First of all, THANK YOU!

Community testing is immensely valuable to us and your help testing Mu is greatly appreciated!

What needs testing

There are two main things we would love to get some help testing:

  1. The Mu micro:bit flashing
  2. The MicroPython runtime for micro:bit 

Testing Mu micro:bit flashing

This is the most important feature you can test!

Older versions of Mu copied a large file into the micro:bit, containing the MicroPython interpreter (large) and your code (smaller in comparison). The new method first checks if MicroPython is already in your micro:bit and if it is, then it only sends your code over.

This new method significantly decreases the flashing time, so now for most users it should only take a few seconds!

However, because the implementation is also significantly different we need some help testing it to make sure we catch any possible bugs before the final version 1.0.0 of Mu.

Testing MicroPython

If you still have some time to test, there is also a newer version of MicroPython packed within Mu.

This version contains a lot of bug fixes, so if it would be fantastic if you could also test that all your programs work just as well as they did before.
At the moment there are no known bugs in this version of MicroPython, so there should be little risk trying it.

Downloading Mu

The version we need help testing is v1.0.0.rc.1 and can be downloaded directly from the Mu website:

There are installers for Windows (32 and 64 bits) and macOS.

Installing Mu

There are different installation instructions per platform:

micro:bit Drivers

In most environments all of the features of the micro:bit should just work without having to install additional drivers.

However, there might be some situations where a serial driver might be needed on on some Window computers to connect to the micro:bit via serial port (for example, to access the REPL).

Please do note that Mu will still work and flash the micro:bit correctly without this driver, but it is very useful to have it installed on these cases as it enables the quicker Flashing and REPL features.

To check if your Windows computer needs to install the driver follow these steps:

  • Download Mu
  • Install Mu
  • Open Mu
  • If Mu asks to select mode, select the micro:bit mode
  • Connect the micro:bit to the computer via USB
  • Wait for the micro:bit to appear as a USB drive in your computer
  • On Mu, click on the REPL button
  • If a panel on the bottom of the window appears (similar to the image below), then it works! No need to install any additional drivers:
  • Otherwise if an error window appears letting you know Mu couldn't connect to the REPL, then you might need to instal the Mbed driver.
    • Remember! The serial driver is no necessary, Mu should still flash your micro:bit successfully, but it works much better with the driver :)

To install the drive you can download it from this URL:

More info can be found at the Mbed website:

Using Mu with the BBC micro:bit

The fantastic Mu website has this great tutorial covering how to use Mu with the micro:bit:

It's highly recommended to check out the Code With Mu website, it contains a lot of useful information!

What do I do if something goes wrong?

This is the important part, if something unexpected happens please let us know!

When reporting a bug, please remember to include the following information:

  • What you were trying to do
  • What steps you took to make this happen
  • What you expected to happen
  • What actually happened
  • Why this difference is problematic (it may not be a bug!)
  • Technical details like the version of Mu you're using, your OS version and other aspects of the context in which Mu was running.
  • Please include a full copy of the Mu log
    • You can get the logs by clicking on the cog icon in the bottom right of the editor window.
    • Click on the logs and use CTRL-A to select all, then CTRL-C to copy and CTRL-V
      to paste the contents into the issue.
  • More info on how to report a bug to Mu:

Then you can follow the "Reset Mu and/or micro:bit to a known state" instructions to continue working with Mu and micro:bit. 

Reset Mu and/or micro:bit to a known state

If something goes wrong, you can follow these steps to reset Mu and the micro:bit to a known state where the micro:bit flashing should work again: 

  • Close Mu
  • Open Mu
  • Select micro:bit mode in Mu
  • Create a new blank script
  • Click Flash
    • This will copy only the MicroPython interpreter into your micro:bit, erasing anything that was there before
  • Open your code again
  • Clicking Flash should work in this state

If you Python code does not work on the micro:bit you can try an older version of MicroPython:

  • Copy your code
  • Go to
  • Paste your code into the online editor
  • Click download
  • Copy the downloaded hex file into your micro:bit
  • This hex file runs your code with MicroPython v0.9 and you can check if there are any differences compared with the version of MicroPython included in Mu

If your code works from the online editor, but it doesn't from Mu, please open an issue on the MicroPython repository ( or send us a support ticket via the method described in the "What do I do if something goes wrong" section above.