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32F429IDISCOVERY external power supply

Question asked by machala.petr on Apr 3, 2015
Hi,
I am trying to design an external power supply for my disco board and I'm a little bit confused how the power supply schemes of STM32F4 relly works...

My idea was to have a possibility of external power supply stabilized to 5V directly to VDD pin and 3xAA battery pack also to VDD pin. The switch between them would be mechanically in the power plug of the external power source (if connected use external, if not use battery). I think this is quite straight forward. This gives me the possibility to switch between the sources...

But one of the features of my application is the use of RTC and I dont want to lose my configuration of the RTC if both sources were disconnected. So I thought that I would make it like a PC, have a small battery only for RTC. Problem is that the VBAT and PDR ON pins are directly connected VDD so I cant have everything...

I see only two options...
1) Have only internal RTC and if both sources aren't present (for example changing batteries) the RTC configuration is lost and has to be reconfigured...
2) Or use external RTC (for example DS1307) and there have a small battery like in a PC that would power only the RTC...

Personally I am starting to like the external RTC possibility because the power management on STM32F4 is quite complex. With the external RTC I could then power down the disco board simply by cutting the power plus for this no additional SW changes are needed...But than can I somehow wakeup the MCU from standby mode (I believe that the Vbat mode is impossible due the wiring) by external RTC (like for example once an hour)? 

Does anyone had a simillar problem? Thanks...

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