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Burning ItsyBitsy 32u4 Bootloader

I bought 22x Adafruit ItsyBitsy 32u4 5v boards this week. Two of them didn’t identify when plugged in, instead registering as an error in Device Manager (I had a libusb-win32 devices device called ATm32U4DFU).

I thought maybe the bootloader was the issue. Here’s how to burn it using an Arduino Uno as an intermediary.

How to burn the bootloader


How to burn

  1. Upload the Arduino as ISP sketch from the Examples > ArduinoISP folder in the IDE.

  2. Wire Arduino Uno to the ItsyBitsy as follows:

    Arduino -> ItsyBitsy
    5V      -> 5V
    GND     -> GND
    10      -> RST
    11      -> MOSI
    12      -> MISO
    13      -> SCK
  3. Download the Caterina bootloader for ItsyBitsy 32u4 5v from Adafruit website

  4. On the command line, set the fuses for programming with:

    avrdude -c avrisp -P COM9 -b 19200 -p m32u4 -U lfuse:r:low_fuse_val.hex:h -U hfuse:r:high_fuse_val.hex:h
  5. Then burn the bootloader!

    avrdude -c avrisp -P COM9  -b 19200 -p m32u4 -U flash:w:Caterina_itsybitsy5V.hex

In theory, its also possible to use the Arduino IDE to do this, but I didn’t have much success (it worked once, then gave me repeated errors).

Adafruit ItsyBitsy 32u4 plugged into the Uno