FreeRTOS tickless mode does exactly what you want. The SysTick timebase is preserved, any tasks delayed for specific periods or timeouts will still respond at the right time, and any external interrupt will exit low power sleep mode. Unlike stop and standby mode there are no limitations on wakeups and clocks.
Sleep mode halts the processor clock, far more power efficient than just slowing down the clock rate. The instruction clock, but not PCLK for peripherals, is halted. The core exits from sleep mode very quickly after any external interrupt occurs. Tickless mode updates the FreeRTOS tick count for total elapsed time while asleep.
What you save here is the power consumed by the core while executing instruction in idle mode, for the total length of the idle period, not just fragments between systick interrupts. Its far less complicated than frequency switching. Saves more power too...after all, zero Mhz will always consume less current.
The sleep mode core implementation is one thing ST does much better than most of its low power M class competitors, especially on the L4 series, and FreeRTOS takes full advantage of it. If you're using battery power don't ignore it.
Jack Peacock
