https://devzone.nordicsemi.com/f/nordic-q-a/76134/what-is-needed-in-ncs-zephyr-to-make-nrf52840-module-fanstel-bc840-to-use-rc-instead-of-external-crystalHi, We&#39;ve been developing our prototype with nRF52840DK. Our Zephyr RTOS based code is working fine there with BLE connections and sensors. Lately we&#39;ve been testing our code on our own PCB designs and running into problems with software booting and freezingen-USTelligent Community 13Tue, 08 Jun 2021 16:23:18 GMThttps://devzone.nordicsemi.com/thread/314276?ContentTypeID=1Tue, 08 Jun 2021 16:23:18 GMT137ad170-7792-4731-bb38-c0d22fbe4515:7e0cb16b-932b-4365-af17-a479311fa115HarriR<p>Hi,<br /><br />Thank you, I did that as well as what is described <a href="https://devzone.nordicsemi.com/nordic/nordic-blog/b/blog/posts/nrf5x-support-within-the-zephyr-project-rtos">here</a>.</p> <div> <div><span></span></div> <pre class="ui-code" data-mode="text">CONFIG_CLOCK_CONTROL_NRF_K32SRC_RC=y CONFIG_CLOCK_CONTROL_NRF_K32SRC_XTAL=n CONFIG_CLOCK_CONTROL_NRF_K32SRC_250PPM=y</pre><br /><br />This seems to have fixed our problems. Zephyr now boots and BLE works just fine. <div><span></span></div> </div><div style="clear:both;"></div>https://devzone.nordicsemi.com/thread/314248?ContentTypeID=1Tue, 08 Jun 2021 14:09:25 GMT137ad170-7792-4731-bb38-c0d22fbe4515:dd36bef8-a8ef-41ec-913f-b147a659d71aSimon<p><span>Try setting these in the prj.conf file:</span></p> <p><span><pre class="ui-code" data-mode="text">CONFIG_CLOCK_CONTROL_NRF_K32SRC_RC=y CONFIG_CLOCK_CONTROL_NRF_K32SRC_XTAL=n</pre></span></p> <p>Best regards,</p> <p>Simon</p><div style="clear:both;"></div>