The definitive guide to ARM Cortex-M0/M0+: Low-power states

Joseph Yiu

September 07, 2016

Joseph YiuSeptember 07, 2016

Editor's Note: In designing deeply embedded systems, engineers face ongoing tradeoffs between power and performance. The rapid emergence of opportunities for personal electronics, wearables and Internet of Things (IoT) applications only exacerbates challenges in reducing power while enhancing performance. The ARM Cortex-M0 and Cortex-M0+ processors have emerged as a leading solution, providing the core for a broad range of microcontrollers designed to meet tough requirements for low-power, high-performance operation. In The Definitive Guide to ARM® Cortex®-M0 and Cortex-M0+ Processors, 2nd Edition, Joseph Yiu offers a comprehensive view of these processors. As blogger Jack Gannsle wrote, these books will "...give you the insight you need to be productive on real projects."

In a series of installments, we offer an excerpt of Yiu's book focusing on the low-power features of these processors and use of those features in building ultralow-power designs:

Low-power states

Wake-up operation

Ultralow-power designs

Low-power requirements

Benchmarking low-power devices

Low-power benchmark setup

Using low-power features

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Adapted from The Definitive Guide to ARM® Cortex®-M0 and Cortex-M0+ Processors, 2nd Edition, by Joseph Yiu (Newnes).


CHAPTER 9. System Control and Low-Power Features

9.5 Low-Power Features

9.5.1 Overview

A number of low-power features are available in the Cortex-M0 and Cortex-M0+ processors. In addition, microcontroller vendors usually also implement a number of low- power modes in their Cortex-M0/M0+-based microcontroller products. This section focuses mostly on the low-power features provided by the Cortex-M0 and Cortex-M0+ processors. Details for microcontroller-specific low-power features are usually available in user manuals or application notes available from the microcontroller vendor Web sites, or in example software packages. Some examples of using device-specific low-power features are covered in Chapter 19.

In general, the Cortex-M processors include the following low-power features:

  • Two architectural sleep modes: normal sleep and deep sleep. The sleep modes can be further extended with vendor-specific speed control features. Within the processor, both sleep modes behave similarly. However, the rest of the microcontroller can typically reduce power by applying different level of device-specific power reduction methods based on these two modes.

  • Two instructions for entering sleep modes: WFE and WFI. Both can be used with normal sleep and deep sleep modes.

  • Sleep-On-Exit (from exception) feature: allowing interrupt driven applications to stay in sleep mode as often as possible.

  • Optional Wake-up Interrupt Controller (WIC): this optional feature allows the clocks of the processor to be completely turned off during sleeps. When this feature is used with state retention technology, found in certain modern silicon implementation processes, the processor can enter a power-down state with extremely low-leakage power, and it is still able to wake up and resume operations almost immediately.

  • Low-power design implementation: various design techniques were used to reduce the power consumption as much as possible. Since the gate count is also very low, the static leakage power of the processor is tiny compared to most other 32-bit microcontrollers.

In addition, various characteristics of the Cortex-M processors also help to reduce power consumption:

  • High performance: the Cortex-M0 and Cortex-M0+ processors performance is often several times higher than many popular 8-bit/16-bit microcontrollers. This allows the same computational tasks to be carried out in shorter time and the microcontroller can stay in sleep modes for longer period of time. Alternately, the microcontroller can run at a slower clock frequency to perform the same required processing task to reduce power.

  • High-code density: By having a very efficient instruction set, the required program size can be reduced and as a result you can use a Cortex-M0 or Cortex-M0+-based micro- controller with smaller flash memory to reduce power consumption and cost.

Because the processor is only a small part of a microcontroller, to get the best energy efficiency and maximum battery life out of a microcontroller product, it is necessary to understand not only the processor but also the rest of the microcontroller. Most microcontroller vendors provide application notes and software libraries to make this easier for software developers.

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