Custom Board with BC40C (nRF5340) - Bluetooth won’t advertise after flashing Peripheral UART sample

[YeQuit]

Hi everyone,

I'm new to Nordic development and currently working with a custom board based on the BC40C module (using an nRF5340).
I’m facing an issue with the Bluetooth functionality.

Problem:
I can flash the Bluetooth: Peripheral UART sample onto my custom board without any errors.
However, the board does not appear in any Bluetooth scanners, including the nRF Connect app.
Interestingly, I have another custom board using the same BC40C module (an earlier version), and the exact same firmware works fine there (Bluetooth is advertising and visible).

Both boards are very similar in design. The main differences are a slightly smaller PCB layout and an extra LED on the newer board (but that shouldn’t make a big difference or cause the problem).

This makes me wonder if I might be missing some configuration or initialization step on the new board.
Could it be that the older board already has something preconfigured, such as another program running on the network core (that enables Bluetooth) to advertise properly?

What works:

• Flashing works fine on both boards.
• The Blinky sample runs perfectly on both as well.
• I’ve been able to erase the chip using nRF Connect for Desktop, and reflashing works every time, so the programming process itself seems fine.

My setup & DK configuration:
I’m using the nRF5340 DK as a programmer, with SB47 shorted, SB19 open, SW9 set to USB, and SW8 switched ON.
The external board is powered with 3V via connector P19, following the official Nordic guide:
https://docs.nordicsemi.com/bundle/ug_nrf5340_dk/page/UG/dk/ext_programming_support_P19.html
I also verified that Pin 3 (GND detect) on P19 is correctly pulled low, which means the DK detects the external board as expected.

What I’ve tried so far:

• Different build configurations (application core only, network core, with and without "non secure" builds)

• Using different nRF5340 DKs for programming

• Scanning with standard Bluetooth apps and nRF Connect

• Erasing and reflashing the new board multiple times

• Testing with another BC40C-based board — the newer board always shows the same Bluetooth issue

In summary:
Even though flashing and simple applications like Blinky work fine, Bluetooth will not start on my new BC40C-based custom board.
On the older, almost identical board, Bluetooth advertising works without problems using the same firmware.
I’m wondering if I’m missing something critical for Bluetooth initialization on the newer board.

Any ideas what could cause this?
I appreciate any hints or suggestions on what else I should check!

Thanks in advance!

[SwRa]

Hi Lars,

The most likely possibility is that the network core is not programmed with the correct image (typically hci_rpmsg). In that case, bluetooth will not work, even if the application core is running fine. Bluetooth functionality is handled by the network core. You can refer this ticket:  devzone.nordicsemi.com/.../327376

If your custom board does not have a 32.768 kHz crystal, you must configure the firmware to use the internal RC oscillator for the low-frequency clock on both the application and network cores. If this is not set, Bluetooth will not function. Also, if your board does not have DCDC inductors, you should disable DCDC in your configuration: CONFIG_BOARD_ENABLE_DCDC=n

These should help resolve the issue.

Best Regards,

Swathy