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RM0365
Reset and clock control (RCC)
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The basic concept consists in providing a relative measurement (e.g. the HSE/LSI ratio): the
precision is therefore closely related to the ratio between the two clock sources. The higher
the ratio is, the better the measurement will be.
9.3 Low-power
modes
APB peripheral clocks and DMA clock can be disabled by software.
Sleep mode stops the CPU clock. The memory interface clocks (Flash and RAM 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 mode stops all the clocks in the V18 domain and disables the PLL, the HSI and the
HSE oscillators.
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, deepsleep 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.