Memory protection strategy

PM0212

10/22

Doc ID 022351 Rev 2

When the PCODE option byte is set, the PCODE area and the TRAP vector are write

protected to prevent a malicious user/program from inserting a “dump” routine inside the

protected code.

When accessing the PCODE area, no other program can be executed (all interrupt and

debug access are disabled). The programmer must consequently ensure that the protected

code embedded in the PCODE area gives back control to the main/end user software at

reasonable periods of time.

3.3

User Boot Code area protection

Whatever the memory content, it is always possible to restart an ICP session after a critical

error by applying a reset and restarting the SWIM communication.

On the contrary, during IAP sessions, the programming software driver must always be write

protected to be able to recover from any critical failure that might happen during

programming (such as power failure).

The pages where the IAP driver is implemented must be located in the write-protected boot

code area (UBC). The application reset and interrupt vectors and the reset routine must also

be stored in the UBC. These conditions allow the user software to manage the recovery

from potential critical failure by applying a reset and restarting the IAP routine from the

protected boot area.

The UBC size is defined by the user boot code (UBC) area option byte. See the following

table for the minimum and maximum size of the UBC area.

3.4

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. 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

Table 3.

Recommended minimum and maximum sizes of the UBC area

Recommended minimum size

of the UBC area

Maximum size of the UBC area

3 pages = 192 bytes

255 pages