VBAT input if used with removable primary battery

[PGE]

Dear All,

We are designing a custom board based on STM32MP1. In some ST documents we can read "if no external battery is used, it is mandatory to connect this pin [VBAT] to VDD."

If I check the developpement board schematic VBAT is simply connected to a connector. Also according to the STM32MP157 datasheet (figure 35) the VBAT input is connected to a switch who connect either VBAT or VDD to VSW domain. So the connection to VDD seems to be managed internally.

We are using a removable primary battery as power source for VBAT. Do we have to provide a mechanisme to connect VBAT to VDD if the battery is removed ?

Many thanks in advance for any clarification

[PatrickF · Accepted solution]

Hello,

I confirm it is only mandatory to connect VBAT to VDD when there is no VBAT battery planned on the system. This is to ensure correct backup domain reset sequences.

If VBAT is used in the system, it could be kept open during the time to replace the battery (which might be done with VDD presence to maintain VSW content).

Nevertheless, the system is robust enough to work with VBAT open as it is the case on STM32MP157C-DK2 for instance (as the battery is not provided in the kit), even if this is not recommended.

I confirm that there is an automated switch between VBAT and VDD for the VSW domain.

Regards.

[PatrickF]

@PGE​ , please click on 'Select as Best' if my reply solved your issue or answered your question.