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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?

Comparing and choosing low-power microcontrollers (MCUs) based on claimed current consumption specifications can be a challenging and confusing task. In most cases, developers selecting an MCU will initially look at the first page of a datasheet to get information about a device, including peripherals, operating speed, package information, number of GPIOs, and power characteristics. This approach works well to determine the overall functionality of a device but is not as useful when trying to gauge low-power characteristics.

To get a full view of low-power operation, developers must take into consideration current consumption, state retention, wake-up time, wake-up sources, and peripherals that are capable of operating while in low-power mode. It is also important to consider any additional functionality or peripherals that can reduce total system power and available evaluation tools that can make an engineer's job easier.

MCU vendors will usually list the lowest power achievable on the first page of the datasheet. Although the device may be capable of achieving the specification in the datasheet, the actual operating mode may not be practical and useful in a real-world application. Some of the non-advertised features of the lowest power mode may include a very slow wake time, no state or RAM retention, or a reduced operating voltage range. To get around the variety of low-power specifications, developers must identify a common operating mode. This common operating mode must consist of two sections: electrical specifications and low-power functionality.

Comparing electrical specifications
The electrical specifications are available in the datasheet, but determining which specifications are relevant may require some digging. Usually the electrical specifications are organised by vendor-specific power mode. This makes assessment slightly more difficult, as it requires knowledge and familiarity with the functionality of each power mode. In general, it is beneficial to define a set of operating conditions and then map them to a power mode. For example, the developer might define the following set of operating conditions:
???Sleep mode current consumption with state and RAM retention
???All other peripherals disabled
???Sleep mode current consumption with RTC running with state and RAM retention
???RTC enabled and running all other peripherals disabled.
???Wake time
???Supply voltage range

Once the operating conditions are clearly defined, it should be easy to determine the applicable vendor-specific power mode.

Additional low power functionality
The second section, low-power functionality, is not as easy to locate in the vendor's documentation and may be spread across the datasheet and reference manual. Examples of low-power functionality include:
???Available wake sources
???How code resumes execution
???Peripherals capable of operating in sleep mode

Once the common operating mode has been clearly defined, developers can begin to examine the documentation in more detail.

While going through this exercise of compiling data, keep in mind that there may be some MCU-specific features that can further optimise an application for ultra-low power. Optimisations may reduce BOM costs, provide longer product life or provide greater design flexibility. For example, an on-chip dc-dc converter can efficiently provide power to the system and decrease power consumption. This can enable the use of smaller batteries, which will decrease the overall BOM costs, or provide power budget flexibility. A variety of wake sources can provide design flexibility and allow the MCU to stay in the lowest power mode as long as possible, further reducing the average current consumption of the application.

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