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Why FRDM-K64F UART does not work with external supply?

Question asked by Francesco Salvatore on Jan 22, 2016
Latest reply on Jan 30, 2017 by Mark Butcher

Hi,

i'm developing on a project that uses a lot of UARTs on the board. When i use my PC's USB port to supply the FRDM-K64F all UARTs works fine, but when i try to supply the board using VIN pin (5V) seems that UARTs communications are loopbacked, like that RX and TX pins were shorted.

 

I tried also to supply the board using USB connector with an external USB charger but it did the same thing.

 

It works correctly only with PC's USB port connected.

 

Thank you!

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