https://devzone.nordicsemi.com/f/nordic-q-a/128614/bluetooth-bootloaderA custom PCBA based on the nRF52832 microcontroller. I am testing two different versions of firmware: Version 1 (Sample code from SDK v11): examples\ble_peripheral\ble_app_hrs\pca10040\s132_with_dfu Version 2 (Integration): Integrate the versionen-USTelligent Community 13Fri, 03 Jul 2026 11:37:25 GMThttps://devzone.nordicsemi.com/thread/568766?ContentTypeID=1Fri, 03 Jul 2026 11:37:25 GMT137ad170-7792-4731-bb38-c0d22fbe4515:b33d5190-a231-41c3-b61f-f2736a801303Hung Bui<p>Hi Kelvin,&nbsp;<br />Please be aware that nRF5 SDK v11 is very old. It&#39;s released 10 years ago I believe.&nbsp;</p> [quote user=""]if flashing Version 2, it only works at the first time. Upon a system reboot or subsequent connection, the board <b>refuses to enter DFU mode a second time</b>. [/quote] <p>Please clarify what you meant by &quot;only works at the first time&quot; . You meant you can turn it on, do a DFU and it boot up with new image ? Which new image did you use?&nbsp;<br />Please double check if the new image has the same BLE attribute table as the Version 2. If the new image doesn&#39;t have DFU buttonless service or the service is not the same location in the table as the old image, the phone may have trouble to access that.&nbsp;<br /><br />You may want to refresh the phone , for example by turning Bluetooth off and on, remove bonding.&nbsp;<br /><br /></p><div style="clear:both;"></div>