Watchdog and low-power modes

The watchdog that resets the MCU when the program hangs, and the sleep, stop and standby modes for battery devices.

Watchdog

When healthy, the counter is reloaded periodically. If the program hangs and cannot reload it, the watchdog resets the MCU.

If it is not told "still alive" within a set time, it resets the MCU.

Type	Characteristics
IWDG (independent)	Separate LSI clock; works even if the main clock dies
WWDG (window)	Resets if you are too late, and also if you are too early

Using it correctly

If you refresh the watchdog inside a timer interrupt, a hung main loop never causes a reset. Refresh it where you have confirmed that the main loop or all tasks are healthy.
With an RTOS, each task sets an "alive" flag and a supervisor task refreshes the watchdog only when it has seen all of them.
After a reset, use the reset-cause flags in RCC->CSR to record whether it was a watchdog reset.

Low-power modes (STM32 in general)

Towards the right the current falls, and the wake-up time and what is lost both grow.

Mode	What stops	How to wake	Current
Sleep	CPU only	Any interrupt	mA
Stop	Most clocks	External interrupt, RTC	µA
Standby	Almost everything; RAM is lost	Wake-up pin, RTC	Below 1 µA

After waking from Stop the clock is back on HSI, so the PLL has to be set up again. Battery life is calculated as "awake time × active current + asleep time × standby current".