MKRZero Connection Errors, the RESOLVED thread techniques aren't working.

I've just started using a MKRZERO for some SPI communications with an A/D for a mixed signal project. The board was communicating and working yesterday and stopped this morning.

I'm running Ardunino 1.8.5
I've tried the 1.6.17 and 1.16.18 board libraries.

If I double click the reset button I get the blinking LED indicating bootloader is active...but I cannot for the life of me get the bootloader reinstalled using the directions on the MKR page including changing the programmer to Atmel EDBG

I've uninstalled/reinstalled the board libraries and IDE. And I've tried the following

I also had the same problem, I have now cured the problem as follows.

  1. Loaded Arduino software onto another PC and burnt an empty sketch to the MKRZero.

  2. Uninstalled SAMD board module from Arduino library manager.

  3. Downloaded USBDeview from web and used to physically remove driver from windows directory.

  4. Reinstalled SAMD board module.

The board has worked correctly since. I hope this helps someone, It definitely seems to be a driver compatibility or corruption problem.


Each time I do this the port number goes up one and it looks like the board took the program..but it won't start running and the USB connection is lost again. This morning I was on Com 8...I'm up to 13 now.

Any other suggestions?


The COM jumping is a known issue but nobody seems to want to take that one up.

I know there was a firmware upgrade for the MKR1000 but am unsure if it is the same as the MKR zero.

You should not need to change the bootloader unless you are doing something quite special with the board ?

If you mean you get it into BOOTLOADER mode (quickly double tap reset) then don't worry as that is more of a fallback for when sketches cause unwanted side effects.
Going into bootloader mode is a method to gain control back of the board.
Usually at that point I recommend a simple sketch such as "blink" or "bareminimum" be uploaded to over write the errant one.

If you are operating under windows then you may at some point find that the COM issues causes some strange effects at around COM 30 to COM 40 and at that point you would need to clear the COM port stack in windows.
For that simply google "how to remove unused com ports" it is a painless process but on rare occasions needed and more so if you have and use a large number of different boards.

So, I've tried that...I can upload the new sketch (blink or a simple Serial Test) and sometimes it's fine until I try and load the SPI based sketch I'm working on. Other times I get the errors on the simple sketch.

For reference...the exact same SPI sketch worked until this started...and still works on a Due board.