offers 20 percent less size, 30 percent less power than the KL02 MCU launched a year before. It is based on the Cortex-M0+ core from ARM and operates across the supply voltage range of 1.7V to 3.6V with a clock frequency of 48MHz. A lot of the power consumption savings have been made by careful design to avoid leakage in the analog and other sections of the die. That is still a long way from the sub-1V regime.

Freescale is pointing out the progess made in miniaturization. "When size is no longer a barrier to incorporating microcontrollers into edge-node devices, we can start to redefine what's possible for the Internet of Things," said Rajeev Kumar, director of business development for MCUs at Freescale.

Meanwhile ARM is making research efforts in a core optimized for near-threshold voltage operation. In August 2013, Mike Muller, CTO of ARM revealed that the company has been researching logic operation at both sub- and near-threshold operation and clock frequencies of the order of tens of kilohertz. At the time Muller said that the work had gone into a development stage with partner companies. At that time Muller said that near-threshold was the more likely implementation choice because it was closer to mainstream models from the foundries.

But there still remains the "chicken-and-egg" problem that without large markets to point at foundries may be reluctant to do library work and without well characterized libraries for 0.8V and 0.6V operation fabless chip companies are reluctant to design near-threshold logic.

Lees is confident that near-threshold circuit operation will come. "It will come but in pieces. It will probably take two to four years," he said.

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