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NUCLEO-L432KC - External power 3v3



I want to make something like a shield for the NUCLEO-L432KC,

The shield will be a can bus interface and power supply from 24v to 3v3. This shield will also permit to connect sensors to the pins of the nucleo board. So all the pins are accessible.

I come here because i read a lot about external powering on nucleo but mostly 5v, i know there are many post on the forum that talk about it, but my question is about 3v3 external supply.


The user manual says we have to disconnect sb14 and sb9 and stlink will not work with external 3v3.


But after some analysis of the schematic this is what i see:

- When the board is not powered by USB or 5v the st-link is not powered, so the nrst output of stlink to the stm32 will pull low, that’s why we have to remove SB9

- Removing SB14 is probably to prevent the 3v3 to go on the output of unpowered regulator and in an input (pa0) of unpowered stlink


but now i’m looking for solutions to flash the stm32 if SB14 and SB9 are disconneced and this is what i see:

- if stm32 is powered by external supply (3v3) with SB14 disconnected AND stlink is powered by USB, the two chips are well powered by different powers but same gnd. So at this point it’s ok.. Right?

- The unique issue that « prevent debugging » is that the nRST is not connected to the st-link with SB9 off ?


But is it really needed to control nRST from st-link to debug/flash?

As i know we can flash stm32 with DIO/CLK pins without reset ?

Am i missing someting ? Is there a problem that i don’t see …?


Because if i am right then, it’s possible to disconnect SB14/9 and:

- When the board is used in production, only the STM32 is powered (through external power on 3v3)

- When we want to flash the board, there are no problems, just plug usb cable and power the stm32 with external power through 3v3 then start programming…