UNO (except WiFi Rev2) and Mega boards use either an Atmega16U2 or Atmega8U2 chip as a USB-to-serial converter. By setting a board to DFU mode you can upload USB-to-Serial firmware to the chip.

1. Connect the board to your computer

2. Find the RESET and GND pins for the ATmega USB-Serial Processor. They are the innermost two pins of the six located close to the USB port.

   

3. Briefly short the pins (follow these instructions for UNO Rev1)

The board will reset to DFU mode.

Short the HWB line (Rev1 models)

If you’re using an Rev1 model, you also need to assert and hold the HWB line:

1. Find the two HWB pads on the back of the board.

   

2. Assert the line by shorting the two pads, e.g. with a piece of wire.

3. Short the RESET + GND pins.

4. Release the RESET + GND pins.

5. Release the HWB line.

The board will reset to DFU mode.

Further reading

• Flash USB-to-Serial firmware in DFU mode.