Modifying single byte or word and also large struct into flash

[Ricko]

Hi,

I would like to change at runtime some user-defined variables stored in Flash (so they remain on the system after power off). I have defined them as const and some are just 16 bits vars and others are more complex large structures. What is the best way to do that?

I came across the Flash Sector erase function but don't want to risk using a whole block erase (i.e. reading block, changing specific byte, word or struct then write back the entire sector with new values). Are there other options?

Also which include files do I need? Still getting familiar with the documentation, I came across three possible header files to include but not sure which to include in my code of these:

#include <stm32l4xx_hal_flash.h>
#include <stm32l4xx_hal_flash_ex.h>
#include <stm32l4xx_hal_flash_ramfunc.h>

could someone please clarify that too.

Thank you 😊

[TDK]

There's no way to modify a flash sector without erasing it first. It is a hardware limitation. The most common solution is to use a sector of flash which is reserved for user settings variables and can be erased as needed.

The top level HAL include should be "stm32l4xx_hal.h". That's it. Other files are included from within there as needed. Modify your stm32l4xx_hal_conf.h file to include other modules if needed.

[AScha.3]

You cannot change anything random in a flash memory.

After erase (block of memory typically) you can write it once - that's it. To write new data, you have to erase the area and then you can write it again.

So to store some values, you can use the battery backed up ram ( if you have a battery on your system) in RTC module, attach an eeprom or use a part of the CPU flash as a eeprom ( there are some examples for this in GitHub or STM examples ).

[Tesla DeLorean]

Journal the structure content over the available erased flash space.

By "journal" I mean write multiple instances of the structure, with say a sequence count and checksum or CRC, so you can walk it later and pull out the most recent valid instance.

Use two pages/sectors, so you always have a valid copy, erase the first page at the point you're ready to write the second instance into the second page.