Solved
Can you ground both VDD50USB and VDD33USB if you are not using USB features?
The datasheet is a bit light on details about what to do with these pins if you are not using USB at all. Can I ground both of them, or does there need to be a supply voltage on VDD33USB?
One of the notes on the Absolute Maximum Ratings section in the datasheet says "All main power (VDD, VDDA, VDD33USB, VDDSMPS, VBAT) and ground (VSS, VSSA) pins must always be
connected to the external power supply, in the permitted range," but I don't know if this means that ground is a valid "external power supply."
For example, in other parts of the datasheet it says that VBAT must be connected to VDD even if a battery is unused. However, it also says VDDSMPS should be connected to GND if unused. So I am unsure about the meaning.