ST STM32F301 6 Series Reference Manual page 101

RM0366 Reset and clock control (RCC)
All U(S)ARTs and I2Cs have the capability to enable the HSI oscillator even when the MCU
is in Stop mode (if HSI is selected as the clock source for that peripheral).
All U(S)ARTs can also be driven by the LSE oscillator when the system is in Stop mode (if
LSE is selected as clock source for that peripheral) and the LSE oscillator is enabled
(LSEON) but they do not have the capability to turn on the LSE oscillator.
Standby mode stops all the clocks in the V18 domain and disables the PLL and the HSI and
HSE oscillators.
The CPU's deepsleep mode can be overridden for debugging by setting the DBG_STOP or
DBG_STANDBY bits in the DBGMCU_CR register.
When waking up from deepsleep after an interrupt (Stop mode) or reset (Standby mode),
the HSI oscillator is selected as system clock.
If a flash programming operation is on going, deep-sleep mode entry is delayed until the
flash interface access is finished. If an access to the APB domain is ongoing, deepsleep
mode entry is delayed until the APB access is finished.
RM0366 Rev 5 101/874
125