This projects allows flashing of Arduino UNO R3 via USB Mass Storage (i.e., file copy or drag and drop without needed to install any software).

It implements it in firmware of the ATmega16u2 - the USB interface chip on the Arduino UNO. This firmware in turn talks to the optiboot bootloader running on the main ATmega328p chip.

The mass storage interface accepts files in UF2 format. The new format (i.e., not .hex and not .bin) is necessary to implement a reliable mass storage flashing in the 512 bytes of RAM of the ATmega. The UF2 repository contains some conversion tools, and recent PXT versions have pxt hex2uf2 command.

There's now a blog post up about how it works and how it came about.

$ git clone https://github.com/abcminiuser/lufa lufa
$ cd lufa/Projects
$ git clone https://github.com/mmoskal/uf2-uno
$ cd uf2-uno
$ make

I'm using LUFA 170418 right now. You need to have avr-gcc etc in PATH. I'm using 4.9.

Check out binary releases.

• place the ATmega in DFU update mode by shorting two pins sticking out closest to the USB plug
• under macOS or Linux install dfu-programer and run make burn
• under Windows use Atmel FLIP

Arduino provides more detailed instructions.

Note: this bootloader is not currently compatible with Arduino IDE. If you want that, go back to the original bootloader (also following the instructions from the Arduino page above).

The firmware exposes a custom raw HID interface, implementing a small subset of HF2 protocol, in particular the serial-forwarding parts. The serial can be accessed using the C-based uf2tool or using PXT command line.

On the Arduino side you have to use Serial.init(115200); in setup(), and not any other baud rate.

MIT

This code is based on the Mass Storage example from LUFA, has a few lines and the RingBuffer from the Arduino USBSERIAL firmware, and lifts some code from the SAMD21 UF2 Bootloader. All these are MIT licensed.