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Hello.
Why when on Vin pin voltage is bigger than 6.5V the atmega8u2 MCU dont work?
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Why when on Vin pin voltage is bigger than 6.5V the atmega8u2 MCU dont work?
This doesn't make sense, what model of arduino are we talking about?

A voltage lower than 6.5V on the Vin will not allow the regulator to work correctly so the output will not be 5V.
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Why when on Vin pin voltage is bigger than 6.5V the atmega8u2 MCU dont work?
This doesn't make sense, what model of arduino are we talking about?

A voltage lower than 6.5V on the Vin will not allow the regulator to work correctly so the output will not be 5V.
I use Arduino Uno.
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I use Arduino Uno.
Then this statement is not true:-
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when on Vin pin voltage is bigger than 6.5V the atmega8u2 MCU dont work?
What makes you think it is?
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when on Vin pin voltage is bigger than 6.5V the atmega8u2 MCU dont work?
What makes you think it is?
Because when i get on Vin higher than 6.5V than voltage on USBVCC is less than 1V and serial wount work!!!
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I use Arduino Uno.

Which model?  The R3 schematic shows that USBVCC is used for the +5V supply if half the VIN supply is less than 3.3V

Are you using a USB cable for serial I/O?  If so, why isn't the host system providing 5V to USBVCC through the USB cable?
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Because when i get on Vin higher than 6.5V than voltage on USBVCC is less than 1V and serial wount work!
Sounds like your power switching circuit is broken.
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