By Mike Salas, Vice President Marketing, Ambiq Micro

Subthreshold power optimized technology (SPOT) has the potential to dramatically reduce the power consumption of the next generation of microcontrollers.

Energy consumption is a critical factor in the design of portable systems, replacing performance as the driver for the industry. Reducing power enables longer battery life and/or allows a smaller, lighter battery to be used to not only minimize cost but to also reduce the size of the device and make it more attractive to users.  Ideally, it may even be possible to begin considering using energy harvested sources to replace batteries entirely.

But to get a dramatic reduction in energy consumption requires a significant shift in the way logic is designed. SPOT (Subthreshold Power Optimized Technology) is a different approach to traditional logic transistor design, operating at voltage levels far below what is considered normal. In traditional designs there is a threshold voltage where circuits are considered “on” and anything below that was considered “off”. While this usually means driving transistors up to 1.8V in order to create an “on” state, it’s not as binary as it looks.

Even below the threshold voltage at around 1.0V there is current that flows, although traditionally this leakage current has not been considered a good thing. However, through the use of Ambiq Micro’s SPOT approach, it is actually possible to derive an ‘on’ signal from that current and since energy consumption is directly proportional to the square of the voltage applied, it is possible to achieve some fairly dramatic power savings (figure 1).  For example, achieving operation at 0.5V enables up to a 13x reduction in power. Using an even more aggressive subthreshold voltage of 0.3V gives a dramatic 36x improvement!

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Figure 1: The on/off current ratio is orders of magnitude smaller in the sub-threshold region

SPOT has been used to develop the Apollo family of microcontrollers, the industry’s first MCU to rely overwhelmingly on subthreshold transistor operation. This has resulted in a solution that can operate down to an industry-leading 30uA/MHz in active mode and 100nA in standby.  Just as interesting, was the selection of an ARM Cortex M4F core for this solution (figure 2).  Unlike other “low power” MCUs who have traditionally chosen to use an ARM Cortex-M0+ core; Ambiq Micro purposely chose the M4F core for two key reasons. The first reason is that the subthreshold circuit level technology enables Ambiq to choose an M4F core instead of an M0+ core with absolutely no power penalty. This is demonstrated by the fact that the M4F-based Apollo MCU power consumption numbers are far lower than even the competing M0+ solutions from all other suppliers. The second reason is due to the fact that some of the major markets Ambiq are targeting – like wearables and IoT – are increasingly dependent on large numbers of sensors and complex algorithms. Having an M4F core is a great advantage for these applications since it is possible to execute instructions far faster than what is possible when using an M0+ solution. The end result is the best solution possible: power consumption levels that are below even competing M0+ solutions combined with the performance of an M4F processor.

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Figure 2: Ambiq Micro Apollo MCU

The design of the Apollo family starts with the assumption that all the logic will use SPOT, with intelligent decisions then made about where that is not feasible or not needed. In some cases a superthreshold voltage is perfectly acceptable, for example if it’s something that just occurs once at boot up, then those transistors can be left entirely in the traditional superthreshold domain as there is no impact on the overall energy consumption of the device. In other areas where the signaling information is required to be attained more quickly, the voltage can be increased to provide sufficient performance. This means Apollo has a very small proportion of the device operating in superthreshold, with the majority of the device operating in the near-threshold and subthreshold domain.

The 24MHz maximum clock speed that was chosen for the Apollo devices was not driven by any limitation of the sub threshold technology but was chosen as the sweet spot in the trade-off between energy and performance – this frequency was more than enough to meet the needs of many key target markets including those in the wearable and Internet of Things (IoT) space.

This was not the first subthreshold device that has been available on the market. Ambiq previously developed a real time clock (RTC) that was introduced in 2013 and has been shipping in volume since then. This effort provided a tremendous amount of learning, particularly about timing and clocking, which was subsequently used in the design of the more ambitious MCU.

A key requirement for SPOT was that the implementation needed to utilize standard, mainstream CMOS technology.  This was actually very hard to do as it required a thorough understanding of the leakage characteristics at low voltages.  These are not accurately modelled by the fabs as they don’t expect people to be operating at voltages that low. This required many years of test chips and wafer shuttles to thoroughly model these subthreshold domains and how they vary with temperature, process drift and the effect of noise. All of these parameters are highly sensitive at these low voltage domains so a lot of work and modelling was done to really understand the effects.

However, modeling the low voltage characteristics was simply the first step required. The harder thing to do was to then create a set of dynamic adaptive circuits to overcome the many problems that existed in these subthreshold domains. Truly understanding the subthreshold effects, and in turn building new models and custom cell libraries, allows patented circuitry to be designed that is both dynamic and adaptive to help overcome some of the negative effects seen in the low voltage domains.

The back end of the process is also a challenge as industry standard testers had to be used and these don’t test at the picoAmp and nanoAmp levels that are generated by the use of Ambiq’s subthreshold technology. Therefore, Ambiq needed to create special load boards and test fixtures that were specially adapted to measure these very low currents while still using industry standard testers. This power measurement challenge extends all the way up to the evaluation kit level, as even lab equipment won’t measure these currents. This led to a custom current ammeter board that is mounted on the kit so that customers can see accurate current levels.

In summary, the implementation of this innovative subthreshold technology requires a new way of thinking across the entire design flow, from the transistor all the way to the evaluation kit. It also required a completely different way of architectural thinking to achieve even greater levels of energy reduction. The Apollo family reduces the energy consumption by a factor of ten to give system designers much more flexibility and battery life in their designs than was previously possible.