Hardware Watchdog; Behavior In Stop And Standby Modes; Register Access Protection; Debug Mode - STMicroelectronics RM0365 Reference Manual

Independent watchdog (IWDG)
26.3.3

Hardware watchdog

If the "Hardware watchdog" feature is enabled through the device option bits, the watchdog
is automatically enabled at power-on, and generates a reset unless the Key register is
written by the software before the counter reaches end of count or if the downcounter is
reloaded inside the window.
26.3.4

Behavior in Stop and Standby modes

Once running, the IWDG cannot be stopped.
26.3.5

Register access protection

Write access to the IWDG_PR, IWDG_RLR and IWDG_WINR registers is protected. To
modify them, you must first write the code 0x0000 5555 in the IWDG_KR register. A write
access to this register with a different value will break the sequence and register access will
be protected again. This implies that it is the case of the reload operation
(writing 0x0000 AAAA).
A status register is available to indicate that an update of the prescaler or the down-counter
reload value or the window value is on going.
26.3.6

Debug mode

When the microcontroller enters debug mode (core halted), the IWDG counter either
continues to work normally or stops, depending on DBG_IWDG_STOP configuration bit in
DBG module.
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RM0365