PaulS:
The Arduino UNO is powered via USB cable only . I've check that the Vcc is about 7.5V. Is there a problem here
The USB voltage is supposed to be 5.0. If you are getting more than that anywhere, yes, there is a problem.
Thanks everybody,
i've found the solution for my problem:
I've check that the Vcc is about 7.5V
Sorry for my missing, it isn't 7.5V for my USB port, just because the error of the volt meter. Use anther voltmeter , it is 5V normally.
After that, i've used the 12V-2A external power for Arduino Uno board. and everythig work fine .
In conclusion, it is the Power problem