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I had some trouble with home assembly of some PCBAs. I've got an STM32F411xC (datasheet here).

Anyway, After reworking it a bit, and some issues elsewhere on the board were uncovered (all related to oversights with soldering technique), the first board was toast...Missing pads and so on..

So, I moved to the next clean board, and started fresh...One component at a time, and testing along the way for continuity, resistance, etc.

The first and most technically troublesome part to put down was the STM32F4 in a 64 pin LQFP, so I dropped that thing down and took a reading across the 3.3V rail and GND, and everything looked good...wide open.

So, imagine this...I only have the STM32F4 on the board, that's it...I put 3.3V on the board from an external supply, and it shorts out, 350mA before the current control kicks in. Now, I've got 26kohm between 3.3V and GND after removing power...

So, after a day of double checking pinout, orientation of the chip (found here), the thing I tried was pulling up the VCAP1 pin and leaving it floating...So, I read a little about it and see that VCAP1 is pretty serious when it says to hang 2x2.2uF or 4.7uF off of it, so I do that...

I get a new IC, desolder the old one, put it down, measure resistance between power and ground, make sure all the pins are really connected to the board well, ring out all the VDD's and VSS's AND slap down two 2.2uF caps on VCAP1.

I turn it on, and still, short circuit.

What the heck is going on here? This circuit is so simple, it's two components...The STM32F411 and a pair or 2.2uF caps...

I'm tellin' ya...You hook this circuit up, it'll short right out and get real hot, real quick. What is the GOTCHA here that I'm missing?

enter image description here

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1 Answer 1

Reset to default
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The minimum circuitry for an STM32F4 includes:
- NRST pull-up.
- BOOT0 pull-down.
- Decoupling capacitor for each power pair.
- Vcap.
- Vbat.

Also. Why is Vcap connected to Vbat and Vref? You're shorting the internal regulator?

You have seen this image, right?

enter image description here

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    \$\begingroup\$ Oh... My... God... I want these two days back. I can't explain it man... I've looked at that very image 100 times in the last sleepless 48 hours... I can't believe I didn't see that nothing was tied to Vcap! I thought that was a raw power input to the internal regulators and the cap was a bypass or filter for said regulator... I can't believe it. Thanks for spotting that! \$\endgroup\$
    – testname123
    Commented Sep 25, 2017 at 6:57
  • \$\begingroup\$ NRST doesn't need a pullup resistor, but a capacitor to ground. \$\endgroup\$
    – followed Monica to Codidact
    Commented Sep 25, 2017 at 7:04
  • \$\begingroup\$ @berendi never cap a reset unless you have a button or jumper on it. \$\endgroup\$
    – Jeroen3
    Commented Sep 25, 2017 at 7:28
  • \$\begingroup\$ That's it baby... You nailed it. Shorting out Vcap to VDD is a great way to fry eggs. Cut that connection and now I can talk to the chip \$\endgroup\$
    – testname123
    Commented Sep 27, 2017 at 0:51

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