Program not running when powering from external source

DavidL_ · ‎2024-09-23

I have an L432KC and to test it, I created a simple blinking program which I uploaded via USB, which works just fine. When connecting my MCU to my external 5V power supply though, my program does not run. Looking at some posts, I found people suggesting to pull down the Boot0 pin to ensure that it loads the user program, I am not able to find the Pin name on the board of the Boot0 pin of the L432KC though.

I'd appreciate it if someone could help me out.
Thanks in advance!

DavidL_ · ‎2024-09-24

Also, is there a difference between the effects of pulling the NRST pin low and clicking the Reset button on the board?

Andrew Neil · ‎2024-09-24

NRST is the reset signal; the 'N' prefix indicates that it's active low; ie, so long as it's low (GND), the chip is held in reset.

@DavidL_ wrote:
There is a physical jumper from GND to NRST

Is there??

Andrew Neil · ‎2024-09-24

@DavidL_ wrote:
is there a difference between the effects of pulling the NRST pin low and clicking the Reset button on the board?

No - both have the effect of pulling NRST low, so both will cause a reset.

DavidL_ · ‎2024-09-24

How would I „break“ this link? What exactly does that mean?

Andrew Neil · ‎2024-09-24

@DavidL_ wrote:
How would I „break“ this link? What exactly does that mean?

It's here:

The photo in the UM shows a zero-ohm resistor fitted:

To break the link, you need to remove that zero-ohm link.

There's no need to be particularly careful with it - you don't need to keep it - so just heat it up with a soldering iron until it comes off.

@SofLit @STTwo-32 I've noted before that this needs to be better documented:

https://community.st.com/t5/stm32-mcus-products/docmentation-of-solder-bridges/m-p/689362

#BreakSolderBridge #SolderBridge #SB

DavidL_ · ‎2024-09-24

Thank you a lot for the detailed explanation!

Just to better understand what and why I'm doing it, I have a few questions:

- Do I understand it correctly that the SB9 is pulling down the NRST signal by default (so setting it to ON in the table you sent), and by breaking this pulldown, it will always be "OFF"?

- Do I always need to do this when I want to be able to use my stm32 nucleo board with an external battery?

- Does breaking the solder bridge somehow effect me being able to program and debug the MCU from my pc? If not, why is it even there in the first place?

Andrew Neil · ‎2024-09-25

@DavidL_ wrote:
- Do I understand it correctly that the SB9 is pulling down the NRST signal by default ace?

No.

SB9 is just connecting the Target microcontroller's NRST input to the ST-Link's NRST output.

Normally, when the board is powered via USB, the ST-Link will correctly drive NRST.

The problem only comes when the ST-Link is not powered - that's when it will just pull NRST permanently low.

@DavidL_ wrote:
Does breaking the solder bridge somehow effect me being able to program and debug the MCU from my pc

Possibly.

In theory, the Target MCU can be programmed using just the SWD lines. But there are a few cases (particularly, when the Target MCU goes into low-power modes) when it is necessary to NRST to access the target.

But, in those cases, you can always re-close the bridge.

DavidL_ · ‎2024-09-25

Ok, so I simply disconnect SB1 and the NRST will not be pulled low anymore when the ST-Link chip isn't powered, thus allowing me to use my MCU with my 5V battery and I can still program it while not in low-power mode. And if there is any problem, I can simply re-connect it, by applying (normal?) solder, is that right?

DavidL_ · ‎2024-09-25

I disconnected the SB9 solder bridge and now everything works! For some reason, It also solved my other problem which I reported here: https://community.st.com/t5/stm32-mcus-boards-and-hardware/failed-to-execute-mi-command/td-p/724162#M21581. I do not fully understand why it solved that one, bc. my MCU worked perfectly fine before (with SB9 connected).

It would be nice if someone had an idea to help me understand.

Thanks for your help!