nRF5 DFU over-the-air bootloader prevents wired flashing

[Jarmo]

When we use nRF5 bootloader provided with SDK the first application flashing must always be done using DFU over-the-air. Only after that it is possible to re-flash application with a wire, using e.g. segger j-link. This applies both, nRF51 and nRF52, and at least SDK8 and SDK12.

This is a problem since when we flash our customer's devices, it is too slow to use DFU over-the-air. We want to do first flashing quickly with the wire.

I guess the bootloader checks existence of an application, and if it's not flashed using DFU over-the-air, it does not start application at all. Even if it is there, but flashed with a wire. How to go around this check, and where it is placed in the bootloader code?

[Wojtek]

This should help. You have to validate bootloader settings.

[Nguyen Hoan Hoang]

That is due to the fact that the app valid flag & crc was not set. When loading with the debugger, it does not set the flag & crc which is located in the DFU section (0x7F000). By loading it via DFU, that flag & crc are set properly. Softdevice always jumps to DFU if it is there. DFU checks for those flags before jumping to app. If it's invalid, DFU stays in DFU. Your app never gets called. You'll never break at main.

[Jarmo]

Okay, so how to get correct correspoding file that was mentioned behind the link, I mean app_valid_settings_dfu.hex ? I guess that is not necessarily compatible with the bootloaders in SDK8 and SDK12, or is it?

[Wojtek]

For SDK 12 secure bootloader, nrfutil gives you proper fix-file when generating dfu package i think. For older one, try the ones from that topic, or just write your own (easier than it looks to be). Something from that topic should work just fine though.

[bjorn-spockeli]

@nikula (Jarmo): If you're using SDK v12.x.0 then you have to generate a bootloader settings hex file with nrfutil v2.1.0 that contains the application crc and sets the valid application flag. You then have to merge this hex file with the bootloader. You can then merge that image with the application and softdevice hex files and flash the combined image using a programmer.