Unable to debug Fanstel BM840 nRF52840 custom board using nRF52840 DK

[Ken58]

I am unable to debug a Fanstel BM840 nRF52840 based simple custom board using a nRF52840 DK. I believe the custom board is basically ok: Before flashing my application onto it, powering it would run a Fanstel BT test program visible over the air. The custom board is a Fanstel BM840 with connectors and nothing else.

Development context:

• Using nRF Connect for VS Code, version 2025.5.152
• nRF Connect SDK 3.0.1
• nRF Connect Toolchain 3.0.1
• nRF52840 DK is connected to the Custom Board through the P20 connector as follows:
  • P20.VDDnRF' to Custom Board VDD
  • P20.VTG to Custom Board VDD
  • P20.SWDIO to Custom Board SWDIO
  • P20.SWDCLK to Custom Board SWCLK
  • P20.RESET to Custom Board RST
  • P20.GND_DETECT to Custom Board GND
  • P4.GND to Custom Board GND
• Custom Board support defined through nRF Connect for VS Code following guidelines discussed in the Nordic video "Adding Custom Board Support in nRF Connect SDK
• Flashing the Custom Board seems successful:

-- runners.nrfjprog: Flashing file: C:\nordic\SlotCars\Car\build_4\merged.hex
[ #################### ] 5.058s | Erase file - Done erasing
[ #################### ] 2.100s | Program file - Done programming
[ #################### ] 2.061s | Verify file - Done verifying
Applying system reset.
Run.
-- runners.nrfjprog: Board with serial number 683268243 flashed successfully.

The DEBUG CONSOLE shows the following:

JLinkGDBServerCL: SEGGER J-Link GDB Server V8.18 Command Line Version
JLinkGDBServerCL:
JLinkGDBServerCL: JLinkARM.dll V8.18 (DLL compiled Mar 5 2025 14:42:43)
JLinkGDBServerCL:
JLinkGDBServerCL: -----GDB Server start settings-----
JLinkGDBServerCL: GDBInit file: none
JLinkGDBServerCL: GDB Server Listening port: 51896
JLinkGDBServerCL: SWO raw output listening port: 2332
JLinkGDBServerCL: Terminal I/O port: 2333
JLinkGDBServerCL: Accept remote connection: localhost only
JLinkGDBServerCL: Generate logfile: off
JLinkGDBServerCL: Verify download: off
JLinkGDBServerCL: Init regs on start: off
JLinkGDBServerCL: Silent mode: on
JLinkGDBServerCL: Single run mode: on
JLinkGDBServerCL: Target connection timeout: 0 ms
JLinkGDBServerCL: ------J-Link related settings------
JLinkGDBServerCL: J-Link Host interface: USB
JLinkGDBServerCL: J-Link script: none
JLinkGDBServerCL: J-Link settings file: none
JLinkGDBServerCL: ------Target related settings------
JLinkGDBServerCL: Target device: nRF52840_xxAA
JLinkGDBServerCL: Target device parameters: none
JLinkGDBServerCL: Target interface: SWD
JLinkGDBServerCL: Target interface speed: 4000kHz
JLinkGDBServerCL: Target endian: little
JLinkGDBServerCL:
=thread-group-added,id="i1"
=cmd-param-changed,param="pagination",value="off"
0x0000a7de in sym_WVJLOWUIPBC2HDDMNEWOD4POANUBUKS6OYFXDSA ()

That is all that happens.  The debugger never starts up on main().  No register contents are visible.

Any idea of what to do next would be greatly appreciated.

Thank you,

Ken

[Vidar Berg]

Hello Ken,

Does your custom board include the optional 32KHz crystal. If not, and if you are using the nrf52840dk as your build target, please add CONFIG_CLOCK_CONTROL_NRF_K32SRC_RC=y to your project configuration (prj.conf) file to select the internal RC oscillator as the clock source for the system timer. 

If the crystal oscillator is selected but no crystal is mounted on the board, the startup code may enter an infinite loop while waiting for the crystal to become ready. However, this does not explain why you are unable to read the register contents. 

Best regards,

Vidar

[Ken58]

Hi Vidar,

Thank you for your response.  My short reply is that adding CONFIG_CLOCK_CONTROL_NRF_K32SRC_RC=y to my prj.conf file solved the problem.  However, I want to understand why.  I have a question emailed out to Fanstel to find out exactly what their BM840 module includes external to the nRF52840.  This was not clear to me from the BM840 data sheet.  After I have their response, I will have additional questions for you.

Ken

[Vidar Berg]

Hi Ken,

Good to hear that it solved the problem - thank you for the update. Most third party modules include the optional DCDC inductors needed to enable the internal DCDC regulator(s), but not the 32K crystal. Including the crystal may be a trade off between having a smaller footprint or lower current consumption. The crystal oscillator provides slightly lower power consumption, partly because it enables the BLE stack to account for lower clock drift when setting the size of its RX windows, but also because it does not need to be periodically calibrated against the HF crystal oscillator. 

You can compare the difference between using the RC oscillator instead of the crystal oscillator by using our online power calculator at  Online Power Profiler for Bluetooth LE  

[Ken58]

Hi Vidar,

Although I haven't heard back from Fanstel, I bet their BM840 module does not have the 32K crystal but does have the DCDC inductors.  A couple questions:

1. How should I have known about CONFIG_CLOCK_CONTROL_NRF_K32SRC_RC=y?  Can you provide me with a link to your documentation I should have read?
2. I am building my application to run on nRF52840 DK, nRF52840 Dongle, and my BM840 based custom board.  What is the proper way to handle CONFIG_CLOCK_CONTROL_NRF_K32SRC_RC=y being used in only 2 of the 3 situations?

Thanks a bunch Vidar,

Ken

[Vidar Berg]

Hi Ken,

Documentation is a bit lacking in this area. I have created a ticket internally to address that. The nrf specific configuration settings for the clock control module are defined here:  https://github.com/zephyrproject-rtos/zephyr/blob/6d7744209b1e91836e463d393352014068e07219/drivers/clock_control/Kconfig.nrf#L33. The problem is that you kind of have to know about it to find it. 

Ken58 said:

What is the proper way to handle CONFIG_CLOCK_CONTROL_NRF_K32SRC_RC=y being used in only 2 of the 3 situations?

Ideally you should define your own SDK board which selects this symbol by default like this third party board does here: https://github.com/zephyrproject-rtos/zephyr/blob/6d7744209b1e91836e463d393352014068e07219/boards/we/proteus2ev/we_proteus2ev_nrf52832_defconfig#L24 This ensures that any project built for this board target will select the RC osc. as the LF clock source.

Vidar