Updated Guide to easely rebuild the bootloader for MKR Boards with a second MKR (WITHOUT SD Card)

If you shot the bootlader on a MKR Board, here's the remedy:

  1. First of all, check if the bootloader is realy dead: Double tap the reset fast (Like a double click on a mouse). If the led dimms on and off, everything is fine, upload your sketch.
    If there is no reaction to double tap, continue:

  2. You need a second MKR board to heal the bricked up. This can be another type of MKR, you can heal a MKR 1500 with a MKR 1400 for example.

  3. Solder 5 cables with pinheads to the bricked up MKR :


  1. Connect the soldered cables to the working MKR
    SWDCLCK connected to pin 2
    SWDIO connected to pin 1
    SWDRESET to pin 0
  • 3.3V to VCC
    GND to GND
  1. Double check the wiring, the solderings, check that the MKR can not short circuit etc.

  2. Connect the working MKR with USB to your computer (Only the working MKR).

  3. Download and install the Adafruit DAP library

  4. Load from Examples-> Adafruit DAP library ->SAMD21->flash_MKR_bootloaders

  5. Upload to your MKR

  6. Check that you set 115200 baud an No Line Ending in the serial monitor

  7. You should see a dialog in the serial monitor to choose your Board. If not -> reset.

  8. Type the number of your Board in the serial monitor input field, press Enter.

  9. You see the update process. After finish, unplug the working MKR, plug the hopefully healed to USB power and double tap the reset button. The yellow LED should begin to pulse now. Unsolder the cables and you are done!

(Based on perts instructions and his image)

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