MarkT:
You can use a different analog reference such as the internal one or the 3.3V rail (which ought both to be stable, but need calibrating.)
Hi Mark, excuse me, I'm at first approach with some concepts in Arduino. What do you mean with this sentence? What's the internal analog reference?
I know there should be a way to mesure the V of USB with an Arduino Sketch... but don't know how. And I cannot figure out what do you mean when you say that the 3.3V rail needs to be calibrated. How can I calibrate it?
Once again, thanks for your reply.
Zongo.