Reset and clock control (RCC) RM0008
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The LSE clock is in the Backup domain, whereas the HSE and LSI clocks are not.
Consequently:
● If LSE is selected as RTC clock:
– The RTC continues to work even if the V
DD
supply is switched off, provided the
V
BAT
supply is maintained.
● If LSI is selected as Auto-Wakeup unit (AWU) clock:
– The AWU state is not guaranteed if the V
DD
supply is powered off. Refer to
Section 6.2.5: LSI clock on page 74 for more details on LSI calibration.
● If the HSE clock divided by 128 is used as RTC clock:
– The RTC state is not guaranteed if the V
DD
supply is powered off or if the internal
voltage regulator is powered off (removing power from the 1.8 V domain).
6.2.9 Watchdog clock
If the Independent watchdog (IWDG) is started by either hardware option or software
access, the LSI oscillator is forced ON and cannot be disabled. After the LSI oscillator
temporization, the clock is provided to the IWDG.
6.2.10 Clock-out capability
The microcontroller clock output (MCO) capability allows the clock to be output onto the
external MCO pin. The configuration registers of the corresponding GPIO port must be
programmed in alternate function mode. One of 4 clock signals can be selected as the MCO
clock.
● SYSCLK
● HSI
● HSE
● PLL clock divided by 2
The selection is controlled by the MCO[2:0] bits of the Clock configuration register
(RCC_CFGR).
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