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An Ultra-Low Voltage CPU

An Ultra-Low Voltage CPU
by Bernard Murphy on 04-18-2017 at 7:00 am

A continuing challenge for large scale deployment of IoT devices is the need to minimize service/cost by extending battery life to decades. At these lifetimes, devices become effectively disposable (OK – a new recycling challenge) and maintenance may amount to no more than replacing a dead unit with a new unit. Getting to these levels requires effort to manage dynamic, leakage and sensor power consumption. Managing leakage has been covered in many articles on FinFET and FDSOI technologies and discussions on aggressive power switching, though this challenge is not as acute in legacy processes. I wrote recently about advances in managing sensor power where at least for some types of sensing, standby power can be reduced to zero.

That leaves dynamic power (the power the system burns when it is doing something other than sleeping) as the primary contributor to battery drain. Dynamic power varies with the square of the operating voltage, so reducing voltage has a major impact. We’re used to seeing modern devices running at 1 volt, in some cases at 0.8 volts and even at 0.6 volts. It’s easy to see why; at 0.6 volts dynamic power should be only 36% of the power consumed at 1 volt. So why not reduce the voltage to 0.1 volts or even lower? That isn’t so easy; digital circuits depend on transistors switching between a ‘0’ state and a ‘1’ state. The normal way they do this is to switch between a definitely-off (grounded) state and a definitely-on (saturated) state. But at very low voltages there isn’t enough voltage swing to get up to the saturated state; you can only get part way up the curve.

Of course getting part way up can still be enough to effectively switch, as long as the level you get to is sufficient to switch the next gate in the chain. But there’s a problem. Variabilities in process, temperature and other environmental factors make it difficult to exactly control the voltage swing for each transistor or how quickly the next transistor will respond. As these variations accumulate it can be very challenging to have a circuit operate reliably at very low voltages.

This is where EtaCompute comes in. I think they can safely claim without contradiction that they have developed the world’s lowest power microcontroller IP, since they can operate these as low as 0.25 volts. This might be mildly interesting (do we really need more processors?) were it not for the fact that they have built their IP based on ARM M0+ and M3 cores, in partnership with ARM. They are very cagey about how they make this work, mentioning only that they use self-timed technology and dynamic voltage scaling (DVS) and they say that this is insensitive to process variations, inaccurate device models and path delay variations. 0.25-volt operation has evidently been demonstrated in a 90LP process. They also offer supporting low voltage IP including a real-time clock, AES encryption, an ADC, a DSP and a PMIC to control DVS.

At these low levels of dynamic power consumption and operating in legacy low-power processes where leakage is presumably not a concern, they assert devices built around this logic can comfortably operate in always-on/always-aware mode (which should be easier to manage and lower cost), driven by small coin-cell batteries and energy harvested through e.g. solar power.

The company is quite new. They were founded in 2015 by co-founders and early execs at Inphi, are based in LA and have raised ~$4.5M so far. They have very credible backing, through their partnership with ARM, also some of the investment comes from Walden International (Lip-Bu Tan). Good idea, good backing, should be an interesting company to watch.

You can visit the website HERE.

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