Optimize STM32MP135F RTC power consumption

[aloiseau]

I have a STM32MP135F with a 3V3 47mF supercap on its VBAT line.

Internal RTC appears to work fine with power outages up to 3 or 4 hours, whereas I hoped one day.

I see in datasheet "Table 27. Current consumption in Vbat mode" that I can tweak VBAT powered sub-components to reduce this set (Backup SRAM, ) and increase RTC backup duration, but I fail to find those settings. Does anyone have it in mind?

I am using Yocto and OpenSTLinux on a board derived from DK2.

Regards

[PatrickF]

Hi @aloiseau 

I think there is no DTS control for this. It is probably hardly coded inside TF-A or opTEE.
Look for PWR_CR2.BREN bit.

Regards

[aloiseau]

Thanks, guess I found it in optee-os-stm32mp recipe code, core/arch/arm/plat-stm32mp1/pm/low_power.c file, init_low_power function, alongside a "Keep backup RAM content in standby" comment. Will try and let you know!

By the way, but this is more a HW questions, my goal is to power RTC from VBAT and a small supercap. I also found PWR_CR2_MONEN bit I can disable to reduce VBAT power consumption (with related code within optee at least), and I guess I will need to look for LSE too, hoping it can be disabled to create a middle point between both first lines of datasheet table 27 (see after). Will see, did not dig yet, my hope is to go as low as 500nA despite datasheet which I hope do not reflect all possible combinaisons. If you have in mind another VBAT-powered line I should think about, I am interested :)

[PatrickF]

Hi @aloiseau 

stopping LSE is not possible as this is the clock for RTC.

Not possible to go in the 500nA range on STM32MP13 while keeping the RTC/LSE running.

So, either you put a larger supercap or you use an external RTC on I2C having extreme low power (like https://www.st.com/en/clocks-and-timers/m41t83.html), with 32khz feed to LSE (usually required for low power modes support by Linux). In that latter situation, VBAT of STM32MP13 could be connected to VDD.

Regards,