RM0008 Power control (PWR)
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4.3.2 Peripheral clock gating
In Run mode, the HCLK and PCLKx for individual peripherals and memories can be stopped
at any time to reduce power consumption.
To further reduce power consumption in Sleep mode the peripheral clocks can be disabled
prior to executing the WFI or WFE instructions.
Peripheral clock gating is controlled by the AHB Peripheral Clock enable register
(RCC_AHBENR), APB1 peripheral clock enable register (RCC_APB1ENR) and APB2
peripheral clock enable register (RCC_APB2ENR).
4.3.3 Sleep mode
Entering Sleep mode
The Sleep mode is entered by executing the WFI (Wait For Interrupt) or WFE (Wait for
Event) instructions. Two options are available to select the Sleep mode entry mechanism,
depending on the SLEEPONEXIT bit in the Cortex-M3 System Control register:
● Sleep-now: if the SLEEPONEXIT bit is cleared, the MCU enters Sleep mode as soon
as WFI or WFE instruction is executed.
● Sleep-on-exit: if the SLEEPONEXIT bit is set, the MCU enters Sleep mode as soon as
it exits the lowest priority ISR.
Refer to Ta bl e 8 and Table 9 for details on how to enter Sleep mode.
Exiting Sleep mode
If the WFI instruction is used to enter Sleep mode, any peripheral interrupt acknowledged by
the nested vectored interrupt controller (NVIC) can wake up the device from Sleep mode.
If the WFE instruction is used to enter Sleep mode, the MCU exits Sleep mode as soon as
an event occurs. This event can be either an interrupt enabled in the peripheral control
register but not in the NVIC, or an EXTI line configured in event mode.
This mode offers the lowest wakeup time as no time is wasted in interrupt entry/exit.
Refer to Ta bl e 8 and Table 9 for more details on how to exit Sleep mode.
Table 8. Sleep-now
Sleep-now mode Description
Mode entry
WFI (Wait for Interrupt) or WFE (Wait for Event) while:
– SLEEPDEEP = 0 and
– SLEEPONEXIT = 0
Refer to the Cortex™-M3 System Control register.
Mode exit
If WFI was used for entry:
Interrupt: Refer to Table 36: Vector table
If WFE was used for entry
Wakeup event: Refer to Section 8.2.3: Wakeup event management
Wakeup latency None
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