Independent watchdog (IWDG) UM0306
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Bits 31:16 Reserved, read as 0.
Bits 15:0
KEY[15:0]: Key value (write only, read 0000h)
These bits must be written by software at regular intervals with the key value
AAAAh, otherwise the watchdog generates a reset when the counter reaches
0.
Writing the key value 5555h to enables access to the IWDG_PR and
IWDG_RLR registers (see Section 10.1.2)
Writing the key value CCCCh starts the watchdog (except if the hardware
watchdog option is selected)