I working with the STM32F446-microcontroller and I need to upgrade firmware and change the option bytes on it using the built-in bootloader (en.CD00167594.pdf, AN2606) using SPI-bus (en.DM00081379.pdf, AN4286). I am able to successfully modify all option bytes, except the read protection level (from 1 to 0). In the reference manual (en.DM00135183.pdf) it says that when changing the read protection level from 1 to 0, then the flash will be mass-erased and I would imagine this takes some time. However, I can't find any special instructions in AN2606 on how to do this through the bootloader and whenever I try to change the read protection level, I end up receiving a NAK in the get-ACK-procedure inside the write-memory-command-sequence. Does anybody know the proper sequence for changing read protection level from 1 to 0?
