STMicroelectronics STM32F05 series Reference Manual page 80

Power control (PWR)
Bit 0 WUF: Wakeup flag
This bit is set by hardware to indicate that the device received a wakeup event. It is cleared
only by a POR/PDR (power on reset/power down reset) or by setting the CWUF bit in the
Power control register (PWR_CR)
0: No wakeup event occurred
1: A wakeup event was received from one of the enabled WKUPx pins or from the RTC
alarm.
Note: An additional wakeup event is detected if one WKUPx pin is enabled (by setting the
80/742
EWUP bit) when its pin level is already high.
Doc ID 018940 Rev 1
RM0091