Recommended bootloader size to support future SDK releases

[Andy]

Hello,

I'm currently using a bootloader based on the one included with SDK 15. We're close to mass production and I need to decide on how much space I should leave for the bootloader.

My question is concerning the size of the bootloader. If on a future SDK release it grows enough so that I can't fit it on the current allocated space, we won't be able to upgrade the SDK because the bootloader won't be compatible with the new SoftDevice and it won't be upgradeable.

I don't know if there are any major changes already planned for the bootloader. I currently have around 7 KB of free space in the bootloader area. I'm hoping that's enough space.

Thanks for your support!

[bjorn-spockeli]

Hi Andy, 

I am not aware of any planned major changes in our current bootloader implementation that will increase the code size outside the current memory footprint. 

We generally strive to keep backwards compatibility with older SDK, which with the bootloader implies that we cannot increase the size beyond the memory footprint of the previous version. That said, we did break compatibility between SDK 11 and SDK 12 when we introduced the Secure Bootloader with signing, but this was necessary to provide a secure DFU solution. 

Again, no major changes to the bootloader is planned, so 7kB should be more than enough headroom for any potential size increases in the future. Should the bootloader size increase beyond the limit, then it is possible to move the bootloader start address on the nRF52 if the access port protection is not enabled, see https://devzone.nordicsemi.com/f/nordic-q-a/18199/dfu---updating-from-legacy-sdk-v11-0-0-bootloader-to-secure-sdk-v12-x-0-bootloader

  This is, however, not completely failsafe, as described in the linked post. 

Best regards

Bjørn

[Andy]

Thanks for your answer Bjorn!