femmeverbeek:
Can't say I understand the lot of it. Isn't it tricky to write in the OS location. Couldn't it just accidently damage the bootloader?
You are not writing to the OS location. This is a function that has been implemented. You can use it to store non-volatile values into flash memory. I suspect the software is smart enough to place it into an empty part of the flash. After all that is what compilers and linkers are really good at.
femmeverbeek:
What about the SPI library. Is it suitable for the 33BLE. The latest added processor is nRF52832 not the nRF52840.
The question is not the nRF device but mbedOS. mbedOS has SPI APIs but I am not sure whether they are compatible.
https://os.mbed.com/docs/mbed-os/v5.15/apis/spi.html
This would require an additional component to be added to your board. Do you think you will need to store more than the calibration values in no-volatile memory? Storing the data in internal flash would be less complex. Well except for the software API of mbedOS which could have been a bit easier for storing a few bytes. 