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I have learned that powering a chip without ground is quite dangerous and might cause permanent damage to the chip or the board in general. I have also seen that happening many times in non isolated DC to DC step down converters and some microcontrollers as well.

How can I prevent that from happening? Assuming I have a board with a non isolated switching regulator and a microcontroller that is connected through a UART to a USB to serial cable or to another board. How can I prevent the current flow if the ground disconnects first or the positive line connected before ground?

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    \$\begingroup\$ You need a hot plug connector which connects ground first and disconnects last. Lots of options available. \$\endgroup\$ – Peter Smith Sep 11 at 12:17
  • \$\begingroup\$ Can't I add a protection or anything? \$\endgroup\$ – MrBit Sep 11 at 12:19
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    \$\begingroup\$ How exactly is applying Vcc before Vss harmful to ICs? I don't see how it would be any different to applying Vss before Vcc, since voltage is a relative quantity and if only one connection is made nothing can actually happen (bar tiny currents into parasitic capacitances that will do approximately nothing whatsoever). \$\endgroup\$ – Hearth Sep 11 at 13:20
  • \$\begingroup\$ I think there's a return path through the USB to UART cable. \$\endgroup\$ – MrBit Sep 11 at 14:07
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Its not quite the rule that ground should be connected first, instead the rule is generally power before data, If you look at a USB plug / socket you will notice the 2 outer power/ground connections are longer than the middle data connections, Same for Micro-SD card,

In these cases by having the device be powered before its connected to data lines means everything is at a known state, there will not be any pins mid way from raising from a low to a high signal.

What it sounds like your describing is a mains ground loop, things like PC power supplies have there ground tied to mains ground, so by having your UART - USB Cable connected, they share a ground, if your ground referenced power supply looses its ground connection to your circuit, It ends up completing its circuit through the wiring in your walls

In reality it is rare for this to damage a circuit, however it can make it act weird as this tends to couple in a fair bit of noise.

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  • \$\begingroup\$ I agree about power before data, but what about when there is a potential difference between the two grounds and this high side connects first? Also, what about ESD? \$\endgroup\$ – Aaron Sep 11 at 16:27
  • \$\begingroup\$ @aaron when there is a potential difference between grounds (poor connection somewhere, or multiple stakes) it ends up the same, any current on that ground wire (SHOULD be minimal) will be imposed as a voltage on your circuit, and depending how big the mismatch, a damaging current can flow through your device (blow the wires or traces like fuses), ESD is generally handled on these interfaces by an ESD diode package on the data pins, if any normal data interface is not protected like this, I would question the manufacturer \$\endgroup\$ – Reroute Nov 6 at 9:22

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