I recently created a pcb with the atmega32u4 to use it as a development board. The controller is connected to a micro usb port. Should I install the arduino bootloader instead of the deafult one? Or keep the default boot loader and find another way of flashing hex files? But bottom line what is the process to flash a hex file to the atmega32u4 via usb?

  • do you want to upload Arduino sketches from Arduino IDE? why do you think there is some bootloader on the 32u4?
    – Juraj
    Jan 3, 2021 at 6:29
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    Pull /HWB high and reset the chip - you're now in DFU mode. Use a DFU programmer to upload your code over USB.
    – Majenko
    Jan 3, 2021 at 11:04
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    @Majenko As I read it you pull HWBE LOW during RESET not hight. See page 341 @ ww1.microchip.com/downloads/en/DeviceDoc/…
    – jwh20
    Jan 3, 2021 at 12:51
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    Maybe. I've never done it.
    – Majenko
    Jan 3, 2021 at 13:12
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    There are four ordering codes for the 32U4. Three of them come with the DFU bootloader on it; one doesn't. They also come with and without external oscillator fusing which may be relevant as well depending on how the board is designed and which programming method is used. You should probably mention which ordering code you have and maybe more about the board.
    – timemage
    Jan 3, 2021 at 16:19

1 Answer 1


Installing the Arduino bootloader would make it easier to upload you sketch from the Arduino IDE.

Installing the Arduino bootloader via DFU is probably the easiest method. Alternatively use ISP, but you'd need a programmer for that, or use a separate Arduino board with the ArduinoAsISP sketch running on it.

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