Issue
Some recent versions of Linux may be missing the firmware files required by the BTD-400. When the BTD-400 is connected to the system, the error below may appear:
Bluetooth: hci0: BCM: Patch brcm/BCM20702A1-0a5c-21e8.hcd not found
History
The chip used in the BTD-400 requires firmware to operate, but has limited permanent storage space. The firmware is therefore loaded from the host PC when the BTD-400 is connected to the system. Prior to 2020, the required firmware was included in almost all desktop Linux distributions. Unfortunately, the firmware is not open-source, and the decision was made to remove it from distributions that require all included code to be open-source.
Solution
The firmware is available at the link below:
Extract the .hcd file contained in the ZIP, then copy the hcd to:
/lib/firmware/brcm
Next, power off the computer, plug in the BTD-400, then power the computer back on. The BTD-400 should now operate as expected.