How I killed my Arduino Nano, lessons learned.

Okay, I don't really have to whine, the Arduino was only a few bucks worth.
It was a Chinese Arduino whith the CH340 serial USB chip.

Since I need to make rather precise measurements, I wanted to take the 3,3V as a reference for AREF.
so I bridged 3,3V and AREF, which are conveniently laid beneath each other.

DONT DO THAT!

At least not immediately.
By default the AREF is set to 5V and the AREF pin also outputs that voltage.
So the AREF pin fed 5V back into the 3,3 V circuitry and blew the CH340 serial USB chip away.
The CPU still works, but the USB is dead.

I am not aware of what would happen if you first flash the software with AREF=external, but I will not try again without at least a resistor to limit the current.

By the way, the CH340 was recognised without need to set manually a driver in both Windows 7 and the current Rasbian linux...

No, actually the problem is you didn't call analogReference(EXTERNAL) in your sketch before doing an analogRead(). The AREF pin is there to do exactly as you wished, you just need to tell the software that.

From the Arduino references:

https://www.arduino.cc/en/Main/ArduinoBoardNano:
The Nano has 8 analog inputs, each of which provide 10 bits of resolution (i.e. 1024 different values). By default they measure from ground to 5 volts, though is it possible to change the upper end of their range using the analogReference() function.

https://www.arduino.cc/en/Reference/AnalogReference:
Warning

Don't use anything less than 0V or more than 5V for external reference voltage on the AREF pin! If you're using an external reference on the AREF pin, you must set the analog reference to EXTERNAL before calling analogRead(). Otherwise, you will short together the active reference voltage (internally generated) and the AREF pin, possibly damaging the microcontroller on your Arduino board.

That is why I mentioned:

DONT DO THAT!

At least not immediately.
By default the AREF is set to 5V and the AREF pin also outputs that voltage.

So one must load FIRST a sketch with AREF=External, then later make the shunt between 3,3v and the REF pin.

The warning is not clearly formulated. "If you're using an external reference on the AREF pin, you must set the analog reference to EXTERNAL before supplying any voltage to the AREF pin. "

When you call analogRead shoud be irrelevant.

Anyhow I will never short 3,3V and AREF again without a resistor.
You never know what might be upoladed inadvertently and then you end up with a bricked USB.

Except it doesn't output 5V, at least not until you do an analogRead().

I just checked this on my Uno, but that's the same microcontroller with the same board wiring to AREF. Unless your Chinese Nano does something funky with the wiring, you shouldn't get 5V on the AREF pin until you call analogRead().

Which means the documentation is correct.

Just go buy another chg340. They are only in 70 cents a piece.

70 cents each? You're getting ripped off - the chip is closer to 25 cents.

But he's using a nano... so repairing it involves SMD rework. With nano clones at 3 bucks a pop, why bother?