Dear all,
my arduino UNO R3 works fine when I connect it to the computer after it's switched on, but if I start up my computer with arduino connected (USB cable plugged) the system cannot boot and freezes. is it normal? I guess not. I tried with different computers and different USB ports, but it behaves the same.
Should I reset the chip, or any other idea?
Thank you for your help
Ant
System Mac OS 10.6.8, arduino UNOr3.
No, that doesn't sound particularly normal. 
What kind of a Mac is it? Some of them had trouble with certain USB devices plugged in at startup. Have you tried any other USB devices or computers to see if the problem is limited to your Arduino or computer?
Mine is a MacBook 2GHz Intel Core 2 Duo, and I tried the same arduino with a MacBoo pro 2GHz intel Core 2Duo, with same result.
With USB hard drives, iPod or printer it boots normally, only the arduino card is causing the issue.
a.
Maybe there is a setting in the bios to disable legacy usbs? or startup something?
startup items I don't think, because the system doesn't even start, it stuck before the system boot, so it can be a bios thing, but how to verify? how can I access the bios on a mac?
(for the sake of completion, it happens with the bare minimum example sketch uploaded, so it is not a code related problem)
A
Admant:
startup items I don't think, because the system doesn't even start, it stuck before the system boot, so it can be a bios thing, but how to verify? how can I access the bios on a mac?(for the sake of completion, it happens with the bare minimum example sketch uploaded, so it is not a code related problem)
A
Does the RX TX blink? maybe the PC is asking the device something and waiting, it is weird but in an old pc I remember having the legacy USB (mouse/keyboard/pendrive) support enabled halted the start when I had a SD card reader plugged
nope. only the green ON and the yellow 13. no traffic between cpu and arduino during startup.
a.