Embedded Flash memory (FLASH) RM0444
92/1390 RM0444 Rev 5
The System memory area is read-accessible whatever the protection level. It is never
accessible for program/erase operation.
Level 0: no protection
Read, program and erase operations within the Main Flash memory area are possible. The
option bytes and the backup registers are also accessible by all operations.
Level 1: Read protection
Level 1 read protection is set when the RDP byte and the RDP complemented byte contain
any value combinations other than [0xAA, 0x55] and [0xCC, 0x33]. Level 1 is the default
protection level when RDP option byte is erased.
• User mode: Code executing in user mode (boot from user Flash memory) can access
Main Flash memory, option bytes and backup registers with all operations.
• Debug, boot from SRAM, and boot from System memory modes: In debug mode
or when code boots from SRAM or System memory, the Main Flash memory and the
backup registers (TAMP_BKPxR in TAMP) are totally inaccessible. In these modes, a
read or write access to the Flash memory generates a bus error and a Hard Fault
interrupt.
Caution: In Level 1 with no PCROP areas defined, it is mandatory to set PCROP_RDP bit to 1 (full
mass erase when the RDP level is decreased from Level 1 to Level 0). In Level 1 with a
PCROP area defined, user code to protect by RDP but not by PCROP must be placed
outside pages containing a PCROP-protected subpage.
Level 2: No debug
In this level, the protection Level 1 is guaranteed. In addition, the CPU debug port, the boot
from RAM (boot RAM mode) and the boot from System memory (boot loader mode) are no
more available. In user execution mode (boot FLASH mode), all operations are allowed on
the Main Flash memory.
Note: The CPU debug port is also disabled under reset.
Note: STMicroelectronics is not able to perform analysis on defective parts on which the Level 2
protection has been set.
Changing the read protection level
The read protection level can change:
• from Level 0 to Level 1, upon changing the value of the RDP byte to any value except
0xCC
• from Level 0 or Level 1 to Level 2, upon changing the value of the RDP byte to 0xCC
• from Level 1 to Level 0, upon changing the value of the RDP byte to 0xAA
Table 17. Flash memory read protection status
RDP byte value RDP complement byte value Read protection level
0xAA 0x55 Level 0
Any values except the combinations [0xAA, 0x55] and [0xCC, 0x33] Level 1 (default)
0xCC 0x33 Level 2
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