RM0312 datasheet(29/275 Pages) STMICROELECTRONICS

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RM0312

Flash program memory and data EEPROM

Refer to the option byte section in the datasheet for more information on option bytes, and to

the STM8 SWIM protocol and debug module user manual (UM0470) for details on how to

program them.

4.5

Memory protection

4.5.1

Readout protection

Readout protection is selected by programming the ROP option byte to 0xAA. When

readout protection is enabled, reading or modifying the Flash program memory and DATA

area in ICP mode (using the SWIM interface) is forbidden, whatever the write protection

settings.

Even if no protection can be considered as totally unbreakable, the readout feature provides

a very high level of protection for a general purpose microcontroller.

The readout protection can be disabled on the program memory, UBC, PCODE and DATA

areas, by reprogramming the ROP option byte in ICP mode. In this case, the Flash program

memory, the DATA area and the option bytes are automatically erased and the device can

be reprogrammed.

By default, the PCODE area is always readout protected except for the interrupt vector

TRAP (see Section 4.4.1: Proprietary code area (PCODE)).

Refer to Table 6: Memory access versus programming method for details on memory

access when readout protection is enabled or disabled.

4.5.2

Memory access security system (MASS)

After reset, the main program and DATA areas (when they exist) are protected against

unintentional write operations. They must be unlocked before attempting to modify their

content. This unlock mechanism is managed by the memory access security system

(MASS).

The UBC area specified in the UBC option byte is always write protected (see Section 4.4.2:

User boot area (UBC)).

Once the memory has been modified, it is recommended to enable the write protection

again to protect the memory content against corruption.

Enabling write access to the main program memory

After a device reset, it is possible to disable the main program memory write protection by

writing consecutively two values called MASS keys to the FLASH_PUKR register (see

Section 4.8.3: Flash program memory unprotecting key register (FLASH_PUKR)). These

programmed keys are then compared to two hardware key values:

•

First hardware key: 0b0101 0110 (0x56)

•

Second hardware key: 0b1010 1110 (0xAE)

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