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STM32 L4 VBAT internal battery charging

Question asked by Christophe MARIN on Apr 16, 2018
Latest reply on Apr 16, 2018 by Christophe MARIN

I'm currently struggling with my Nucleo STM32L476RG board about VBAT battery charging feature.

- I removed the SB45 on baord bridge to unbound the VBAT pin from the power supply

- I wired a 100uF/4V tantalum capacitors, which offers more than 5mn of backup for the RTC (yes, that's enough for my application).

I'm now trying to use the VBAT internal battery charging feature by setting the appropriates bits:

    SET_BIT(PWR->CR4, PWR_CR4_VBRS);
    SET_BIT(PWR->CR4, PWR_CR4_VBE);

 

But this seems to have no effect: as soon as I remove the link from my backup capacitor to the power supply, the voltage slowly drops down, as if there were no internal battery charging feauture enabled. (Yes, forgot to mention, there is a Schottky diode from capacitor to the Vdd, and I cut the link from the cap and the diode).

 

Any idea about what I'm currently missing ?

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