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#406 SerialInterface/UsbUartCH340G

Build a USB to TTL serial interface on a breadboard using the CH340G interface chip, and use it to program Arduino sketches on an ATmega328P.


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The CH340G is a USB to UART Interface chip. It is often used as a cheap alternative to more established brands and products such as the FTDI FT232RL. The CH340G is commonly found in:

  • Arduino Nano clones
  • USB to Serial adapters

In this project, I'd like to see how easy it is to build a USB to Serial interface on a breadboard. To test it out, I'll use it program Arduino sketches on an ATmega328P.

CH340G Key Specs

CH340 is produced by WCH 江苏沁恒股份有限公司 (Jiangsu Yuheng Co., Ltd.). It is a series of USB adapters - with variants providing serial, parallel or IrDA interfaces. The CH340G supports serial with common flow control signals.

Key features:

  • USB 2.0
  • Supports common flow control signals RTS, DTR, DCD, RI, DSR and CTS.
  • Supports RS232, RS422 and RS485 with external level shifting components.
  • Baud rate range from 50bps to 2Mbps.
  • 5V and 3.3V operation.


Pin Name Description
1 GND Ground. Connect to the ground pin of USB bus
2 TXD UART Data Transmit output
3 RXD UART Data Receive input
4 V3 Internal 3.3V reference for USB physical layer
5 UD+ USB D+ signal
6 UD- USB D- signal
7 XI Crystal oscillator input
8 XO Crystal oscillator output
9 CTS# UART flow control: Clear to Send
10 DSR# UART flow control: Data Set Ready
11 RI# UART flow control: Ring In
12 DCD# UART flow control: Data Carrier Detect
13 DTR# UART flow control: Data Terminal Ready
14 RTS# UART flow control: Request to Send
15 R232 Auxiliary RS232 enable. Active high, internal pull down
16 VCC Supply rail for the chip


  • V3: Decouple with a 4.7-20nF capacitor when in 5V operation, or tie to VCC when in 3.3V operation.

Chips and DIPs

I have some CH340G chips in SOP-16 package (from a seller on aliexpress). I've mounted one on a DIP adapter, and found a micro-USB breakout:


Examining a Commercial CH340 USB-Serial Adapter

I have a CH340G USB adapter that I got along with an Arduino mini, so I took a closer look to see if it usde the CH340G chip in any unexpected ways.

Tracing the circuit, it turns out to be very straight-forward. A few things to note:

  • the 3.3V/5V selector actually just enables/disables a 662K 3.3V regulator to supply the outbound pwer rail. It does not switch the operating voltage of the CH340G itself (or the data levels)
  • includes a power indicator LED, and a transmit indicator LED
  • has 1kΩ resistors inline the RX/TX lines; not particularly sure why (not mentioned in the CH340G data sheet)
  • lots of ceramic bypass capacitors. I'm not sure of the actual capacitance values.



Building the CH340G Interface on a Breadboard

On one breadboard, I have a basic 5V CH340G setup:

  • RX and TX LEDs; Power LED
  • DTR, TXD and RXD connected on the serial side

On another breadboard, I have an ATmega328 (with Arduino bootloader) wired up for programming. See LEAP#405 ATmegaSerialProgrammer for more on programming over serial.




Here's a quick demo (uploading a sketch). I'm programming the ATmega328 with the UsbUartCH340G.ino sketch - a simple blink variant that also sends bursts of serial output (to exercise the RX LED).


Credits and References