RM0033 Rev 9 501/1381
RM0033 Window watchdog (WWDG)
507
18 Window watchdog (WWDG)
This section applies to the whole STM32F20x and STM32F21x family, unless otherwise
specified.
18.1 WWDG introduction
The window watchdog is used to detect the occurrence of a software fault, usually
generated by external interference or by unforeseen logical conditions, which causes the
application program to abandon its normal sequence. The watchdog circuit generates an
MCU reset on expiry of a programmed time period, unless the program refreshes the
contents of the downcounter before the T6 bit becomes cleared. An MCU reset is also
generated if the 7-bit downcounter value (in the control register) is refreshed before the
downcounter has reached the window register value. This implies that the counter must be
refreshed in a limited window.
18.2 WWDG main features
• Programmable free-running downcounter
• Conditional reset
– Reset (if watchdog activated) when the downcounter value becomes less than
0x40
– Reset (if watchdog activated) if the downcounter is reloaded outside the window
(see Figure 194)
• Early wakeup interrupt (EWI): triggered (if enabled and the watchdog activated) when
the downcounter is equal to 0x40.
18.3 WWDG functional description
If the watchdog is activated (the WDGA bit is set in the WWDG_CR register) and when the
7-bit downcounter (T[6:0] bits) rolls over from 0x40 to 0x3F (T6 becomes cleared), it initiates
a reset. If the software reloads the counter while the counter is greater than the value stored
in the window register, then a reset is generated.
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