STM32U585 Flash overwrite

[BRaff]

Hi, I used to write bytes at 0 in flash bytes that are erased (0xFF) but this is no more possible in the flash of the STM32U585 because of ECC that protects 16 bytes chunks I suppose.

However now I have an 8K sector programmed with some data except for a portion of 16 bytes (at 16 bytes aligned address) that are still 0xFF.but I could not write zeros in it.

Why isn't it possible?

[TDK]

You can only write to each part of the flash once after erasing. Even if you write 0xFF, you can no longer write anything else, even though it reads as 0xFF.