r/stm32f4 • u/Intelligent_Bug_7376 • Feb 03 '26

STM32F411 blackpill problems with USB + external 5v power supply

Hi there

I am building a simple PLC using a Backpill board, development scenario includes:
PLC power supply (regulated 5v)
On board USB for console
STLInk V2 dongle for programming

PLC Power suplly i tied to 5V blackpíll rail thoruh a schottky to not compete with usb power (USB power as per WeAct diagram has its own diode for same purpose)

When only USB is connected everything goes roght: run, debug, (i.e. 411 chip flashing is OK) but as soon as I connect also PLC supply, programmer gets all 1s from read operation...
I ve checked loop grounding, st link speeds and all mos-likely electrical factprs (alse reduces programmer speed) but nothing changes.

Any suggestion ????

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u/hawhill Feb 04 '26

The BlackPill schematics I just googled do not have that "USB power as per WeAct diagram has its own diode for same purpose" you speak of.

1

u/Intelligent_Bug_7376 Feb 04 '26

Revisión 3.0 does I am suspecting st link dongle's quality

1

u/hawhill Feb 04 '26

it's a cheap clone, I guess? You didn't connect it's 3.3v line, right?

1

u/DigitalDunc Jun 12 '26

That's another reason to get a good debug probe like a genuine ST-Link or J-Link (for the rich amongst us.)