[SOLVED] Crazyflie 2.1 BL – Only M2 LED Blinking, Cannot Connect to Bootloader

[yuanpngn]

Hi everyone,
We accidentally flashed our Crazyflie 2.1 Brushless with the wrong firmware, and now we’re having trouble recovering it.

Current Behavior

• Startup: Only the M2 LED blinks blue, while the lighthouse deck LEDs remain solid.
• Crazyradio PA: Detected on macOS (Vendor ID 0x1915, Product ID 0x7777), shows up in cfclient as radio://0/0/2M.
• Bootloader Tool:
• Program button stays grayed out until clicking Initiate Bootloader (cold boot)
• After clicking, log shows:
• “Resetting the firmware, disconnect”
• Then “Not connected.”
• During this, Crazyradio PA LED blinks green/yellow but stops once “not connected” appears.
• M3 LED: Never blinks, even after releasing the power button.

What We’ve Tried:

• Installed cfclient and cflib in a clean Python venv
• Verified Crazyradio PA is recognized by system_profiler
• Attempted multiple cold-boot recoveries — always fails with the same “not connected” message
• Already contacted someone who experienced the same situation, but following their approach did not resolve the issue

Recording:
Here’s a full recording of what we see in cfclient: https://github.com/user-attachments/assets/0df107f0-6b2f-4df0-aa03-58bce0d00874

Question

• It seems like the STM32 bootloader might be missing since M3 never blinks. Do I need to re-flash the bootloader with a debug probe, or is there another recovery method I should try first?

Can you help me get this Crazyflie flying again?

[ArisMorgens]

Hello @yuanpngn!

Sorry that you're facing issues with the Crazyflie 2.1 Brushless. This behavior seems weird. Could you explain what is the wrong firmware that you flashed? Was it for the Crazyflie 2.x, or something custom?

I would suggest trying to enter the bootloader mode from an unpowered state, meaning that you hold the button, connect the battery and wait until the 2 blue LEDs start blinking. Then, in the cold boot (recovery) tab, press initialize bootloader cold boot and see if you can click Program after selecting the cf21bl firmware.

Let us know if this works or if we need to further investigate the issue.

[yuanpngn]

Hello @ArisMorgens !

Yes, I accidentally flashed the firmware for the Crazyflie 2.1 (cf21) instead of the Crazyflie 2.1 Brushless (cf21bl). I followed your instructions, held the button while connecting the battery, waited for the two blue LEDs to start blinking, and used the cold boot recovery tab to initialize the bootloader and select the cf21bl firmware, but the issue still persists. Only one blue LED keeps blinking, and I still can’t proceed with programming.

Would appreciate any further guidance you can share.

[ataffanel]

Hi,

Only one Blue LED blinking means that the STM32 bootloader has been somehow erased. The procedure to restore it is documented there: https://www.bitcraze.io/documentation/repository/crazyflie-firmware/master/development/dfu/#bootloader-recovery

Can you tell me a bit more about the history of this Crazyflie thoug, has it been programmed using USB DFU and/or an SWD debug probe or have you only ever programmed it using the radio bootloader?

[yuanpngn]

Hi everyone,

I’d like to close the loop on this thread and share that the issue with our Crazyflie 2.1 Brushless has now been resolved.

As mentioned earlier, we had accidentally flashed the wrong firmware (cf21 instead of cf21bl), which caused the radio interface to change and left the boot process stuck with only the M2 LED blinking. With guidance from Arnaud and the Bitcraze team, we carefully followed the updated bootloader recovery documentation (instead of the outdated wiki instructions). Using the DFU recovery steps described here:
👉 https://www.bitcraze.io/documentation/repository/crazyflie-firmware/master/development/dfu/#bootloader-recovery

we were able to successfully re-flash the correct brushless firmware and restore normal behavior.

The Crazyflie is now booting correctly and we’re back to flying again.

A huge thank you to Arnaud, the Bitcraze support team, and everyone who shared advice along the way. Your help was crucial in getting us unstuck, and we really appreciate the time and support you gave.