when I hook my arduino up to a usb 1.1 hub - I get around 4,3 volts on vcc
if you connect the batterys to the vin barrel - you are not going to get the same voltage on vcc - far from it. The battery voltage goes through the voltage regulator.
however - this does not garantuee a clean 5v vcc
I finally figured out a simple trick, measure the 3,3v line on an analog pin and use that to calculate the voltage on Aref.
Thanks to coding badly and lefty for some great posts on the subject.
way ahead of you there ...when I use the computers usb ports all is fine!just this usb 1.1 hubI have not looked into it that much further - it doesnt bother me since I figured out a way to measure the vccwhich actually was a good idea since I need this when I want to run my arduino off batteries.I just got fascinated by the whole bandgap idea because it doesnt involve sacrificing an analog pin.thanks for your reply, and thanks for sharing your bandgap experiments