
Reset and clock control (RCC)
RM0351
218/1830
DocID024597 Rev 5
6.3 Low-power
modes
•
AHB and APB peripheral clocks, including DMA clock, can be disabled by software.
•
Sleep and Low Power Sleep modes stops the CPU clock. The memory interface clocks
(Flash and SRAM1 and SRAM2 interfaces) can be stopped by software during sleep
mode. The AHB to APB bridge clocks are disabled by hardware during Sleep mode
when all the clocks of the peripherals connected to them are disabled.
•
Stop modes (Stop 0, Stop 1 and Stop 2) stops all the clocks in the V
CORE
domain and
disables the three PLL, the HSI16, the MSI and the HSE oscillators.
All U(S)ARTs, LPUARTs and I
2
Cs have the capability to enable the HSI16 oscillator
even when the MCU is in Stop mode (if HSI16 is selected as the clock source for that
peripheral).
All U(S)ARTs and LPUARTs 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). In that case the LSE remains always ON in Stop mode
(they do not have the capability to turn on the LSE oscillator).
•
Standby and Shutdown modes stops all the clocks in the V
CORE
domain and disables
the PLL, the HSI16, the MSI and the 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 leaving the Stop modes (Stop 0, Stop 1 or Stop 2), the system clock is either MSI or
HSI16, depending on the software configuration of the STOPWUCK bit in the RCC_CFGR
register. The frequency (range and user trim) of the MSI oscillator is the one configured
before entering Stop mode. The user trim of HSI16 is kept. If the MSI was in PLL-mode
before entering Stop mode, the PLL-mode stabilization time must be waited for after wakeup
even if the LSE was kept ON during the Stop mode.
When leaving the Standby and Shutdown modes, the system clock is MSI. The MSI
frequency at wakeup from Standby mode is configured with the MSISRANGE is the
RCC_CSR register, from 1 to 8 MHz. The MSI frequency at wakeup from Shutdown mode is
4 MHz. The user trim is lost.
If a Flash memory programming operation is on going, Stop, Standby and Shutdown modes
entry is delayed until the Flash memory interface access is finished. If an access to the APB
domain is ongoing, Stop, Standby and Shutdown modes entry is delayed until the APB
access is finished.