Dual bank DFU with code for external uC

Question

I am using two microcontrollers for my application, one of which is the nRF52. What I would like to do is transfer the firmware for the nRF and external uC as a single bundle using the dual bank update approach.
The bootloader should then verify and program the firmware for the nRF and exernal uC in sequence.
Is there an example for this, and how could the bootloader distinguish the difference between the two applications when they are transferred as a single bundle?

Andy · Accepted Answer

I have done this to use the NRF chip to flash an STM chip, but I use separate bundles. My guess is that you won't need to update both chips every single time, so I would suggest to keep it simple and separate the bundles. 
 To distinguish the different firmwares, I first tried using a flag which you first needed to set by writing a code to the DFU Control Point. If the flag was set, the firmware was meant for the STM, if not, for the NRF. Then I had a situation where the flag wasn't set for some reason and "bricked" my device because I accidentally flashed STM firmware onto the NRF, so I decided to change my implementation. 
 My final approach was to use different key signatures for NRF and STM DFU packages and to set the flag accordingly. 
 I considered a third approach of modifying nrfutil to include a flag to indicate whether the DFU package was meant for the NRF or the STM, but I found the different keys to be an easier implementation.

Dual bank DFU with code for external uC

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