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I'm trying to use a CH340G chip with an ATmega328P that is running on 3.3V to have a simple serial to USB interface. The available datasheets for this chip seem to be very minimal so I've tried finding out more in forum posts. Some links provide more information but it's not completely clear:

http://fobit.blogspot.com/2014/11/ch340g-in-eagle.html
http://actrl.cz/blog/index.php/2016/arduino-nano-ch340-schematics-and-details/
https://forum.arduino.cc/index.php?topic=409415.0

When comparing the usage of CH340G with the popular FTDI FT232RL one thing seems peculiar: In those user-provided example schematics the Tx pin of the CH340G is connected to the Tx pin of the ATmega328P and the Rx pin to Rx, when for FT232RL they would be crossed.

The schematic supplied by the fobit blog suggests:

Atmega328P Tx -> CH340G Tx
Atmega328P Rx -> CH340G Rx

CH340G D+ -> USB connector D+
CH340G D- -> USB connector D-

Is this correct? Even though using a 3.3V 1117 voltage regulator for VCC, wouldn't one need additional resistors on the D+ and D- to drop the USB voltage of 5V to 3.3V on these data lines as well? If so, which value has to be used as it's unclear how much current flows on the data lines? Neither the user supplied 3.3V CH340G schematic nor the datasheet mention that.

A minimal translated datasheet can be found here and some relevant info is on page 4:
http://www.handsontec.com/dataspecs/ch340g.pdf

A schematic for a 3.3V setup found on http://fobit.blogspot.com/2014/11/ch340g-in-eagle.html http://i.imgur.com/vUIm3SF.png

Thank you in advance.

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    \$\begingroup\$ Don't use this chip in an original design. The issues aren't worth saving a dollar, and you are somewhat obviously not ready to design for mass production where that would add up. \$\endgroup\$ – Chris Stratton Jan 8 at 17:16
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I'm surprised you found a datasheet on CH430G. There are no US distributors of the chip, I won't use it in my designs. FT232 has an IO voltage pin where the IO level can be selected.

wouldn't one need additional resistors on the D+ and D- to drop the USB voltage of 5V to 3.3V on these data lines as well?

I'm pretty sure the D+/D- lines are not 0/5V but +/-3.3V similar to RS485. Half duplex communication between PC and the device.

The D+/D- mines often have like a 22 or 33 ohm series resistor for termination reasons (internal on the FT232) and also transzorbs from signal to Gnd to prevent ESD damage.

CH340 chip have built in USB bus pull-up resistors and on-chip signal termination, UD+ and UD- pins should be connected to the USB bus lines directly.

So, no series resistor needed.

2 TXD Output UART Data Transmit output. 3 RXD Input UART Data Receive input.

So 2 goes to uC Rx input, 3 goes to uC Tx output. If your uC is 3.3V powered, then power the chip from 3.3V as well.

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