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[on:]

I'm sure this makes sense to you folks, but I'm adrift. What are all these voltages? Why does it compare 1/2 of Vin with VCC30 to determine what to do with USBVCC and +5V? What is the method to what appears to be this madness?

Thanks!

[Reply #1 on:]

You might want to add some context to your question.  I'm at a loss to what you are asking.  I'm even more at a loss to understand what the question has to do with installation or troubleshooting.

[Reply #2 on:]

What a great description. Thanks!

I admit, most of this made sense, but you've done a great job of explaining the rest. So, if Vin is less that 2 X VCC30, then  USBVCC is connected to +5V? Otherwise, the power is supplied from the on-board regulator, from Vin. And VCC30 is generated by the USB chip? It makes sense now, but was not obvious to me. Thanks!

[Reply #3 on:]

And VCC30 is generated by the USB chip?

As KE7GKP pointed out, it depends on the type of Arduino.  Older designs used the FTDI chip's internal 3.3V regulator.  Newer boards, such as the UNO, use a dedicated on-board regulator.