Products ranging from smart meters to wireless sensor nodes, mobile health monitors to intruder alarms, designers need to take every step possible to limit energy consumption.
MCUs need to operate reliably over a period of years or even decades while using the limited energy resources available from batteries or an energy harvesting system.
Since energy is 'power times time', using the 32-bit Cortex-M3 in preference to a less powerful alternative can prove more energy efficient: particularly if the designer opts for an MCU such as Energy Micro's Tiny Gecko, that even in active mode consumes only 150µA/MHz. In short, 'low power' does not necessarily mean 'low energy'.
The task for the designer is therefore to find the lowest-power sleep mode that provides adequate response to anticipated (if unscheduled) events.
Energy Micro's EFM32 Gecko family of energy-optimized 32-bit MCUs provides a good example of the basic requirements of each mode available.
The EFM32 family's energy mode transitions
EM0 "Run Mode": 180 µA/MHz
EM1 "Sleep Mode": 45 µA/MHz
EM2 "Deep Sleep Mode": 900 nA
RTC, Power-on Reset, Brown-Out Detection
RAM and CPU retained
EM3 "Stop Mode": 600 nA
RAM & CPU retained