Dead STM32F407VET6 "Black board"

[ozcar]

Greetings,

I'm posting this in case it might be useful to somebody - how to avoid the "mistakes" I made.

Years ago I got a few STM32F407VET6 "Black boards", which are as described here:

https://os.mbed.com/users/hudakz/code/S ... /shortlog/

I have not used them for any serious permanent project, but I often use them for testing various things. Way back, one of them stopped working. Windows was complaining that USB device was not recognised and Device Manager said it could not read device descriptor (even with boot pins set for DFU mode). ST-Link could also not connect to it.

At the time I checked a few things, like the 5V and 3.3V supplies, but could not find any obvious problem. I recently decided to take another look. I noticed that the processor was very slightly warm compared to other working boards. I did not think to measure the current drawn (mistake number one perhaps) but I know now that a working board draws less than 20mA when in DFU mode with nothing else attached to the board.

Mistake number two was to then replace the processor. Replacing LQFP100 ICs is outside my comfort zone, but after painstakingly checking all the pins I thought I had done a reasonable job. The only problem was that the replacement behaved exactly like the original!

I then found that one of the VCAP capacitors was shorted. There are two of these C21 and C22, both 10 μF, one connected to VCAP_1 and the other to VCAP_2 - see the schematic linked to from the above page. The two VCAP pins appear to connected together within the processor, so C21 and C22 are effectively connected in parallel.

Signs of this problem would be the board drawing more than about 20mA. With the board powered off, the resistance from VCAP_x to ground was showing less than 1 ohm. With the board powered up the voltage on the VCAP pins was near to zero (on an OK board I see around 1.26V there). It is probably easier to check the resistance and voltage across the capacitors than try to get a needle probe on to the processor pins.

I have no idea if the internal regulator is protected against such indignities, but the processor seems to have survived and the board is working OK after I replaced the bad capacitor. I have not done any extensive testing though. The original processor may be just fine too, but I'm not about to put it back to find out.