RTC is not keeping time during low power mode

[viniciusfre]

I am configuring the CPU to wake up from UART reception and/or GPIO interrupt:

echo enabled > /sys/devices/platform/soc/4000f000.serial/tty/ttySTM1/power/wakeup

echo enabled /sys/devices/platform/soc/4000f000.serial/power/wakeup

And it goes to sleep with:

echo "mem" > /sys/power/state

After 1 hour in sleep mode, the time is 20 minutes slow. This is not happening in Run mode.

Additional Information:

• Linux 5.4.31
• 3.0V RTC battery backup

[PatrickF]

Hi @viniciusfre​ 

Need more details to analyze further.

Which board are you using ?

Are you sure about your 32k crystal ?

Did you keep default RTC LSE settings ?

Did you ensure RTC time is set with system time before going to low power mode ?

See https://wiki.st.com/stm32mpu/wiki/How_to_use_the_RTC#How_to_set_a_hardware_clock_using_the_hwclock_tool

Linux timedatectl command could help you to understand.

Regards,

Patrick

[viniciusfre · Accepted solution]

Hello PatrickF,

Thanks for your response. That brought me to the solution. We need to "Set the System Time from the Hardware Clock" after wake up. The drift between system clock and RTC after wake up:

# date

Tue Mar 1 15:39:04 UTC 2022

# hwclock -r (RTC)

2022-03-01 15:49:04.894246+00:00

What it is fixed with: hwclock -s

[PatrickF]

Nice to see it is ok.

Regards.

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