sd_flash_page_erase() time execution on nRF52840

[era]

Hi!

We develop our product on nRF52840 using S113 softdevice, version 7.0.1. From time to time, we need to erase some part of flash, so we use sd_flash_page_erase() function. We measured time execution of this function and it usually takes about 50 us to be executed.

Also, there is an information in nRF52840 documentation that page erase takes 85 ms maximum.
https://infocenter.nordicsemi.com/index.jsp?topic=%2Fps_nrf52840%2Fnvmc.html

Does this function only schedule page erase, which is executed later in smaller parts? Is there any risk, that this function call would take much longer (e.g. 85 ms for whole page erase).

Best regards,
Michal

[Turbo J]

era said:

We measured time execution of this function and it usually takes about 50 us to be executed.

Yes, this function does not actually erase a page. It just schedules an erase during the next available radio time slot.

Which may be much later than anticipated in situations with very high radio usage. The application will be informed (via SD event) when the page erase transaction finshed. This can take significantly more time than 85 ms.

[Jørgen Holmefjord]

Hi,

See the following sections in the softdevice specifications:

• Flash memory API
• SoftDevice timing-activities and priorities

The API call itself is not expected to take a long time, but it can get interrupted by a timing-critical softdevice interrupt, which may take up to 250 us. See Bluetooth Low Energy processor usage patterns for more details.

Best regards,
Jørgen

[era]

Thanks. After page erase scheduling, the erase is performed from priority 0, as Flash erase/write operations are defined as softdevice timing-critical. So this means that I cannot preempt this process and it takes up to 90 ms. Can anything be done about this? Or do I need to be prepared that after page erase scheduling my code can be completely halted for so long?

[Turbo J]

The nRF52840 has partial flash erase, around 1ms each time.

[era]

But does it work with softdevice enabled? I think the only way is to erase page in chunks using timeslots, right? SD has no api for partial erase.