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In ATmega32u2's reference manual, there's a diagram for USB-powered application like shown below.

Should I, or should I not, connect the GROUND to VSS/UVSS line? And why? If not, then what to do, just connect GROUNDs together?

Bus-powered application of ATmega32u2

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Yes, connect ground to UVSS. The ground symbol is used to indicate a common zero voltage reference point. Because you are powering the MCU via USB, this common zero voltage should be the same as the supply and therefore connected to the 0V (UVSS) line.

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The VSS notation is apparently commonly used in modern digital circuits as alternative notation to GND (references: [1], [2], [3]).

I've also noted, that on the ATmega32u2 pinout (manual, Fig.1-1 on p.2), there's neither VSS nor UVSS mentioned, but there's both GND and UGND, which supports this interpretation.

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