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To restore or change the USB-to-serial firmware on a supported Arduino Uno or Mega, enter DFU mode by briefly connecting RESET to GND on the USB-serial processor’s six-pin header, then flash the HEX file for your exact board revision. First identify the chip: not every Uno or Mega uses an ATmega8U2. Arduino’s DFU instructions were edited October 9, 2024 and rechecked October 4, 2026.
Check the board and USB-serial chip before flashing
This procedure applies to Arduino Uno Rev3 and earlier, Arduino Mega, Mega 2560 Rev3, and MegaADK Rev3 models that use an ATmega8U2 or ATmega16U2 as the USB-to-serial converter. It does not apply to Uno WiFi Rev2, Uno R4 WiFi, or Uno R4 Minima. See Arduino’s board support guidance for the supported-board scope.
The chip depends on the revision. Arduino’s board pages identify Uno Rev1/Rev2 and Mega 2560 Rev1/Rev2 as ATmega8U2 boards; later revisions use the ATmega16U2. Use the chip that your board actually has when identifying it or running a command. Do not select ATmega8U2 just because it appears in the title.
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Choose the HEX file for the exact model and revision
| Board | Firmware file |
|---|---|
| Mega 2560 Rev3 | Arduino-usbserial-atmega16u2-Mega2560-Rev3.hex |
| MegaADK Rev3 | Arduino-usbserial-atmega16u2-MegaADK-Rev3.hex |
| Uno Rev3 | Arduino-usbserial-atmega16u2-Uno-Rev3.hex |
| Uno Rev1 or Rev2 | Arduino-usbserial-uno.hex |
| Mega | Arduino-usbserial-mega.hex |
Arduino includes these files in the AVR Boards platform under firmwares/atmegaxxu2/arduino-usbserial. In an installed Arduino15 package tree, look under Arduino15/packages/arduino/hardware/avr/<version>. The platform source can also be downloaded and extracted. See Arduino’s USB-to-serial firmware instructions for file and flashing details.
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- RELIABLE USB PROGRAMMING AND CLEAR WIRING: The ATmega16U2 USB interface supports sketch uploads and serial communication, while clearly labeled headers help simplify connections to jumper wires, shields and modules
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Enter DFU mode
- Connect the board to the computer.
- Locate the six-pin group near the USB port, associated with the ATmega USB-to-serial processor.
- Briefly connect the RESET and GND pins. The board should reset into DFU mode.
A jumper wire can make the brief connection; Arduino does not specify a particular wire type or accessory. Consult Arduino’s DFU entry instructions if you need the board-specific pin illustration.
Uno Rev1 only: use the HWB pads as well
Uno Rev1 uses an additional sequence. On the back of the board, locate and hold the two HWB pads shorted; while holding them, briefly connect RESET to GND; release RESET and GND, then release HWB. Do not use this extra-pad sequence as a general instruction for other Uno or Mega revisions.
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- USB Connectivity for Easy Programming: The built-in USB port allows for direct programming and serial communication, enabling a simple connection to your computer for sketch uploading and debugging through the Arduino IDE.
- Compatible with Arduino IDE: Full compatibility with the Arduino IDE ensures easy access to a vast array of libraries, code examples, and community-driven projects, making the Uno a great choice for both beginners and experienced makers.
- Widely Used in Education & Prototyping: The Arduino Uno is a standard in educational environments, widely used for learning and teaching electronics and programming. It's perfect for prototyping, robotics, IoT projects, and more.
Flash on macOS or Linux with dfu-programmer
- Install or download
dfu-programmerthrough the relevant project or package-manager route. - Connect the board and enter DFU mode.
- Check which chip responds. Try
dfu-programmer atmega16u2 readanddfu-programmer atmega8u2 read. Older versions may usedumpinstead ofread. - Use the chip name that responded in the following commands, and substitute the path to the HEX file for your board revision:
dfu-programmer <atmegaxxu2> erasedfu-programmer <atmegaxxu2> flash <firmware path> - Disconnect the board and reconnect it.
The angle-bracketed text is a placeholder: replace <atmegaxxu2> with the responding chip identifier, such as atmega8u2 or atmega16u2, and <firmware path> with the selected HEX file’s path. These are Arduino’s documented command steps, not a guarantee that every computer or board will respond identically.
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- Connect the board and enter DFU mode.
- Open Device Manager and identify whether the device is an ATmega8U2 or ATmega16U2.
- Install FLIP if needed, then open FLIP and select the identified device. Under Cable, choose USB and open the connection.
- Select the device, load the HEX file for the exact board model and revision, and leave Erase, Program, and Verify selected.
- Choose Program Target Device Memory. Arduino says FLIP should display “Programming done” when it completes.
- Disconnect the board and reconnect it.
If the device is missing from libusb-win32-devices, Arduino’s instructions suggest checking Other Devices for an unknown device and installing the libusb-win32 driver from C:Program Files (x86)AtmelFlip 3.4.7. That folder path is specific to the installation described by Arduino and may differ on your PC. If FLIP reports a missing JVM, Arduino also notes that a Java Runtime Environment may be needed. Consult the Arduino FLIP procedure for its Windows setup guidance.
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- CH340C USB-TO-SERIAL INTERFACE: The onboard CH340C handles USB communication for sketch uploads and serial monitoring, while clearly labeled digital, analog and power headers help simplify wiring to modules and shields
- USB OR EXTERNAL POWER: Run the board from the included USB cable or a recommended 7-12 V external DC supply, then expand with compatible shields and modules for robotics, data logging, automation and custom embedded projects
- BOARD AND USB CABLE INCLUDED: Comes with 1 ELEGOO UNO R3 controller board and 1 USB-A to USB-B data cable; breadboard, jumper wires, sensors, shields and power adapter are not included
What USB recognition symptoms can—and cannot—tell you
When USB-to-serial firmware is missing or corrupted on a supported board, the computer may show entries such as “Unknown Device,” “Composite Device,” “ATmega16u2 DFU,” or “Atmega8U2 DFU.” Arduino lists these as possible presentations and recommends restoring the firmware through DFU mode. They are clues, not proof that firmware is the only fault or that flashing will repair unrelated board damage. If the board remains unrecognized after the board-specific firmware procedure, the cited guidance does not establish that this recovery step addresses the underlying cause.
For the symptom list, see Arduino’s USB recognition troubleshooting article.
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Which flashing route should you use?
| Route | Platform documented by Arduino | Interface | Setup consideration |
|---|---|---|---|
| FLIP | Windows | Graphical | May require a libusb-win32 driver; the applicable device must be selected in FLIP. |
| dfu-programmer | macOS and Linux | Command line | Requires the tool and the correct chip identifier before erase and flash commands. |
Both routes require DFU mode, the HEX file for the exact board revision, and disconnecting and reconnecting after flashing. Arduino’s documented Windows-versus-macOS/Linux paths are described in its firmware update article; check current tool availability and operating-system compatibility before installing software.
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