Read protection on STM32L451CET6 stuck at level 1

[Akshat-hu-vro]

I am using a stm32l451cet6 micro controller chip in my PCB. I want to make the flash memory read protected. For this I use the following code snippet - 

while (HAL_FLASH_Unlock() != HAL_OK) {
                             ;
                      }
   while (HAL_FLASH_OB_Unlock() != HAL_OK) {
                             ;
                      }

  OptionsBytesStruct.OptionType = OPTIONBYTE_RDP; //Configure the RDP
   OptionsBytesStruct.RDPLevel = OB_RDP_LEVEL_1;
   while (HAL_FLASHEx_OBProgram(&OptionsBytesStruct) != HAL_OK) {
                             ;
                      }
   HAL_FLASH_OB_Launch();

 Now this works perfectly and the board goes to level 1 protection. Now after a button press I want to change the read protection  to level 0 from level 1. I use the following code snippet to change the RDP level back to 0.

void __attribute__((__section__(".RamFunc"))) RDP_Regression(void) {
      __disable_irq(); // Disable interrupts


      while (HAL_FLASH_Unlock() != HAL_OK) {
                                 ;
                          }
       while (HAL_FLASH_OB_Unlock() != HAL_OK) {
                                 ;
                          }

      OptionsBytesStruct.OptionType = OPTIONBYTE_RDP; //Configure the RDP
       OptionsBytesStruct.RDPLevel = OB_RDP_LEVEL_0;
       while (HAL_FLASHEx_OBProgram(&OptionsBytesStruct) != HAL_OK) {
                                 ;
                          }
       HAL_FLASH_OB_Launch();
}

But this code does not bring back the RDP level back to 0. The board is sill in level 1 protection. I tried removing the SRAM attribute, but it still doesn't work. And once in level 1 protection, the board does not execute the LED blinking and buzzer operations. The code works fine without the read protection. What am I doing wrong? Please help!

[STea]

Hello @Akshat-hu-vro ,

Could you please check this article note that the function for RDP regression is as follows:

void __attribute__((__section__(".RamFunc"))) RDP_Regression(void) {
      __disable_irq(); // Disable interrupts
      printf("Mass Erase Start\r\n");

      HAL_FLASH_Unlock();  // Unlock the flash memory
      HAL_FLASH_OB_Unlock(); // Unlock the option bytes

      *(__IO uint8_t*) OPTCR_BYTE1_ADDRESS = OB_RDP_LEVEL_0;

      HAL_FLASH_OB_Launch();
}

could you also check the MSI frequency value as the Option byte loading can fail if the MSI frequency is greater than 8 MHz see ERRTA section 2.2.21 .

 Regards

[TDK]

RDP 1 -> RDP 0 requires a chip erase, so your program will not run after that. Is that acceptable?