I just wanted to note a couple of observations that I've made for those of you who are using the PI 400 and interfacing with the Arduino Uno R3 via USB. I spent about 2 hours troubleshooting some of the issues of connecting to and uploading sketches to the Arduino. I have two Uno R3's (one official and the other by DFRobot) and I used 3 different cables, all in working condition. This is what I've found when trying to use the USB 2.0 port for both Arduino boards:
-Arduino IDE would automatically recognize either board, but only sometimes
-The times that the IDE wouldn't recognize either board, I had to manually select the COM PORT
-Even when I did manually select the COM PORT, uploading would hang, or I would get avrdude error messages, or the sketch would execute even though the IDE said it was still uploading
What HAS actually worked for me was leaving my mouse plugged into the USB 2.0 port and using the remaining two 3.0 ports for the Arduino (I have two breadboards with projects on them, but only one Arduino plugged in at a time). I've plugged in either board, uploaded various sketches, and have not experienced any issues with automatic board recognition, error messages, or hanging uploads of any kind using the 3.0 ports. I have no insight as to why this works, but hopefully this saves someone some time and frustration trying to figure out why things aren't working with their Pi 400. Cheers, Tim...