Unable to debug Fanstel BM840 nRF52840 custom board using nRF52840 DK

RE: Unable to debug Fanstel BM840 nRF52840 custom board using nRF52840 DK

Ken58 — Mon, 16 Jun 2025 18:12:52 GMT

Thanks again Vidar! I could not ask for better support. Great job!

Ken

RE: Unable to debug Fanstel BM840 nRF52840 custom board using nRF52840 DK

Vidar Berg — Wed, 11 Jun 2025 09:39:56 GMT

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.

[quote userid="74045" url="~/f/nordic-q-a/121964/unable-to-debug-fanstel-bm840-nrf52840-custom-board-using-nrf52840-dk/538517"] What is the proper way to handle CONFIG_CLOCK_CONTROL_NRF_K32SRC_RC=y being used in only 2 of the 3 situations?[/quote]

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

RE: Unable to debug Fanstel BM840 nRF52840 custom board using nRF52840 DK

Ken58 — Sat, 07 Jun 2025 16:59:49 GMT

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:

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?
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

RE: Unable to debug Fanstel BM840 nRF52840 custom board using nRF52840 DK

Vidar Berg — Fri, 06 Jun 2025 06:22:27 GMT

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

RE: Unable to debug Fanstel BM840 nRF52840 custom board using nRF52840 DK

Ken58 — Thu, 05 Jun 2025 22:01:39 GMT

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

RE: Unable to debug Fanstel BM840 nRF52840 custom board using nRF52840 DK

Vidar Berg — Tue, 03 Jun 2025 12:30:34 GMT

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