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If Arduino UNO (Rev3 or earlier) or Mega is not recognized by the computer

Missing or corrupted USB-to-Serial firmware can cause UNO (Rev1 and Rev3) and Mega boards to be recognized as any of the following:

  • Unknown Device
  • Composite Device
  • ATmega16u2 DFU
  • Atmega8U2 DFU

Resolve the issue by flashing the USB-to-Serial firmware in DFU (Device Firmware Update) mode.

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