PM0212

Memory protection strategy

Doc ID 022351 Rev 2

9/22

3.1

Readout protection

On STM8TL5xxx microcontrollers, the readout protection is set by writing any value except

for 0xAA in the ROP option byte. It is disabled by reprogramming the ROP option byte with

0xAA, and resetting the device. To unprotect the device, the ROP must be written 2 times:

the first writing, with any value, launches a global erase which includes the option byte; the

second writing, AA, unprotects the device, and then the AA value remains in the ROP byte.

Please note the EOP should be checked each time.

The readout protection can only be disabled in ICP/SWIM mode.

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

ICP mode (using the SWIM interface) is forbidden. 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, a software that allows the user to dump the Flash program memory content make

this readout protection useless. Table 5: Memory access versus programming method

describes possible accesses to each memory areas versus the different modes and readout

protection settings.

3.2

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 2 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.

Note:

Values 1 and 2 do not protect any address, but the PCODE option byte is write-protected.

Except for the interrupt vectors which can be read directly, the PCODE area can be read

only through TRAP interrupt in ICP/SWIM (with readout protection disabled), user and IAP

mode.

Table 2.

PCODE size

PCODESIZE[0:7]

Interrupt vector table

Protected page

Protected addresses

0

All interrupt vectors can be

written

0None

3

Only TRAP is write-protected

1

0x8080-0x80BF

4

Only TRAP is write-protected

2

0x8080-0x80FF

...

0xFF

Only TRAP is write-protected

253

0x8080-0xBFBF