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Selecting low-power MCUs by the numbers

Posted: 14 Aug 2013 ?? ?Print Version ?Bookmark and Share

Keywords:microcontrollers? MCUs? low-power? RTC? RAM?

Allowing firmware to scale the internal supply voltage is another optimisation knob available to the developer. If an MCU is operating at a slow frequency, it may be possible to decrease the supply voltage and save power. Selective clock gating allows hardware blocks to be disconnected from the active circuits, preventing inactive peripherals from consuming power. These types of features are not comprehended by supply current specifications that are commonly used to rank low-power MCUs, but are critical to achieving the lowest overall system power consumption.

Reducing complexity using tools
As MCUs become more and more configurable to achieve the lowest power consumption, they also can become more complex. To cope with this increased complexity, developers should take a close look at the evaluation platforms available for an MCU and the overall ease of implementing a solution. For example, the development board and software tools used to program the MCU should be intuitive and easy-to-use. Hardware that is difficult to understand or use is not likely to lead to an easy firmware development process.

From a firmware perspective, MCU vendors should supply firmware examples that can recreate specifications from the data sheet. If advertised current consumption specifications cannot be recreated on an evaluation platform, it is likely that it will be just as difficult (if not impossible) to configure the MCU to achieve these numbers on custom hardware. Giving customers a variety of code examples that can be used as a starting point for their code development can reduce time-to-market and help engineers learn to use a device.

Graphical configuration tools can aid in development and help the developer gain a deeper understanding of an MCU. When developing low-power applications, it is helpful to know where the total consumed power is going. This information is useful because it highlights what aspect of a design needs to be further optimised and can also help the developer understand the overall architecture of the device. Ideally, low-power configuration tools could give tips on further reducing power as well as highlight any configuration errors that were detected throughout the configuration process.

For example, the Power Estimator utility within Silicon Labs' AppBuilder graphical configuration tool provides Power Tips that give configuration guidance and a power-budget pie chart showing how much power is consumed and which peripherals are consuming the power. As configuration changes are made, the pie chart automatically updates.

Evaluating and selecting an MCU for a low-power application requires more than a quick scan of the first page of the datasheet. Determining which MCU provides the lowest overall system power requires developers to know the device's supply current specifications, as well as any system-level optimisations that can reduce the overall supply current.

Unfortunately, each MCU vendor specifies operating conditions differently and in some cases advertises a low-power number that is available in an unusable mode. Using a common operating mode to compare MCUs will prevent developers from being misled by vendor claims of ultra-low-power operation.

Once the electrical characteristics of a device are understood and quantified, developers should take a look at the evaluation platform and software tools available. These considerations are crucial in getting an engineering team up and running quickly and should be included in the final MCU selection process.

About the author
Evan Schulzis a microcontroller product manager at Silicon Labs, focusing on the company's 32bit MCU portfolio. Previously serving as an applications engineer in the company's MCU group, Mr. Schulz joined Silicon Labs in 2008 as an associate applications engineer. He holds a bachelor's degree in Electrical Engineering from The University of Texas at Austin.

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