RTC Battery power for STM32L432 MCU and Nucleo-32 boards ?

[eBirdman]

Posted on May 31, 2018 at 03:40

I am considering to use Nucleo-L432KC board and I need to use it's RTC with maintained time. User manual shows Vbat power in some internals of MCU but there is no pin to provide this power.  Looks like the main chip's power source Vdd is the only source powering RTC block. Consequently Nucleo32 board with it also does not provide battery power for RTC backup even though it does have a crystal for RTC.  Does this mean that RTC built in this MCU was not meant to be maintained in off-power mode and must be setup after every power-up or is there something I am missing?

#rtc #backup-battery #stm32l432 #nucleo32

[T J]

Posted on May 31, 2018 at 03:52

usually the smaller sized processors don't have a VBat pin

your particular chip L432K only comes in 32 pin device.

You may want to look for another chip.

[T J]

Posted on May 31, 2018 at 04:41

the die is likely to support the VBat pin, but the package does not.

:(

the benefit of using STM32 is that mostly, the cpu's are interchangeable.

but its likely that no 32 pin will have VBat.

[eBirdman]

Posted on May 31, 2018 at 04:35

Something was meant by the developers by devising the internally generated power shown as Vbat for the RTC module. What is the idea behind that? Why did they show Vbat if there is no functionality associated with it? Look at the Power supply overview picture from STM32L432 - see Vbat ? (Or is this picture pasted from another MCU datasheet ) How the user suppose to use this information?

[Amel NASRI]

Posted on May 31, 2018 at 12:09

Hi

tech.008

‌,

There is a typo in STM32L432Kx devices as there is no VBat pin in low pin-count packages. We say this explicitly in

http://www.st.com/content/ccc/resource/technical/document/reference_manual/group0/b0/ac/3e/8f/6d/21/47/af/DM00151940/files/DM001519pdf/jcr:content/translations/en.DM001519pdf

:

VBAT is internally bonded to VDD for small packages without dedicated pin.

VBAT pin is not available on low pin-count packages, VBAT is internally connected to VDD.

The datasheet will be updated to fix this error.

-Amel

[eBirdman]

Posted on May 31, 2018 at 14:53

Thank you Amen for clarification. This also implies that all the efforts of STM developers adding RTC provision to the tiny Nucleo_L432 board by allocating RTC crystal is usable only for the duration of the power ON. Since RTC clock does not keep time after power-off, RTC must be reset to a real time after each power-on... Not really a full RTC implementation without ability to keep time...

[AChow]

Hi,

I am working on stm32l433RC nucleo board.

I am implemented RTC with date and time.

I wanna backup RTC values (means when there is no power to the MCU RTC should be work with battery power )

please help me how to implement this one on nucleol433rc development board

Thanks in advance