SparkFun Pro Micro (Qwiic, USB-C, 5V/16MHz) USB Enumeration Failure

Development Boards Arduino
1

I’m working with a SparkFun Pro Micro (Qwiic, USB-C, 5V/16MHz, ATmega32U4) and ran into a full USB enumeration failure.

:wrench: What I’m Trying to Do

I’m building a USB HID device using the Pro Micro. However, the board never enumerated over USB, even when first plugged in. There was no COM port, no blinking, and no sign of USB activity. I tried double-reset, etc. I primarily used a Windows 11 machine but also checked my MacBook Pro without success.

Assuming the bootloader or fuses were corrupted, I reflashed the bootloader externally using a Raspberry Pi as an ISP programmer.

:hammer_and_wrench: Steps I Took

  • Wired Raspberry Pi as an ISP programmer using avrdude and linuxspi with GPIO for RESET.
  • Successfully flashed two bootloaders for testing:
    • Caterina-promicro16.hex
    • Caterina-Leonardo.hex (from ArduinoCore-avr)
  • Successfully flashed fuses recommended for 5V/16MHz operation:
    • lfuse = 0xFF
    • hfuse = 0xD8
    • efuse = 0xCB
    • lock = 0x2F
  • Used slower SPI clock (-B 10) to ensure reliable flashing.
  • Verified that both bootloader flashes completed without errors (avrdude confirmed verification).
  • Swapped multiple USB cables:
    • Tried several USB-C to USB-C cables (including ones that work with other devices)
    • Then switched to known good USB-C to USB-A data cables
  • Tried multiple computers and USB ports (USB 3 and USB 2).
  • Carefully monitored Windows Device Manager during plug/unplug and reset cycles:
    • No new COM port.
    • No “Unknown Device.”
    • No temporary bootloader port even after double-tapping RESET.

:red_question_mark: The Problem

Despite successful bootloader and fuse flashing, the board does not enumerate at all over USB.
There is no COM port, no USB device detected, and no evidence of USB negotiation.

The board powers up (5V verified), and ISP programming works perfectly, but USB is completely silent.

:magnifying_glass_tilted_left: What I’ve Concluded

I suspect a hardware fault — possibly:

  • Damage or fault on the USB D+/D− lines
  • A defective USB-C connector
  • Internal ATmega32U4 USB PHY failure (unlikely, but possible)

Because ISP programming and memory verification work 100% reliably, the ATmega32U4 core appears functional.

:toolbox: Reference Details

  • Product: SparkFun Pro Micro (Qwiic, USB-C, 5V/16MHz)
  • SKU: DEV-15795
  • Purchased from: Amazon (Sold by SparkFun)
  • Tested on Windows 11 and Raspberry Pi OS
  • Arduino IDE used for uploading sketches normally

:hammer_and_wrench: What I’d Like to Know

  • Is this a known issue with this board batch or model?
  • Is there anything else I should try to confirm hardware failure?
  • Would this qualify for an RMA or warranty replacement?

Thank you very much for your time and help —
Happy to provide additional photos, wiring diagrams, or flash logs if needed.

Best regards,
Teslamax

2

It sounds like a defective device; was it purchased from us? If so head over to www.sparkfun.com/returns (contact vendor if purchased elsewhere) and we’ll get ya squared away

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It was purchased through Amazon.

SparkFun Qwiic Pro Micro - USB-C… Amazon.com: SparkFun Qwiic Pro Micro - USB-C (ATmega32U4) - Compatible with Arduino Development Board 5V/16MHz microcontroller AP2112 3.3V Voltage Regulator Castellated PTH pin Pads Reset Button Maximum 6V Input : Electronics

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I bought it through Amazon, but the return period has closed.