Flash code protection

AN4246

6/11

DocID024207 Rev 1

This command line option translates the literal pool operations with alternative instructions.

These instructions build the register values without any data read access. It is mainly

needed for loading registers with variable addresses. As an alternative method is less

efficient, this option translates these operations in a slightly less effective code. However,

the loss of performance is limited (below 5%), which is acceptable for the protected parts of

the code.

The PCROP sector is selected by using the same option bytes as the Write protection. As a

result, these 2 options are exclusive each other. However, the sectors protected against

reading (PCROP) are also protected against writing/erasing. Therefore, the PCROP may be

considered as a superset of the sector write protection.

In order to activate the PCROP (change the function of the nWRP option bits), the

SPRMOD option bit must be activated. This operation is irreversible.

Also in PCROP mode, a sector which was set to be read-protected cannot be reset to the

unprotected state. As a result, new sectors may be added to the read protected area (when

RDP is set to Level 0 or 1), but the protected ones cannot be unprotected, either erased or

modified.

Depending on the RDP level, there is a possible workaround for recovering a protected chip.

If the STM32 is in RDP Level 1 and the RDP option byte is set to Level 0, the user’s Flash

area will be totally erased. This is the only case where the SPRMOD and nWRP bits may be

reset and all the protected sectors may be unprotected.

However, as this operation is always associated to the global erase of the user Flash area,

the code protection is not affected.

When the RDP is set to Level 2, all the option bytes are frozen and can no longer be

modified. As a result, the protected sectors never can be erased or modified, so the

protection becomes permanent.