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RM0365
Power control (PWR)
112
8.3 Low-power
modes
By default, the microcontroller is in Run mode after a system or a power Reset. Several low-
power modes are available to save power when the CPU does not need to be kept running,
for example when waiting for an external event. It is up to the user to select the mode that
gives the best compromise between low-power consumption, short startup time and
available wakeup sources.
The device features three low-power modes:
•
Sleep mode (CPU clock off, all peripherals including ARM
®
Cortex
®
-M4 core
peripherals like NVIC, SysTick, etc. are kept running)
•
Stop mode (all clocks are stopped)
•
Standby mode (1.8V domain powered-off)
In addition, the power consumption in Run mode can be reduce by one of the following
means:
•
Slowing down the system clocks
•
Gating the clocks to the APB and AHB peripherals when they are unused.
8.3.1 Slowing
down system clocks
In Run mode the speed of the system clocks (SYSCLK, HCLK, PCLK) can be reduced by
programming the prescaler registers. These prescalers can also be used to slow down
peripherals before entering Sleep mode.
For more details refer to
Section 9.4.2: Clock configuration register (RCC_CFGR)
Table 23. Low-power mode summary
Mode name
Entry
wakeup
Effect on 1.8V
domain clocks
Effect on
V
DD
domain
clocks
Voltage
regulator
Sleep
(Sleep now or
Sleep-on -
exit)
WFI
Any interrupt
CPU clock OFF
no effect on other
clocks or analog
clock sources
None
ON
WFE
Wakeup event
Stop
PDDS and LPDS
bits +
SLEEPDEEP bit
+ WFI or WFE
Any EXTI line
(configured in the
EXTI registers)
Specific
communication
peripherals on
reception events
(USART, I2C)
All 1.8V domain
clocks OFF
HSI and
HSE
oscillators
OFF
ON or in low-
power mode
(depends on
Power control
register
(PWR_CR)
Standby
PDDS bit +
SLEEPDEEP bit
+ WFI or WFE
WKUP pin rising
edge, RTC alarm,
external reset in
NRST pin,
IWDG reset
OFF