RM0016 datasheet(44/467 Pages) STMICROELECTRONICS

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Flash program memory and data EEPROM

RM0016

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DocID14587 Rev 14

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. Furthermore, on medium and high density STM8S and STM8AF, the debug

module (DM) cannot start code execution by the CPU when the readout protection is active,

and the CPU is stalled.

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.

Removing the readout protection

The readout protection can be disabled on the program memory, UBC 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.

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 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 4.4.3:

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 4.8.6: 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)

The following steps are required to disable write protection of the main program area:

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