catatung:
Hmm, is this a misprint?"The FTDI FT232RL chip on the Nano is only powered if the board is being powered over USB. As a result, when running on external (non-USB) power, the 3.3V output (which is supplied by the FTDI chip) is not available and the RX and TX LEDs will flicker if digital pins 0 or 1 are high."
Ah, but you failed to mention you were talking about a Nano board in your post title or first posting Vs any of the standard shield based format Arduino boards. Lets look at the Nano's version 3 schematic.
Looking at the schematic the FTDI chip uses power named +5V, the same power name as the 328P shows, so if the 328P has power when using external Vin power via the on-board +5vdc regulator, both the 328p and the FTDI appear to be powered up and functional. There is a isolation diode shown (D1) connecting USB voltage and +5V voltage but that would not cut off power to the FTDI So the statement they make:
"The FTDI FT232RL chip on the Nano is only powered if the board is being powered over USB. As a result, when running on external (non-USB) power, the 3.3V output (which is supplied by the FTDI chip) is not available and the RX and TX LEDs will flicker if digital pins 0 or 1 are high."
appears indeed to be in error, or the statement applies to an earlier version of the Nano that might have had a different voltage arrangement, or this schematic has fundemental errors in how the power is actually used Vs what it shows? As I don't own a Nano I can only go this far in analysis of what reality is Vs what is stated and shown in the documentation. So again from what I see in the schematic it appears to me that USB serial should still be functional even if using Vin to power the board.
Lefty