RM0402 Rev 6 99/1163
RM0402 Power controller (PWR)
108
In Stop mode, the following features can be selected by programming individual control bits:
• Independent watchdog (IWDG): the IWDG is started by writing to its Key register or by
hardware option. Once started it cannot be stopped except by a Reset. See
Section 21.3 in Section 21: Window watchdog (WWDG).
• Real-time clock (RTC): this is configured by the RTCEN bit in the Section 6.3.20: RCC
Backup domain control register (RCC_BDCR)
• Internal RC oscillator (LSI RC): this is configured by the LSION bit in the
Section 6.3.21: RCC clock control & status register (RCC_CSR).
• External 32.768 kHz oscillator (LSE OSC): this is configured by the LSEON bit in the
Section 6.3.20: RCC Backup domain control register (RCC_BDCR).
The ADC can also consume power during the Stop mode, unless it is disabled before
entering it. To disable it, the ADON bit in the ADC_CR2 register must be written to 0.
Note: If the application needs to disable the external clock before entering Stop mode, the HSEON
bit must first be disabled and the system clock switched to HSI.
Otherwise, if the HSEON bit is kept enabled while the external clock (external oscillator) can
be removed before entering stop mode, the clock security system (CSS) feature must be
enabled to detect any external oscillator failure and avoid a malfunction behavior when
entering stop mode.
Exiting Stop mode
The Stop mode is exited according to Section : Exiting low-power mode.
Refer to Table 21 for more details on how to exit Stop mode.
When exiting Stop mode by issuing an interrupt or a wakeup event, the HSI RC oscillator is
selected as system clock.
When the voltage regulator operates in low-power mode, an additional startup delay is
incurred when waking up from Stop mode. By keeping the internal regulator ON during Stop
mode, the consumption is higher although the startup time is reduced.
Table 21. Stop mode entry and exit
Stop mode Description
Mode entry
WFI (Wait for Interrupt) or WFE (Wait for Event) while:
– No interrupt (for WFI) or event (for WFE) is pending,
– SLEEPDEEP bit is set in Cortex
®
-M4 with FPU System Control register,
– PDDS bit is cleared in Power Control register (PWR_CR),
– Select the voltage regulator mode by configuring LPDS bit in PWR_CR.
On Return from ISR:
– No interrupt is pending,
– SLEEPDEEP bit is set in Cortex
®
-M4 with FPU System Control register,
– SLEEPONEXIT = 1,
– PDDS bit is cleared in Power Control register (PWR_CR).
Note: To enter Stop mode, all EXTI Line pending bits (in Section 10.3.6:
Pending register (EXTI_PR)), all peripheral interrupts pending bits,
the RTC Alarm (Alarm A and Alarm B), RTC wakeup, RTC tamper,
and RTC time stamp flags, must be reset. Otherwise, the Stop
mode entry procedure is ignored and program execution continues.
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