atmegaatmelavrmicrochipusb
On the ATmega32U4 MCU there are several pins related to USB:
In the datasheet and in almost all reference schematics I find on the Internet, it's explained how to connect those pins when you need the USB.
On my board, instead, I don't need the USB at all.
Hence, I want to understand how to connect those pins.
I'm not sure if I can just leave them floating or else.
What you are looking for is called a "USB Power Injector".
Many commercial (cheap) offerings exist. Here's an example. They are available in all USB connector types: A, B, mini-B, micro-B, etc.
In your second image, the scale is just 10mV. Hence, I would not read too much into that pattern. It's possible that you are just picking up noise. Power supply decoupling caps should not cause you much concern, as long as they are in parallel, and are not blown. Could you provide more detail on how much power the part is drawing when you connect it to the USB port? Does the part overheat?
Best Answer
The datasheet does have that information.
You can make a self-powered USB device as per the datasheet diagrams and imagine the USB cable is unplugged, which gives enough information how the pins should be connected without USB connection.