https://devzone.nordicsemi.com/f/nordic-q-a/33477/52840-usb-secure-dfuDear Engineer, We are working on 52840 usb dfu,program softdevice s140_6.0, and usb secure bootloader, evaluate board is pca10056,v0.9.0 Led1,led3 turn on once load bootloader. nrfutil version is 3.2.1, in infocenter,usb dfu page,step 9 saiden-USTelligent Community 13Tue, 17 Apr 2018 12:58:16 GMThttps://devzone.nordicsemi.com/thread/128643?ContentTypeID=1Tue, 17 Apr 2018 12:58:16 GMT137ad170-7792-4731-bb38-c0d22fbe4515:bcaf2ff4-f016-4664-a869-8af3e6809b10Bj&#248;rn Kvaale<p>I am using nrfutil version 3.4.0 &amp; when I enter the command nrfutil dfu --help, I get help information for the usb_serial command. Could you try upgrading to the latest nrfutil version &amp; testing again?</p><div style="clear:both;"></div>