Write user data to NVM

[Dekel]

Hi,

I'm working with nRF5340 and would like to store my own data in the NVM (2 bytes in total).

I couldn't figure out which flash partition is relevant for this and what is its address. I looked at the memory map (P.20), but it doesn't specify that.

I would appreciate your help with doing so, and also to understand if using nrfx_nvmc library is relevant or should I user another method.

Thank you!

[Vidar Berg]

Hi,

Since the minimum erase unit for the flash is 4kB, you would need to reserve at least 4kB of flash even if you only need 2 bytes of storage (actually 4 bytes, since that is the minimum write size).

If you are using any of the SDK samples with wireless connectivity, it is a good chance that it is already using the Settings Subsystem to manage storage of persistent application data. In that case, I would recommend you use the same module for storing your own data to allow you to use the existing storage partition in flash (You can run the 'Memory report' action in the vs code extension to see where in flash the storage partition is placed).

Best regards,

Vidar

[Dekel]

Thanks for your response.

If I understand it correctly, I would still need to implement the get & set functions by myself, since settings.h is a Zephyr library. Hence, I would still need to use nrfx_nvmc.h

Therefore, my question persists - how do I reserve a partition for this purpose and which addresses are allowed for that in nRF5340?

Thanks.

[Vidar Berg]

When using Settings, you store and load values from flash using only the settings API. The Settings subsystem builds on top of the NVS filesystem, which again builds on top of the flash drivers. nrfx_nvmc is the lowest layer.

If you want to allocate a separate flash partition and manage the flash writes and erases yourself, you may take a look at the zephyr/samples/drivers/soc_flash_nrf sample which demonstrates how to use the zephyr flash API. Your custom flash partition can be added by creating a configuring a new static partition: https://docs.nordicsemi.com/bundle/ncs-latest/page/nrf/scripts/partition_manager/partition_manager.html#adding_a_static_partition 

[Dekel]

Vidar Berg said:

You can run the 'Memory report' action in the vs code extension to see where in flash the storage partition is placed

that's a nice feature, it introduced me with partition "EMPTY_0" at the end of the flash area, which works great with nrfx_nvmc.h!

So for code simplicity, I'll continue with this library, unless you would recommend differently.

[Vidar Berg]

I don't foresee any problems with this approach, but I recommend locking down the memory layout so the EMPTY_0 placeholder partition does not suddenly get moved or removed due to a configuration change in your project. You can do this by making a copy of the generated $BUILD_DIR/partitions.yml file, renaming it to pm_static.yml, and placing it in your project's root directory.

Link: https://docs.nordicsemi.com/bundle/ncs-latest/page/nrf/scripts/partition_manager/partition_manager.html#static_and_dynamic_configuration 

You can also change the 'EMPTY_0' name in your pm_static file to a more descriptive name, such as 'app_data'. The size and address of this partition can be found programmatically using the symbols defined in pm_config.h (included with #include <pm_config.h>).

[Dekel]

Vidar Berg said:

I recommend locking down the memory layout so the EMPTY_0 placeholder partition does not suddenly get moved or removed due to a configuration change in your project.

great tip! I've implemented that and it looks great, thank you!