Arduino UNO digital output voltage (5V) - how it gets the 5V from USB power?

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hi folks. when arduino (eg uno) gets power usb cable only, how digital output pins 5v?

the outputs @ 5.0 v (from multimeter measurements) when output pins high. if usb voltage 5v, thinking there voltage drops internal arduino result in less 5v output. in advance.

quote
if usb voltage 5v, thinking there voltage drops internal arduino result in less 5v output.
why think that? not case.

there no voltage drop if there no current flow. put 220r resistor high input ground , measure voltage again see voltage drop 5v.


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