Memory protection strategy

PM0054

16/28

Doc ID 15433 Rev 9

4.4

Unwanted memory access protection

The unwanted memory access protection consists of writing two 8-bit keys in the right order

into dedicated MASS key registers.

Writing the correct sequence of keys in the program memory MASS key register

(FLASH_PUKR) enables the programming of the program memory area excluding the UBC

and the PCODE area (when available). If wrong keys are provided, a reset must to be

generated to be able to reprogram the right keys.

Once the write memory protection has been removed, it is possible to reactivate the

protection of the area by resetting the PUL bit in FLASH_IAPSR.

To enable write access to the data EEPROM area, another specific MASS key register

(FLASH_DUKR) and a different key sequence must be used. Once the data

EEPROM/option byte area is unlocked, it is possible to reactivate the protection of the area

by resetting the DUL bit in FLASH_IAPSR.

If wrong keys have been provided to the FLASH_PUKR register, the device must be reset

before performing a new key program sequence. However, when wrong keys are provided to

the FLASH_DUKR register, new keys can be entered without the device being previously

reset.

On low density STM8L devices, the size of the DATA area can be configured through the

DATASIZE option byte for a given product.

In order to be as effective as possible, the application software must lock again the

unwanted memory access protection as soon as the programming is completed. Otherwise,

the protection level of the MASS is significantly reduced. To activate the MASS protection

again, the user must reset the corresponding bits in the FLASH_IAPSR register (DUL bit for

data EEPROM or PUL bit for Flash program memory).

Note:

The mechanism to lock and unlock unwanted memory access protection is identical for

option bytes and data EEPROM (see Table 9: MASS).

Before starting programming program memory or data EEPROM, the software must verify

that the area is not write-protected by checking that the PUL or DUL bit is effectively set.