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Power Management
At the microcontroller (MCU) level there are several ways to reduce power usage. Some methods for reducing power consumption are:
- Slow down the clocks, especially the main clock for the core (CPU and main logic) of the device.
- Reduce the supply voltage to the core of the device.
- Stop the main clock.
- Turn off power to the core.
- Turn off power to the clock generator(s).
- Turn off clocks to unused peripherals and peripheral bus controllers.
- Turn off power to unused peripherals.
- Put memory into a low-power (RAM data retained) “not instantly accessible” mode.
- Turn off power to unused sections of memory.
- Turn off power to the entire device except an external “wake-up” pin.
Each of these methods has consequences. The most significant consequence is that it can take up to several 10s of microseconds to recover or respond to an IRQ or event. Sometimes this is tolerable, other times it is not.
Most MCUs have some sort of “sleep modes”. These sleep modes invoke some combination of the methods above. Typically these modes progressively turn off clocks/power to sets of major functions of the device (ie. [CPU], [CPU+Main Logic], [CPU+Main Logic+Clock Gen], etc.). These sleep modes don’t generally address Active Mode clock speed and core voltage or turning off clocks and/or power to unused parts of the device. All of this is also passive - the application has to manage power on its own by setting each of these parameters when appropriate. This leads to non-portable application code - power management has to be implemented in the application for each specific MCU.
Understanding how the sleep modes and power reduction methods are implemented for each MCU, how all of the methods interact and affect a specific MCU, and how to architect power management into the application is an arduous task that the application programmer shouldn’t have to go through.
Therefore the new power management api was born. A specific implementation called pm_layered is provided to automatically switch to sleep modes. The pm_layered module will be explained in the next sections.