RTC backup 3VDC battery wiring on stm32mp157c-dk2 board?

[albertr]

I have seen some stm32mp157 boards from third parties do have a backup 3VDC battery attached to their RTCs, so it won't lose the time when powered off. Is there any way to connect a backup battery to the RTC circuit on the stm32mp157c-dk2 board?

-albertr

[PatrickF]

To disable RETRAM and BKPSRAM, you need to modify TF-A source code as this is currently hard coded (always enabled before entering in low power modes).

[albertr]

Hi @PatrickF​ ,

I don't use trusted boot chain, I'm using basic boot (u-boot-spl.stm32 + u-boot.img). Anything I need to change in u-boot or in its device tree to disable it?

-albertr

[PatrickF]

In uBoot SPL (Basic Boot), as low power is not supported, the BREN and RREN bits should not be set (they are cleared on a VSW/VBAT domain reset), so you should have nothing to do to get lower power when in VBAT mode.

We recommend to use TF-A+uBoot (Trusted Boot), which is better supported and provide all features. Basic Boot only exist as currently required for DDR Tools.

In TF-A, changes are in plat/st/stm32mp1/stm32mp1_low_power.c

in enter_cstop() function remove the following lines:

                      /* Keep retention and backup RAM content in standby */
                       mmio_setbits_32(pwr_base + PWR_CR2, PWR_CR2_BREN |
                                              PWR_CR2_RREN);

[albertr]

Thanks for clarification, @PatrickF​ !

-albertr

[albertr]

Thanks again!

-albertr