RE: Hook into MBR boot sequence to run custom code

Austin — Wed, 15 Aug 2018 01:22:40 GMT

Although binary patching the MBR is not necessarily a good option, I have tested this approach for a very simple case which sets a GPIO pin before entry to the bootloader.

The patch process uses the following steps:

Write a small bit of C code containing the required operations to perform as part of boot. This should be very simply.
Generate a set of assembler instructions by compiling the C code against the nRF5 SDK with GCC.
Use the generated opcodes from the assembler as input to a patch process which will put the patch assembler instructions into upper MBR address memory.
Patch the MBR reset vector to point to the patch instructions.

When the MCU boots, the reset vector will now run the patch code. In the attached example, the patch sets GPIO P0.15 to a high level on boot. The last instruction in the patch jumps back to the original reset vector location to allow normal MBR functionality to run.

Use this approach with caution as the MBR is the only component which is never updated as part of any firmware upgrade process when using the Nordic DFU modules.

See the attached files for additional instructions and an example of how the patching process could work. The patch code and tools will require manual modification to support each specific application. e.g. Changes will be needed for processors other than nRF52840, S140.

devzone.nordicsemi.com/.../patch_2D00_mbr_2D00_tools.tgz

RE: Hook into MBR boot sequence to run custom code

bjorn-spockeli — Thu, 31 May 2018 09:25:12 GMT

Hi Austin,

Yes, there will be a small window during the bootloader copy procedure where you will not have be able to control the GPIO pin states, the size of the window will depend on the size of the bootloader as you've pointed out. My comments to you suggestions are:

1. Yes, I agree that you should avoid binary patching.

2. This would be the best option, but I am afraid that we do not distribute the MBR source code as this considered Nordic proprietary. However, I will forward your request for the MBR source to the Product Manager responsible for the SoftDevice. It might be possible to share something under NDA, but I would not count on it.

3. As far as I know adding such a feature to the MBR is not a part of the SoftDevice roadmap and unless more of our customers request such a feature this will most likely not be added in a future SoftDevice release.

Best regards

Bjørn