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?
ATmega32u2 – How to Ground a USB-Powered ATmega32u2
groundingschematics
Related Solutions
First of all, this is a nonstandard and unhelpful way of calling out component values. Don't do this on schematics you draw! (In normal practice, "R220" means "resistor number 220" and the value is shown separately.)
But somebody with experience might be able to figure it what's going on. Let's see how I do...
C180 must mean 18 pF. Those are crystal load capacitors, which are commonly in the 10 pF - 20 pF range.
C105 means 1 uF. These are bypass capacitors and 1 uF is a common value and the datasheet-recommended value.
R220 probably means 22 ohms, from the example circuits in the 32u2 datasheet.
I found this drawing in the datasheet:
Also notice the supply connections. The micro has its own internal regulator, so the Vcc doesn't need to be connected to anything else (except its bypass capacitor.)
None of the components are super critical. 10% tolerance should be fine.
Once again, don't use this as example of how to draw schematics.
(edited to revise 0.1uF guess to 1 uF.)
According to the datasheet for the ATmega32u2, the UCAP pin is the output of the internal 3.3V voltage regulator. Check out the 3rd page for a block diagram and you can see for yourself.
From the diagrams that you posted, one grabs its VCC strait form the USB bus for 5V IO operation while the other connects to the internal 3.3V regulator for 3.3V signaling.
Choosing between which one to use will depend on what you intend to connect to the microcontroller. Other hardware may only do 3.3V signaling for example and will not be tolerant to 5V. Other hardware may only operate with 5V supply and thus maybe output data at 5V.
Be sure to read the datasheets carefully for anything you intend to hook up and that will guide you in what IO voltage to use.
Best Answer
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.