We have used STM32F427IIH6 MCU to create custom board. We assembled first proto batch of 10 boards.
All boards were perfectly flashing. While testing one board stopped flashing.
Based on error message we checked write protection option byte status.
We got to see RDP status is FF.
Its very strange as all other boards are showing AA.
Only this particular boards showed FF. That too after flashing it multiple times for running various test cases.
My question here is - Whats the reason behind showing RDP status FF ( where there is no such option )?
How to restore it AA ( Level 0 , no protection )?
We tried ST Link V2 & V3 ( both with latest firmware updated ) but no luck.
To change the level protection to 0, you just have to change the value of the RDP bit to AA on cube programmer. But the Flash memory and the backup SRAM will be mass-erased. For more details, take a look at the part 3.7.3 of the RM0090. Also, those two publications may help you to change the RDP value (post1) and understand a potential reason of this problem (post2).
If you can't set it back to 0xAA, my guess is faulty hardware. Perhaps power rail isn't sufficient, perhaps something else is wrong. Especially on a BGA chip that would be my first guess. You can xray the chip, which is somewhat expensive, but most board houses can do it.
0xFF is a valid setting, it's level 1 protection. But also consider the case where st-link isn't able to read the chip correctly and may be reporting a bogus value.