RM0440 Rev 4 205/2126
RM0440 Embedded Flash memory (FLASH) for category 2 devices
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5.4.2 Option bytes programming
After reset, the options related bits in the Flash control register (FLASH_CR) are write-
protected. To run any operation on the option bytes page, the option lock bit OPTLOCK in
the Flash control register (FLASH_CR) must be cleared. The following sequence is used to
unlock this register:
1. Unlock the FLASH_CR with the LOCK clearing sequence (refer to Unlocking the Flash
memory).
2. Write OPTKEY1 = 0x08192A3B in the Flash option key register (FLASH_OPTKEYR).
3. Write OPTKEY2 = 0x4C5D6E7F in the FLASH_OPTKEYR register.
The user options can be protected against unwanted erase/program operations by setting
the OPTLOCK bit by software.
Note: If LOCK is set by software, OPTLOCK is automatically set too.
Modifying user options
The option bytes are programmed differently from a main memory user address.To modify
the user options value, follow the procedure below:
1. Check that no Flash memory operation is on going by checking the BSY bit in the Flash
status register (FLASH_SR).
2. Clear OPTLOCK option lock bit with the clearing sequence described above.
3. Write the desired options value in the options registers: Flash option register
(FLASH_OPTR), Flash PCROP1 Start address register (FLASH_PCROP1SR), Flash
PCROP1 End address register (FLASH_PCROP1ER), Flash WRP area A address
register (FLASH_WRP1AR), Flash WRP area B address register (FLASH_WRP1BR),
4. Set the Options Start bit OPTSTRT in the Flash control register (FLASH_CR).
5. Wait for the BSY bit to be cleared.
Note: Any modification of the value of one option is automatically performed by erasing both user
option bytes pages first and then programming all the option bytes with the values contained
in the flash option registers.
Option byte loading
After the BSY bit is cleared, all new options are updated into the flash but they are not
applied to the system. They have effect on the system when they are loaded. Option bytes
loading (OBL) is performed in two cases:
– when OBL_LAUNCH bit is set in the Flash control register (FLASH_CR).
– after a power reset (BOR reset or exit from Standby/Shutdown modes).
Option byte loader performs a read of the options block and stores the data into internal
option registers. These internal registers configure the system and cannot be read with by
software. Setting OBL_LAUNCH generates a reset so the option byte loading is performed
under system reset.
Each option bit has also its complement in the same double word. During option loading, a
verification of the option bit and its complement allows to check the loading has correctly
taken place.
During option byte loading, the options are read by double word with ECC. If the word and
its complement are matching, the option word/byte is copied into the option register.
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