Why is the value of NRF_DFU_APP_DATA_AREA_SIZE 8192 bytes on the nRF5281x?

[loquat]

I want to use bootloader for nRF52811.

On nRF5 SDK 17,

exmaples/dfu/secure_bootloader/pca10056e_s112_ble/config/sdk_config.h
  #define NRF_DFU_APP_DATA_AREA_SIZE 8192

But,
exmaples/dfu/secure_bootloader/pca10056_s140_ble/config/sdk_config.h
  #define NRF_DFU_APP_DATA_AREA_SIZE 12288

I use FDS in my application. (3pages flash)

Should I change the value of NRF_DFU_APP_DATA_AREA_SIZE from 8192 to 12288?

Why is it 8192 bytes instead of 12288 in nRF52811?

[Einar Thorsrud]

Hi,

Isolated, there is no right or wrong value of NRF_DFU_APP_DATA_AREA_SIZE, as long as it is a multiple of a flash page size in bytes. The only thing that decides which value you should set NRF_DFU_APP_DATA_AREA_SIZE in the bootloader's sdk_config.h file to is how many pages you use for application data (FDS and/or other). If you use 3 pages for application data (FDS) in your case, then you must set NRF_DFU_APP_DATA_AREA_SIZE to 1024 words * 4 bytes per word * 3 pages = 12288 bytes. If you set a higher value than needed, then you may not be able to perform DFU even though there is actually enough room. And if you set a lower value you risk corrupting the application data during a DFU update, as the bootloader may use the flash area.

If you only need a single FDS page for data storage and one for swap, then you can keep NRF_DFU_APP_DATA_AREA_SIZE as 8192 in the bootloader sdk_config.h and set FDS_VIRTUAL_PAGES to 2 in the application's sdk_config.h. If you need two data pages of FDS, then you need to set NRF_DFU_APP_DATA_AREA_SIZE to 12288 instead, and FDS_VIRTUAL_PAGES to 3 (or higher numbers if you need it, though you will soon run out of flash on the nRF5281x devices).

[loquat]

Thanks.

This means that if your application has FDS_VIRTUAL_PAGES set to 3, then NRF_DFU_APP_DATA_AREA_SIZE must be set to 12288 or higher, regardless of nRF52811 / nRF52832?

[Einar Thorsrud]

Yes, that is the case. This has no dependency on the IC variant (other the practical issue than that the number of available flash pages is quite small on the nRF52811).

[loquat]

OK. Thank you.