Flash memory interface PM0059
16/29 DocID15687 Rev 5
The user option bytes can be protected against unwanted erase/program operations by
setting the OPTLOCK bit by software.
Modifying user option bytes
To modify the user option value, follow the sequence below:
1. Check that no Flash memory operation is ongoing by checking the BSY bit in the
FLASH_SR register
2. Write the desired option value in the FLASH_OPTCR register
3. Set the option start bit (OPTSTRT) in the FLASH_OPTCR register
4. Wait for the BSY bit to be cleared
Note: The value of an option is automatically modified by first erasing the user configuration sector
and then programming all the option bytes with the values contained in the FLASH_OPTCR
register.
2.6.3 Read protection (RDP)
The user area in the Flash memory can be protected against read operations by an
entrusted code. Three read protection levels are defined:
• Level 0: no read protection
When the read protection level is set to Level 0 by writing 0xAA into the read protection
option byte (RDP), all read/write operations (if no write protection is set) from/to the
Flash memory or the backup SRAM are possible in all boot configurations (Flash user
boot, debug or boot from RAM).
• Level 1: memory read protection.
It is the default read protection level after option byte erase. The read protection Level
1 is activated by writing any value (except for 0xAA and 0xCC used to set Level 0 and
Level 2, respectively) into the RDP option byte. When the read protection Level 1 is set:
– No access (read, erase, program) to Flash memory or backup SRAM can be
performed while the debug feature is connected or while booting from RAM or
system memory bootloader. A bus error is generated in case of read request.
– When booting from Flash memory, accesses (read, erase, program) to Flash
memory and backup SRAM from user code are allowed.
When Level 1 is active, programming the protection option byte (RDP) to Level 0
causes the Flash memory and the backup SRAM to be mass-erased. As a result the
user code area is cleared before the read protection is removed. The mass erase only
erases the user code area. The other option bytes including write protections remain
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