#406 Custom CH340G USB to UART interface

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.

Notes

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.

Pinout:

Notes:

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

Drivers and Setup for MacOSX

• see the offical MacOSX setup notes
• if the Arduino has an FTDI instead of CH340G, if necessary install the FTDIUSBSerialDriver_v2_2_18.dmg
• see the arduino forum - general discussion and updates on the issue of USB drivers

CH340G Drivers

If the CH340G serial chip is not recognised my MacOSX, a driver is required. See:

• How to use cheap Chinese Arduinos that come with with CH340G / CH341G Serial/USB chip - a great guide.
• Updated driver (Sierra-compatible) is available at github.com/adrianmihalko/ch340g-ch34g-ch34x-mac-os-x-driver
• There also seems to be a professional drive kit available (at cost) from https://www.mac-usb-serial.com/

I’ve “cached” copies of the drivers I’ve used in the past in the drivers folder:

• CH341SER_MAC.ZIP
• CH34x_Install_V1.3.zip - latest version I’ve used

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).

A Protoboard CH340G Demonstration

A final prototype before doing anything more serious - I’ve switched to SMD components and built up for testing on some protoboard with DIP adapters. A few changes to the design:

• added a 500mA resetable polyfuse on the USB supply
• connected both DTR and CTS lines to the 6-pin device adapter
• bumped up the LED current-limiting resistors to 2.7kΩ

The device pinput sequence I’ve selected is as follows. This suits some devices, but may need wires crossed for others.

Here’s a sketch of the layout I used:

Credits and References

• CH340G datasheet
• FTDI FT232RL
• 2TY S8550 PNP transistor datasheet
• 662k voltage regulator datasheet
• RS-232 - wikipedia
• Serial port - wikipedia
• ..as mentioned on my blog