AI driven near threshold design for low power microcontroller

STMicroelectronics has leveraged near-threshold design techniques to enhance the energy efficiency of its latest low-power microcontroller series tailored for remote applications.

The STM32U3 microcontrollers are specifically designed for low-power applications like utility meters, healthcare devices, and industrial sensors. They come equipped with encryption key protection and in-factory provisioning to comply with upcoming European cybersecurity regulations.

Utilizing the ARM Cortex-M33 microcontroller family, ST has implemented a strategy that involves operating with a small digital signal swing close to the threshold of the CMOS transistors. This approach helps minimize dynamic power consumption, although it can be challenging due to process technology variations impacting transistor yield. To address this, ST has introduced AI-driven adaptive voltage scaling at the wafer level to offset process discrepancies in the foundry.

As a result, the dynamic power consumption has been reduced to 10µA/MHz with a static current of 1.6µA. The latest core, running at speeds of up to 96MHz, achieves an impressive Coremark-per-milliwatt score of 117. This represents nearly double the efficiency of ST’s previous STM32U5 series and five times that of the STM32L4 series.

The STM32U3 microcontrollers also feature up to 1MB of Flash memory with dual-bank capabilities and 256kB of SRAM. This configuration enables the devices to operate using coin cell batteries, ambient solar energy, or thermoelectric sources. Additionally, these MCUs find applications in consumer products such as smart watches, wearables, and hearables.

• Ultra-low power STM32U5 microcontroller uses autonomous peripherals

According to Patrick Aidoune, General-Purpose MCU Division General Manager at STMicroelectronics, “Using recent advancements in near-threshold design, the new devices significantly reduce dynamic power consumption, doubling efficiency compared to our previous generation. This contributes to companies’ sustainability objectives.”

The STM32U3 microcontrollers are engineered to securely store encryption keys in permanent memory using a coupling and chaining bridge (CCB) to eliminate vulnerable fetch operations. This marks the first application of this technology within the STM32 MCU family.

ST provisions any attestation credentials during manufacturing before the devices leave the factory. This approach enhances security and simplifies provisioning processes. The security features, including SESIP3 and PSA Level3 certifiable assets like cryptographic accelerators, TrustZone isolation, random generator, and product lifecycle management, aid ST customers in achieving compliance with forthcoming Cyber Resilience Act (CRA) regulations in Europe.

Two product variants are available, with or without a hardware cryptographic accelerator. Manuel Frech, a Product Development Engineer at smaXtec, shared his experience with the STM32U3 series, stating, “The STM32U3 allows us to elevate our hardware for animal health monitors. The active mode power consumption is extremely low, only a few µA/MHz, enabling us to reduce energy usage for current data processing algorithms while introducing new features. The advanced low-power modes facilitate deep sleep when data processing is idle. The newly integrated STOP3 mode, along with its wakeup capabilities, efficiently maintains low power consumption.”

The STM32U3 series is available in commercial (-40°C to 85°C) and extended industrial (-40°C to 105°C) temperature ranges. The microcontrollers are currently in production and can be purchased starting from $1.93 for orders of 10,000 units.