Datasheet

DocID030584 Rev 2 13/84
STM8S001J3 Functional overview
23
There are two levels of write protection. The first level is known as MASS (memory access
security system). MASS is always enabled and protects the main Flash program memory,
data EEPROM and option bytes.
To perform in-application programming (IAP), this write protection can be removed by
writing a MASS key sequence in a control register. This allows the application to modify the
content of main program memory and data EEPROM, or to reprogram the device option
bytes.
A second level of write protection, can be enabled to further protect a specific area of
memory known as UBC (user boot code). Refer to
Figure 2.
The size of the UBC is programmable through the UBC option byte (Table 12), in increments
of 1 page (64-byte block) by programming the UBC option byte in ICP mode.
This divides the program memory into two areas:
• Main program memory: 8 Kbyte minus UBC
• User-specific boot code (UBC): Configurable up to 8 Kbyte
The UBC area remains write-protected during in-application programming. This means that
the MASS keys do not unlock the UBC area. It protects the memory used to store the boot
program, specific code libraries, reset and interrupt vectors, the reset routine and usually
the IAP and communication routines.
Figure 2. Flash memory organization
Read-out protection (ROP)
The read-out protection blocks reading and writing the Flash program memory and data
EEPROM memory in ICP mode (and debug mode). Once the read-out protection is
activated, any attempt to toggle its status triggers a global erase of the program memory.
Even if no protection can be considered as totally unbreakable, the feature provides a very
high level of protection for a general purpose microcontroller.
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