Unity Booting Issue | How to fix the bootloader with STLINKv2

When do I need this?

First make sure it’s the bootloader which is broken, not the firmware. This is not for a broken firmware, always try to flash the firmware again using the Focbox app first! The main symptom of a broken bootloader is that it does not turn on at all. Often a light goes on, and turns off after a few seconds. The Unity comes with the bootloader pre-flashed, so if the Unity just comes out of the box this is also not required (unless it’s broken out of the box ofc).

What do I need?

  • An STLINKv2 serial debugger. Get the original (or a really good fake), as fakes are sometimes wired wrong.
  • A bunch of female to male jumpers. I used these 25, as I had some laying around. They were slightly too large (they do fit, but it’s for one-time use), so get one size smaller if you ordering for this.
  • A Windows computer.
  • The ST-LINKv2 41 utility.
  • The Bootloader binary 57. This is the bootloader and firmware bundled together. 

How do I do this?

  1. Disconnect the unity from the battery and any other connectors.
  2. Remove the orange cover.
  3. This reveals the unity with a separate PCB on top. The separate PCB is the Bluetooth module.
  4. Remove the Bluetooth module. Take note of the connector the module is plugged into. This is the connector we will be using to flash the bootloader.
  5. Take the jumpers, and connect them as follows to the ST-LINK (phase wires are oriented at the top). Take a look at your ST-LINK for the corresponding connections.


NOTE: You don’t need to plug your unity into your battery. It is powered by the ST-LINK. 

  1. Connect the ST-LINK to your computer.
  2. Open the ST-LINK utility.
  3. Click connect on the top row (Looks like a mains plug with a lightning bolt). This should connect to the unity. It should look somewhat similar to this:
  4. Click on Open file and select the bootloader binary.
  5. Click on Program & Verify on the top row. This should show something like this:
  6. Check the start address. (Unlike the photo) it should show 0x08000000.
  7. Click start!
  8. After this is done, disconnect the ST-LINK inside of the Utility. This is the Mains plug with the red cross on it.
  9. Disconnect the ST-LINK from the computer.
  10. Unplug the Jumpers from the Unity.
  11. Put the Bluetooth module back in its position.
  12. Put the orange cover back.
  13. Connect the phase wires, remote and battery back into the Unity as normal.
  14. Try it out!

Now you should be able to use the Unity like normal.

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