MCU battle: Low power is the name of the game

Many microcontroller vendors focus their energy on the growing market for battery-powered and mobile devices, which aim to be part of the Internet of Things (IoT).

Using the EEMBC ULPBench power benchmark, introduced last year, MCU vendors have become engaged in a game of leapfrog, announcing new products with ever-improving benchmark results and (temporarily) claiming leadership in ultra-low power processing.

While this may seem like a marketing game, developers will ultimately be the winners as vendors refine their techniques for saving power.

In the past, a low-powered MCU also meant low performance, but vendors have been challenging this correlation by offering increasingly powerful MCUs for low power applications. Developers, however, faced a problem in evaluating these offerings. Traditional specifications such as operating current in mW/MHz and sleep-mode leakage currents became increasingly difficult to evaluate in the face of the multiple power states that devices offered, and in the face of inconsistency in the industry in the descriptions and specifications used to characterise low-power operation.

Additionally, many applications called for long periods of MCU dormancy punctuated by rapid bursts of responses to events. In such applications parameters such as wake-up time and the time needed to execute the response before returning to sleep factor heavily into the total energy an MCU will use in practice. The factors can make it challenging to assess low-power claims accurately.

Providing developers with a methodology to "reliably and equitably measure MCU energy efficiency" is the stated goal of the ULPBench benchmark first released to the EEMBC consortium in March 2014. The benchmark was made public and augmented with a standardised hardware measurement platform—EnergyMonitor—last October, and the combination forms the basis for the results MCU vendors are now touting.

"This is one of the strictest benchmarks we've ever done in terms of setup and such," EEMBC President Marcus Levy said in an interview with EE Times. "The benchmark has the MCU perform 20k clock cycles of active work once a second, and sleep the remainder of the second. This way each processor performs the same workload, which levels the playing field with regard to executing the benchmark."

The final score is 1,000 divided by the median value for average energy used during each of 10 benchmark cycles. A larger value therefore represents less energy consumed.

The EEMBC ULPBench measures energy used by an MCU's core processor during a series of sleep-process-sleep cycles.

Using this benchmark, MCU vendors have recently begun publishing their results and leapfrogging one another to temporarily claim a leading position. This recent run arguably started with the STMicro release of its STM32L4 microcontroller family in mid-February. The products are based on the ARM Cortex-M4 processor and achieved a ULPBench score of 123.5. A week later, Texas Instruments released its Cortex-M3-based SimpleLink C26xx wireless MCU family with a score of 143.6.

This week TI surpassed its own earlier result by announcing the MSP-432 family based on the Cortex M4F. It achieved a ULPBench score of 167.4. While TI was briefing the media on this product, however, Atmel quietly published a ULPBench score of 185.8 for its SAM-L21 MCU based on the Cortex M0+, a product announced last year that was scheduled to be released at about this time. It's reasonable to expect that a formal announcement of the product's score and availability will be made soon.

All of these scores are likely to be eclipsed, however, by the advent of Ambiq Micro's Apollo MCU, whenever it earns its benchmark score. The Cortex-M4F-based device is based on sub-threshold logic technology, which uses a much smaller voltage swing between logic levels than conventional CMOS logic. This reduced swing yields both faster switching and up to a ten-fold lower dynamic power consumption than conventional logic, according to Ambiq's VP of marketing Mike Salas.

Still, the low-power leapfrog game is likely to continue for some time. "Low power is an area where everyone is pouring a lot of R&D into," Andreas Eieland, director of product marketing for Atmel's MCU business unit, told EE Times, "and it has taken on a much faster pace than before. We know we're the lowest power now, but you never know where your competition is in its efforts. So, we're already looking at the next step."

- Rich Quinnell
??EE Times