Memory protection strategy

PM0054

14/28

Doc ID 15433 Rev 9

4.1

Readout protection

On low density STM8L101x microcontrollers, the readout protection is:

●

enabled by writing 0xAA in the ROP option byte

●

disabled by reprogramming the ROP option byte with any value except 0xAA and

resetting the device.

On low density STM8L05x/15x, medium density STM8L05x/15x and

STM8AL31xx/STM8AL3Lxx and high density STM8L05x/15x/16x microcontrollers, the

readout protection is:

●

enabled by writing any value except 0xAA in the ROP option byte

●

disabled by reprogramming the ROP option byte with 0xAA, and resetting the device.

To unprotect the device, the ROP must be written twice: the first write, with any value,

launches a global erase which includes the option byte; the second write, 0xAA,

unprotects the device, and then the 0xAA value remains in the ROP byte.

Please note the EOP should be checked each time.

Readout protection can only be disabled in ICP/SWIM mode.

When readout protection is selected, reading or modifying the Flash program memory in

ICP mode (using the SWIM interface) is forbidden. When available, the data EEPROM

memory is also protected against read and write access through ICP.

Erasing the ROP option byte to disable the readout protection causes the Flash program

memory, the DATA area and the option bytes to be erased.

Even though no protection can be considered as totally unbreakable, the readout protection

feature provides a very high level of protection for general purpose microcontrollers. Of

course, software that dumps the Flash program memory content makes this readout

protection useless. Table 10 describes possible accesses to each memory areas versus the

different modes and readout protection settings.

4.2

Proprietary code area protection

The memory pages containing peripherals software libraries must be located in the

proprietary code area (PCODE).

The size of the PCODE area can be configured through the PCODE option byte

(PCODESIZE) in ICP/SWIM mode. Refer to Table 7 for details on PCODE minimum and

maximum size. Once programmed, the PCODE option byte cannot be erased, the size of

the PCODE area remains fixed and its content protected from read and write operations

whatever the mode.

Table 7.

PCODE size

STM8 microcontroller family

Minimum size

Maximum size

Low density STM8L05x/15x

3 pages (192 bytes)

127 pages (last page always left free for

main program)

Medium+ density STM8L15x

1 page (256 bytes)

128 pages

High density STM8L05x/15x/16x

1 page (256 bytes)

255 pages (last page always left free for

main program)