July 9, 2012 -- Few applications today are immune from the need to reduce energy consumption. For designers of portable and battery-powered equipment, this is a constraint that impacts the top-line performance and usability of their products. One of the main contributors to the total energy consumption of any system is the performance of the microcontroller (MCU) at its heart. As a consequence, designers have sought to minimize MCU power consumption, focusing on three key areas.

The first is the active energy consumption — the amount of useful processing work the MCU can do for a given energy input — which is obviously important. A ready measure of this contribution is offered by the MCU's current consumption per MHz, with a benchmark figure for a 32-bit Cortex M3 processor being the 150µA/MHz achieved by the EFM32 Tiny Gecko MCUs from Energy Micro.

The processor also needs a range of sleep and deep-sleep modes that require as little current as possible. Again taking the example of the Gecko series processors, an attainable figure for stop mode is around 20nA.

In practice, however, the third aspect of power consumption is probably the most important. A sleep mode is of little use if there is no way for the processor to monitor external events while asleep, in order to know when to wake up. In practice, therefore, most MCUs spend most of their time in an intermediate "waiting for input" mode.

By Anders Guldahl. (Guldahl as an application engineer at Energy Micro SA.)