Tickless Idle
Tasks and Scheduling established that the tick is a periodic interrupt — SysTick, by default 1000 Hz — that the kernel uses as its only window onto elapsed time. That periodicity is exactly what makes an RTOS system a bad candidate for the deep sleep modes earlier in this folder, for a reason that has nothing to do with how much idle time there is: a system that wakes once a millisecond to increment a counter never gets to stay in a low-power mode for longer than a millisecond, no matter how little else it has to do. Sleep Modes showed that Stop mode's wake latency comes from a regulator switch-back and, at the deepest setting, a flash re-power — real time that has to be paid on every entry and exit. Pay it a thousand times a second and the average current never gets anywhere near the deep-sleep floor in the table on that page; it is dominated by the wake-and-sleep-again overhead, not by the sleep current itself.