Auto-Wakeup From Low-Power Mode; Table 28. Shutdown Mode - ST STM32L4x6 Reference Manual

Power control (PWR)
Shutdown mode
Mode entry
Mode exit
Wakeup latency
5.3.11

Auto-wakeup from low-power mode

The RTC can be used to wakeup the MCU from low-power mode without depending on an
external interrupt (Auto-wakeup mode). The RTC provides a programmable time base for
waking up from Stop (0, 1 or 2) or Standby mode at regular intervals. For this purpose, two
of the three alternative RTC clock sources can be selected by programming the
RTCSEL[1:0] bits in the
•
Low-power 32.768 kHz external crystal oscillator (LSE OSC)
This clock source provides a precise time base with very low-power consumption.
•
Low-power internal RC Oscillator (LSI)
This clock source has the advantage of saving the cost of the 32.768 kHz crystal. This
internal RC Oscillator is designed to add minimum power consumption.
To wakeup from Stop mode with an RTC alarm event, it is necessary to:
•
Configure the EXTI Line 18 to be sensitive to rising edge
• Configure the RTC to generate the RTC alarm
To wakeup from Standby mode, there is no need to configure the EXTI Line 18.
To wakeup from Stop mode with an RTC wakeup event, it is necessary to:
•
Configure the EXTI Line 20 to be sensitive to rising edge
•
Configure the RTC to generate the RTC alarm
To wakeup from Standby mode, there is no need to configure the EXTI Line 20.
The LCD Start of frame interrupt can also be used as a periodic wakeup from Stop (0, 1 or 2)
mode. The LCD is not available in Standby mode.
The LCD clock is derived from the RTC clock selected by RTCSEL[1:0].
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Table 28. Shutdown mode

WFI (Wait for Interrupt) or WFE (Wait for Event) while:
– SLEEPDEEP bit is set in Cortex
– No interrupt (for WFI) or event (for WFE) is pending
– LPMS = "1XX" in PWR_CR1
– WUFx bits are cleared in power status register 1 (PWR_SR1)
On return from ISR while:
– SLEEPDEEP bit is set in Cortex
– SLEEPONEXT = 1
– No interrupt is pending
– LPMS = "1XX" in PWR_CR1 and
– WUFx bits are cleared in power status register 1 (PWR_SR1)
– The RTC flag corresponding to the chosen wakeup source (RTC
Alarm A, RTC Alarm B, RTC wakeup, tamper or timestamp flags) is
cleared
WKUPx pin edge, RTC event, external Reset in
Reset phase
Backup domain control register
DocID024597 Rev 3
Description
®
-M4 System Control register
®
-M4 System Control register
NRST
(RCC_BDCR):
RM0351
pin