Flash memory interface

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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