The USB Serial Adapter plugs into a USB port on your computer and provides you with a hardware serial port at the other end. Compatible with both 3.3V and 5V logic, and pin-compatible with the popular FTDI cable header.

Board Features

Micro USB socket. Connect to your computer using the supplied micro USB cable.

I/O breakouts. Access to the MCU I/O pins, accessible by replacing the firmware in the MCU.

ATmega16u2 MCU. Microcontroller that converts between the virtual USB serial port and the hardware serial port.

ICSP header. Load new firmware into the MCU to use the USB Serial Adapter as a general-purpose microcontroller dev board.

RX LED. Illuminates when receiving data on the serial port.

TX LED. Illuminates when transmitting data on the serial port.

Power LED. Illuminates when power is provided to the serial port.

6-pin serial header. Hardware serial port in FTDI-cable compatible format. The ends labelled "BLACK" and "GREEN" correspond to the coloured wires in a typical FTDI cable to indicate orientation.

DFU jumper. Shorting this jumper puts the MCU into DFU (Device Firmware Update) mode.

Output voltage switch. Selects 3.3V or 5V output to the device connected to the serial port.

Driver Installation

The USB Serial Adapter is designed to appear as a virtual serial port to your computer, in the exact same way as a typical Arduino board.

For Linux and MacOS X, no drivers are needed. The USB Serial Adapter will be detected and supported automatically.

For Windows, you must install a ".INF" configuration file just as you would for an Arduino board. If you have previously installed the Freetronics .INF file when using an Eleven, EtherTen, or any of our other boards, you probably don't need to do anything and the USB Serial Adapter will be detected and supported automatically when you first plug it in. If you have never used a Freetronics board before, please follow the steps shown here for installation of the USB driver. This only needs to be done once.

Connecting The Hardware

1. Connect the USB Serial Adapter to your computer using the supplied micro USB cable.

2. Check that the output voltage switch is in the correct position for your device. The selected voltage will be supplied to your device through the 6-pin header on the pin labelled "VOUT".

3. Connect the USB Serial Adapter to your project using the built-in 6-pin header.


The USB Serial Adapter will appear to your computer as a virtual serial port. You should see it listed as an available port in your software shortly after you plug it in. It automatically handles different serial speeds transparently, so select the serial port speed in your software that your end device (connected to the 6-pin header) expects to see.

Replacing The Firmware

The USB Serial Adapter comes pre-loaded with the same firmware installed on Arduino boards into the ATmega16u2 used to provide the USB interface. In fact the USB Serial Adapter can be considered to be equivalent to that part of the Arduino, with the rest removed.

However, you can also use the USB Serial Adapter as a tiny AVR development board, installing whatever firmware you like into the ATmega16u2 MCU. The ICSP header is compatible with the USBasp ICSP Programmer, so you can reflash the board with your own project and take advantage of the USB interface and I/O breakouts for your own purposes.