Question about CH340G and 3.3v pin

Other Hardware General Electronics
1

On many UNO and Nano clones that use CH340G, the 3.3v pin is often connected to 3.3v. From what I can find out about CH340G, if it's used in 5v mode, the 3.3v pin is supposed to be connected to ground through a 100nF capacitor. And when it's connected to 3.3v supply, they are supposed to run at 3.3v mode with suitable level input/output.

But it seems like an odd design, USB is always 5v and if UNO or Nano was running in 3.3v mode, there wouldn't be any 5v unless the board's connected to USB. And when it's connected to USB, it'd always be 5v mode. On some schematics I am able to find, the 5v pin (VCC input) of CH340G is connected to all of UNO or Nano board. I've found one version where CH340G is connected to USB 5v only and the rest of the board had separate 3.3v but that was a Leonardo.

It seems to me that having 3.3v mode on a 5v only board is pointless? Or maybe it's poorly wired and that CH340G is supposed to only get 5v from USB and not from the board like the Leonardo clone schematic I found?

2

The CH340G wasn't designed for the Arduino, it's a general purpose USB/serial translator. The purpose for the 3.3V output is to provide a voltage to interface the chip with 3.3V logic on the serial side. Arduino doesn't use that, but it can be used as a very low current 3.3V supply instead. So it replaces a 3.3V regulator and saves the part and the space on the board. Of course there is always 5V since the USB provides it.

3

The Nano 'steals' the on-chip regulated 3.3volt supply of the CH340 for the Nano's 3.3volt pin.
The Nano itself isn't using that supply. It's only available to the user for sensors, etc.
Don't draw more than about 30mA from that supply, to avoid volt-drop and thus possible USB<>Serial problems.

All the Unos with CH340 I've seen use a separate/independent 3.3volt regulator, powered from 5volt USB.
Capable of supplying 3.3volt/150mA. Again, this supply is not used by the Uno itself.
Leo..