nRF52840 single- vs. dual-bank behavior

[Daniel Fedai Larsen]

Hi,

I'm working on enabling dual-bank updates in our projects (we're using nRF5 SDK v15.3.0), and during this process I've stumbled onto some behavior that confused me a bit. I was testing updating the bootloader from single-bank to dual-bank and found that the application was still present and was started after the update. This surprised me as I thought with single-bank updates the application would always be overwritten by the DFU package. I tested replacing the proper application with dummy data and re-did the test and the dummy data was overwritten (as expected). I then found the following:

https://docs.nordicsemi.com/r/bundle/nrf5_sdk_v15.3.0/page/lib_bootloader_dfu_banks.html specifies that for single-bank updates

"The DFU bootloader checks if a dual-bank update is possible. Only if it is not (because the free memory is not sufficient), it will revert to a single-bank update."

To enable dual-bank updates the macro NRF_DFU_SINGLE_BANK_APP_UPDATES should be set to 0. However, if the macro NRF_DFU_FORCE_DUAL_BANK_APP_UPDATES isn't also set to 1 https://docs.nordicsemi.com/r/bundle/nrf5_sdk_v15.3.0/page/group_nrf_dfu_validation_config.html?section=ga9bd60b6464d75f738bb06845ec8b86a9 specifies

"If not enabled then if there is not enough space to perform dual-bank update application is deleted and single-bank update is performed."

The way I understand this is that only setting NRF_DFU_SINGLE_BANK_APP_UPDATES to 0 will make the bootloader will behave the same way as if it was set to 1: it will prefer dual-bank updates, but fall back to single-bank updates if there isn't enough room to fit the DFU package. Am I missing something? What's the point of separating NRF_DFU_SINGLE_BANK_APP_UPDATES and NRF_DFU_FORCE_DUAL_BANK_APP_UPDATES?

Thanks,
Daniel

[Einar Thorsrud]

Hi Daniel,

NRF_DFU_SINGLE_BANK_APP_UPDATES and NRF_DFU_FORCE_DUAL_BANK_APP_UPDATES are not equivalent. With NRF_DFU_SINGLE_BANK_APP_UPDATES, you actualy enforce single bank updates where possible. Whereas NRF_DFU_FORCE_DUAL_BANK_APP_UPDATES prevents single bank, also in caes where this means an update is not possible (due to lack of available space do dual bank).

To use dual bank when possible, but revert to single bank when not (which is what makes sense for most applications), let NRF_DFU_SINGLE_BANK_APP_UPDATES be 0 and also NRF_DFU_FORCE_DUAL_BANK_APP_UPDATES be 0. This way, dual bank updates are used whenever possible, but if not, single bank is used.

[Daniel Fedai Larsen]

Right, but it seems that setting NRF_DFU_SINGLE_BANK_APP_UPDATES to either 1 or 0 results in the same behavior (if we assume NRF_DFU_FORCE_DUAL_BANK_APP_UPDATES is always set to 0): in both cases dual-bank updates can happen. Am I wrong?

[Einar Thorsrud]

Hi,

From what I can see NRF_DFU_SINGLE_BANK_APP_UPDATES set to 1 should make application and SoftDevice update always use single bank. Bootloader updates however will always be dual bank, as the bootloader cannot update itself in place.

[Daniel Fedai Larsen]

Thanks for the quick replies!

But what about this statement from docs.nordicsemi.com/.../lib_bootloader_dfu_banks.html

[Einar Thorsrud]

Hi,

This is the general normal case, yes. The exception is if you have NRF_DFU_SINGLE_BANK_APP_UPDATES set to 1. But this is not much used. And moreover, if you have BLE as transport for DFU the bootloader depend on the SoftDevice, and then the SoftDevice can also not be updated in single bank manner.

Let me note that setting NRF_DFU_SINGLE_BANK_APP_UPDATES to 1 is not much used (and frankly I had forgot it existed), as for most products it makes sense to use dual bank whenever possible.