DFU Upgrade of only Bootloader (not SoftDevice)

[Bruno Randolf]

I am in the situation that I have 1000+ deployed devices with a buggy bootloader in a production environment. The current application version triggers this bug, which means that as soon as the bootloader is entered it crashes and the application is started again. So I can not upgrade the application any more. I need to upgrade the bootloader first.

The problem is that the devices have a nRF52810 which does not have enough flash space to allow for SD+BL updates.

Is there any way I can do a BL update only (without the softdevice)? I know nrfutil does not support this right now, but what are the chances that I could change/hack nrfutil or the DFU package ZIP file to add the BL without the SD?

Why does this restriction exist at all, why doesn't nrfutil allow us to make BL+APP or BL only upgrade packages?

[Einar Thorsrud]

Hi,

I have a few questions in order to understand more:

• What bootloader have you flashed on the device and what changes have you made?
• Which transport do you use (BLE or UART)?
• What does your flash layout look like?

Generally, there is no problem upgrading just the bootloader without upbraiding the SoftDevice, as long as the new bootloader is built for the same SoftDevice. However, in order to support bootloader updates you must have enough free space in the application are to temporarily store the bootloader, and you must have a MBR params page. If either of these two are missing, bootloader update is not possible. 

nrfutil does allow generating a upgrade image of the bootloader only, and that is also supported by the nrF5 SDK bootloader. The only combination that is not allowed is bootloader + application, so in this case you need to use two separate upgrade images and to one after the other.

[Bruno Randolf]

Thank you for your answer!

I use an adapted version of the secure bootloader from the SDK, but I have introduced a bug when WDT is activated by the application. The current application uses WDT so the bootloader crashes every time I enter it. So there is no way to update neither APP or BL. I think this case is hopeless...

I use the BLE transport. The flash layout is single bank and there is not enough space on nRF52810 to update the BL+SD...

I wonder why the bootloader + application combination is not allowed, as that would help in this case, when I upgrade older versions. As you said I have to update BL first and then the application, but it would be good to be able to do that in a single command as with APP+SD+BL upgrades.

[Einar Thorsrud]

Hi,

I see. The primary issue here is entering DFU mode without the WDT resetting the bootloader, then. One obvious possibility comes to mind, though it might not be possible. Which methods does the bootloader support for entering DFU mode? I ask because after a WDT reset, the WDT is also reset, i.e. it is disabled. So if you can start the bootloader immediately after a WDT reset without the app running first (and enabling the WDT), that could be a workaround. For instance if you have a button or pin that can be asserted during bootup to enter DFU mode?

[Bruno Randolf]

Thank you - That's a very good idea! My bootloader supports button pin 16, GPREGRET and buttonless. But unfortunately P0.16 is not accessible from the outside.

[Einar Thorsrud]

I see. Then I suspect the only remaining option is to access the SWD pins and program using a debugger, though that may not be a real option for devices in the field.

[Bruno Randolf]

Yes, unfortunately...

Another issue is the upgrade of APP+BL: That would be really useful to upgrade older versions. It should work the same way as APP+SD+BL updates, just without SD. The BL is updated first and then the application in one go. If we have to update BL first and then reconnect to MAC address + 1 to separately upgrade the APP that gets unmanageable really quickly if you have many devices in an environment.

[Einar Thorsrud]

I see. APP+BL is not supported though, and not possible. If you are making a app for your end customers to use in the field you could integrate this so that the operation looks like a single operation from the GUI user perspective, though.