Ambiq leads low-power MCU race

With so much development for the Internet of Things (IoT) targeting battery-powered operation, it's no wonder that MCU vendors have been engaging in a game of low-power leapfrog with one another, vying for the title of best in low-power performance. But with the announcement of verified EEMBC ULPBench benchmark results for Ambiq Micro's Apollo MCU at ARM TechCon this week, that game may be all but won. The Cortex M4F-based Apollo MCU achieved twice the score of the prior title holder.

ULPBench is an industry-standard means of measuring MCU energy efficiency that mimics typical low-power system behaviour. The benchmark works in conjunction with a standardised hardware device that monitors the MCU's energy consumption. Both the benchmark and monitoring device were developed by the Embedded Microprocessor Benchmark Consortium (EEMBC), which also validates vendor test results. According to EEMBC president Markus Levy, the benchmark calls for the MCU to perform 20k clock cycles of active work once a second and sleep for the remainder of that second, repeating the cycle multiple times to ensure accurate results. The benchmark 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 and because the benchmark counts clocks rather than using a fixed time interval, the results compensate for clock speed differences among processors.

EEMBC ULPBench

Results posted on the EEMBC website show the prior leader in low power performance was the STMicroelectronics STM32L476RG MCU with a score of 187.7. Ambiq Micro held a public demonstration at ARM TechCon of its Apollo MCU achieving a whopping 377.5.

The secret behind Apollo's performance is the unique subthreshold technology that Ambiq has developed. Rather than having the device's logic transistors switching between saturation and off states, Ambiq's transistors operate exclusively at voltages that remain below the switching threshold. As a result they use up to 10x less current than conventional transistors when changing logic states.

A comparison of ULPBench results shows that MCU vendors have been making small, incremental improvements in their processors' low-power performance in their efforts to claim the top spot. With this two-fold leap forward, Ambiq has set the mark at a point other vendors will likely find extremely difficult to match. "MCU vendors have counted on process lithography reduction for making significant improvements in their low-power performance," said Ambiq's VP of business development Vince Murdica, "so they'll probably look to that again to try to catch us." But the Apollo MCU is made using an older, wider process lithography than most of the prior low-power leaders, Murdica added. "So as they go down the process curve to lower power, we'll be able to follow along and get the same kind of benefit." Until another vendor makes a fundamental change in their base technology the way subthreshold design has for Ambiq, Murdica expects, the low-power leadership crown will stay with Ambiq.

- Rich Quinnell
  EE Times