Enabling debug mode on a Bluetooth controller

This resource relies on unofficial or undocumented functionalities. There is no known maintenance of these tools so it is not possible to ensure that they may work correctly in all scenarios.

Some manufacturers seem to include undocumented debug or diagnostic modes in their devices.

These diagnostic modes are not standardised but enable functionalities not available in common modes of operation. Examples of such functionalities are the ability to display all sent and received Link Layer packets.

Some known procedures for enabling these debug modes are listed below.


Supported Devices
Cypress CYW20735B1
Cypress CYW20819A

This procedure is only documented on Linux. All the devices listed above expose a UART interface when connected to a machine. To be recognised as Bluetooth devices, it is necessary to run the btattach process to flag them as bluetooth capable. This may be done using the Bluetooth Attach Service tools.

When running the btattach process, some devices expose a special interface via debugfs. To enable diagnostic mode, write a 1 to the vendor_diag file for your device.

echo 1 > /sys/kernel/debug/bluetooth/hciX/vendor_diag

If our board does not expose the vendor_diag file, there is the possibility to enable diagnostic mode via a manufacturer-specific HCI command. This can be done with hcitool:

hcitool cmd 0x3f 0xf0 0x01