Reset and clock control (RCC) RM0390
124/1328 RM0390 Rev 4
The LSE clock is in the Backup domain, whereas the HSE and LSI clocks are not. As a
consequence:
• If LSE is selected as the 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 the 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 122 for more details on LSI calibration.
• If the HSE clock is used as the 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 1.2 V domain).
Note: To read the RTC calendar register when the APB1 clock frequency is less than seven times
the RTC clock frequency (f
APB1
< 7xf
RTCLCK
), the software must read the calendar time and
date registers twice. The data are correct if the second read access to RTC_TR gives the
same result than the first one. Otherwise a third read access must be performed.
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
Two microcontroller clock output (MCO) pins are available:
• MCO1
You can output four different clock sources onto the MCO1 pin (PA8) using the
configurable prescaler (from 1 to 5):
–HSI clock
– LSE clock
– HSE clock
– PLL clock
The desired clock source is selected using the MCO1PRE[2:0] and MCO1[1:0] bits in
the RCC clock configuration register (RCC_CFGR).
• MCO2
You can output four different clock sources onto the MCO2 pin (PC9) using the
configurable prescaler (from 1 to 5):
– HSE clock
– PLL clock
– System clock (SYSCLK)
– PLLI2S clock
The desired clock source is selected using the MCO2PRE[2:0] and MCO2 bits in the
RCC clock configuration register (RCC_CFGR).
For the different MCO pins, the corresponding GPIO port has to be programmed in alternate
function mode.
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