Many embedded applications spend most of their time waiting for something to happen—receiving data on a serial port, watching an I/O pin change state, or waiting for a time delay to expire. If the processor is still running at full speed when it is idle, battery life is consumed while very little is being accomplished. So in many applications, the core is only active during a very small amount of the total time, and by placing it in a low-power mode during the idle time, the battery life can be extended considerably.

A good approach is to have a task-oriented design and to use an RTOS. In a task-oriented design, a task can be defined with the lowest priority, and it will only execute when there is no other task that needs to be executed. This idle task is the perfect place to implement power management. In practice, every time the idle task is activated, it sets the core into a low-power mode. Many microprocessors and other silicon devices have a number of different low-power modes, in which different parts of the core can be turned off when they are not needed. The oscillator can for example either be turned off or switched to a lower frequency. In addition, individual peripheral units, timers, and the CPU can be stopped. The different low-power modes have different power consumption based on which peripherals are left turned on. A power debugging tool can be very useful when experimenting with different low-level modes.

You can use the Function profiler in C-SPY to compare power measurements for the task or function that sets the system in a low-power mode when different low-power modes are used. Both the mean value and the percentage of the total power consumption can be useful in the comparison.