nRF9160 Questions related to Immutable, MCUBoot & application

Question

Hi Lorenzo, 
 I have gone through most of the documentation related to Immutable (SB or b0), McuBoot and application and i have the following queries: 
 
 which one should i choose while building my application b0+MCUBoot+app or b0+app or MCUBoot+app?(to avoid confusion i am treating SPM and app together) 
 when i build my application with b0+MCUBoot+app, what is the procedure to serial upgrade MCUboot image? please point me to an example? 
 I find some threads on serial application DFU but it needs the device to be pushed into recovery mode, cant it be done when the application is running?(consider a use case where my metering application is running from one slot, then a new image is ungraded to the second slot, once the upgrade completes successfully pass the controller to the bootloader to flash & activate the image) 
 
 regards 
 kk

shibshab · Answer

1. - If you want an upgradeable bootloader you need b0 and MCUBoot (update the thing that updates the app) - else if you have an application which consists of SPM and app you cannot use b0 - app - else if you have an app which does not contain SPM you are free to choose between b0 - app and mcuboot - app 2. - This would be done using smp over UART. Transfer the signed mcuboot image over serial to the secondary slot, and then on the next reboot MCUBoot perform the swap to the alternate b1 slot (s0 or s1). - See this page for more information on the sample: docs.zephyrproject.org/.../README.html - Some modifications would be needed for this to be capable of updating MCUBoot in addition to the application. Specifically, you would need some way of communicating whether you want the MCUBoot candidate linked against slot S0 or S1. Inspiration for how this is handled elsewhere can be seen here: github.com/.../fota_download.c 3. - This is the "serial recovery" feature, it can not be done while the application is running. Instead (as you say) it requires some manual action during boot-up so that MCUBoot will start in a special mode. For your use case, if the image is to be transferred over serial, you would need the feature discussed in question 2.