RDP for STM32C0

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1. RDP level management

The STM32C0 silicon device life cycle is based on the RDP mechanism implemented for the FLASH interface, as specified in the STM32C0 reference manual (RM0490^[1], section 4.5.1).

RDP protection level	Debug	Comments
Level 0: device open	Allowed	The boot address can be either the user flash memory, bootloader in system memory, or embedded SRAM, depending on the boot mode configuration.
Level 1: device memories protected	Limited	The boot address can be either the user flash memory, bootloader in system memory, or embedded SRAM, depending on the boot mode configuration. Debug access to the user flash memory, embedded SRAM, and backup registers is not allowed.
Level 2: device closed	None	The boot address must target the user flash memory. The flash memory user option bytes are read-only, so RDP level 2 cannot be changed.

The figure below illustrates the level transitions:

STM32C0 RDP level management

Changing the read protection level:

From level 0 to level 1: write any other values than 0xCC or 0xAA in the RDP register.
From level 0 or 1 to level 2: write 0xCC in the RDP register.
From level 1 to level 0: write 0xAA in the RDP register.

Warning

As specified in section 4.5.6 of RM0490^[1], setting BOOT_LOCK in association with RDP level 1 stops the device debug capabilities.

It is mandatory to insert a debug reactivation in the boot sequence to reactivate the debug capabilities and allow the regression from level 1 to level 0.

Refer to the example code in system_stm32c0xx.c from the STM32C0 firmware package OEMiSB:

/* Software addition added to keep Debug enabled after Boot_Lock activation and RDP=1  */
#ifdef ENABLE_DBG_SWEN
  if (((FLASH->SECR & FLASH_SECR_BOOT_LOCK) == FLASH_SECR_BOOT_LOCK)           \
      && (((FLASH->OPTR & FLASH_OPTR_RDP) != 0xCCU)                            \
          && ((FLASH->OPTR & FLASH_OPTR_RDP) != 0xAAU)))
  {
    FLASH->ACR |= FLASH_ACR_DBG_SWEN;  /* Debug access software enabled to avoid the chip
                                         to be locked when RDP=1 and Boot_Lock=1        */
  }
#endif /* ENABLE_DBG_SWEN */

2. RDP password regression

Regression allows the reopening of the product by returning to RDP level 0. A full user flash memory mass erase is applied prior to the reopening.

Mass erase (full or partial) is only triggered by the RDP regression from level 1 to level 0.

3. References

↑ ^1.0 ^1.1 , RM0490

[multiple-1] 1.0 ^1.1 , RM0490

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