nRF5340 Custom board - make it headphone device for mobile phones

[Vivek_Core]

Hi,

I am using a custom nRF5340 board and referring to the Zephyr sample application located at:

ncs\v2.6.0\nrf\applications\nrf5340_audio

My setup and testing environment are as follows:

• Android phone with Bluetooth 5.3 support (LE Audio capable)

• iPhone 16 Pro for additional testing

Steps Performed:

1. Created custom board files for the non-secure (_ns) variant.

2. Disabled certain parts of the code to make it compatible with my custom hardware.

3. Built the application as a Unicast Server for the csb_cpuapp_ns board and successfully flashed it onto the custom board.

4. The device advertises correctly with the name "NRF5340_AUDIO" and pairs successfully with my phone.

Issue:

After pairing, the device is not recognized as a headphone or microphone by the phone.

When making a phone call, the “NRF5340_AUDIO” device does not appear in the audio output/input selection menu.

I have verified this through the RTT logs attached below.

I have also attached the sample code that I am using right now for the custom board and build it for the csb_cpuapp_ns.

*** Booting nRF Connect SDK v3.5.99-ncs1 ***
-- [00:00:00.253,601] <inf> fw_info: 
   nRF5340 Audio nRF5340 Audio DK cpuapp           
   NCS base version: 2.6.0           
   Cmake run : Wed Nov 12 12:40:35 2025
-- [00:00:00.279,815] <inf> bt_mgmt_ctlr_cfg: Controller: SoftDevice: Version 5.4 (0x0d), Revision 8591
-- [00:00:00.279,998] <inf> bt_mgmt: Local identity addr: E6:25:48:D0:53:EB (random)
-- [00:00:00.283,996] <inf> bt_mgmt_adv: Local addr: 70:E2:C2:69:A3:F9 (random)
-- [00:00:00.284,515] <inf> bt_mgmt_adv: Advertising successfully started
-- [00:00:33.948,883] <inf> bt_mgmt: Connected: 45:2E:46:94:28:ED (random)
-- [00:00:33.948,913] <inf> main: Connected
-- [00:00:34.124,176] <wrn> bt_l2cap: Ignoring data for unknown channel ID 0x003a
-- [00:00:38.594,665] <inf> main: Security changed
-- [00:00:38.689,300] <err> bt_content_ctrl_media: Discovery of MCS failed (-61)
-- [00:00:45.884,948] <inf> bt_mgmt: Disconnected: 08:C7:B5:B5:48:46 (public) (reason 0x13)
-- [00:00:45.884,979] <inf> main: Disconnected
-- [00:00:45.886,474] <inf> bt_mgmt_adv: Local addr: 70:E2:C2:69:A3:F9 (random)
-- [00:00:45.886,535] <inf> bt_mgmt_adv: Adv directed to: 08:C7:B5:B5:48:46 (public).
-- [00:00:45.887,115] <inf> bt_mgmt_adv: Advertising successfully started
-- [00:00:47.167,999] <inf> bt_mgmt: Directed adv timed out with no connection, reverting to normal adv
-- [00:00:47.170,623] <inf> bt_mgmt_adv: Local addr: 70:B5:B6:F1:51:71 (random)
-- [00:00:47.171,478] <inf> bt_mgmt_adv: Advertising successfully started
-- [00:00:51.711,425] <inf> bt_mgmt: Connected: 08:C7:B5:B5:48:46 (public)
-- [00:00:51.711,456] <inf> main: Connected
-- [00:00:51.945,037] <inf> main: Security changed
-- [00:00:52.005,096] <err> bt_content_ctrl_media: Discovery of MCS failed (-61)
-- [00:00:52.064,880] <wrn> bt_l2cap: Ignoring data for unknown channel ID 0x003a

Question:

What could be the reason that the "NRF5340_AUDIO" device is not detected as a headphone or microphone on the phone after successful pairing? 

Any help would be much appreciated.

Regards,
Vivek K

nrf5340_audio_csb_board.rar

[Elfving]

Vivek_Core said:

Now, other than Google Pixel, is there any other android phone with which I can test LE Audio?

I am not sure, and I do not have any extensive list here. It is also bound to change quickly, as more and more vendors provide support for it. I just tried asking ChatGPT though, who mentioned Samsung S23, and a Xiamoi 14T.

Vivek_Core said:

FYI: I sent hex file to one of my colleague and he tested on his own Google Pixel 8 pro. I informed me that after pairing completed successfully, device not showing as headphone or microphone. Basically, same issue as I have faced here.

Alright, that is more interesting. You should make sure that he tries the configuration for headsets, and he might also need CONFIG_TRANSPORT_CIS. Another thing is that LE Audio might not be available by default, on the Pixel 7 atleast you had to explicitly enable it in the developer options on the phone.

EDIT: Sorry I seemed to have overlooked that you wanted to follow that old tutorial. That is fine. I assume you are using the buildprog script and all cmd arguments he is using in that one? Note that the script does not just create a hex, so it would be best if you could lend your co-worker the DK you are using as well.

Regards,

Elfving

[Vivek_Core]

I am not using buildprog to build the code. Instead, I am building it directly from VS Code by selecting the custom hardware as the board target—nothing else.

We are both using the same merged_domains.hex file generated in the /build/zephyr/ directory.

Also, CONFIG_TRANSPORT_CIS & CONFIG_STREAM_BIDIRECTIONAL were already enabled during our testing.

Regards,
Vivek K

[Elfving]

I see. And you are sure that it is enabled on the phone? It might be that it is by default, but I see some mention here that one has to go into Developer Options to get it working.

Regards,

Elfving

[Vivek_Core]

Okay. Let me check it in the developer options. I’ll get back to you on this.

Regards,
Vivek K