I build a prototype board with a STM32G431KBT6 on it. This board simply drives some peripherals (led panel, motor) based on what commands I'm sending it over the Virtual COM port.

When connected to my laptop, I have a perfectly working prototype.

  • I have USB enumeration
  • I can send/receive commands over the VCP
  • It perfectly drives the peripherals

But all this changes when I connect the prototype to my client's PC This PC isn't connected to the internet and probably can not find the right VCP port yet. But it should enumerate in my opinion. Nothing is shown in the device manager.

The weirdest thing of it all is that it also doesn't enumerate on my laptop and lab PC anymore

So basically as soon as my client's PC is connected to USB of the prototype, the device isn't recognized anymore.

So I thought, clearly there is something wrong with his USB cable, but when I connect an Arduino Nano on his USB cable, it is shown in device manager.

So how can Windows enumeration permanently damage the STM32 chip?

I did a full erase of the device and reprogrammed it, but nothings seems to work other then resolder a new chip.

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  • \$\begingroup\$ Windows can't permanently damage the STM32 chip. There must be a software fault so it just does not enumerate with some computers, or connecting them caused a hardware fault, such as ESD pulse or surge when potential between two devices equalized, such as from power supplies. Please provide schematics and PCB layout to find out if there is any problems in the design. \$\endgroup\$
    – Justme
    May 18, 2021 at 17:25
  • \$\begingroup\$ I added part of the schematic, prototype is powered by a 12V DC adapter \$\endgroup\$ May 18, 2021 at 18:04
  • \$\begingroup\$ So, is the 12V adapter 2-prong ungrounded one, or 3-prong grounded one? If the latter, was it connected to ungrounded or grounded outlet (I don't know what kind of sockets you have and if this is possible where you live)? Did you measure any AC or DC voltage difference between the devices? Is the laptop power supply grounded or ungrounded? Is the laptop chassis grounded or ungrounded? \$\endgroup\$
    – Justme
    May 18, 2021 at 19:05
  • \$\begingroup\$ The 12V adapter has a grounded 230V connector, but has 2-prong on 12V side. So basically ungrounded. The host pc is a Elo touch panel PC with it's own 19V DC adapter. This one probably is grounded properly. So a proper grounded USB shield is meeting my 0V from 12V DC adapter as soon as someone connects the usb cable. \$\endgroup\$ May 18, 2021 at 20:56
  • \$\begingroup\$ Probably? So it would make perhaps sense to know what kind of power supplies the devices have before connecting them together with USB cable. Unless you measure it, there is no way you can know if a power supply output is grounded, floating at high impedance, or if it has a hundred volts of high frequency switching noise as common mode voltage. If both are grounded, there might be a current loop via mains earth wiring. If there is high common mode voltage, connecting plugs together might even show visible sparks, and then ripple current would flow via USB ground wire. \$\endgroup\$
    – Justme
    May 18, 2021 at 21:06

1 Answer 1


This is an EMI signal integrity problem caused between the tower ground noise and the external SMPS noise with lack of high-quality double shield on USB. Change any of the 3 influences to mitigate the issue. Also adding extra low ESL braided ground cables and CM chokes and ground shielding can help on both the offending and offended cables once the exact path is determined.

  • \$\begingroup\$ Yes pretty thin USB cable if you ask me. My prototype is powered via 12v DC adapter and my client's PC is a Elo touch panel with its own DC adapter (probably 19V) My boards ground (which is from DC 12) is meeting the touchpanel's ground as soon as someone is connecting the USB \$\endgroup\$ May 18, 2021 at 18:14

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