Chapter 1 Device Overview MC9S12ZVHY/MC9S12ZVHL Families
S12ZVHY/S12ZVHL Family Reference Manual Rev. 1.05
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Freescale Semiconductor
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The application program programmed into the microcontroller has the capability to write to the
backdoor key locations.
The backdoor key values themselves would not normally be stored within the application data, which
means the application program would have to be designed to receive the backdoor key values from an
external source (e.g. through a serial port).
The backdoor key access method allows debugging of a secured microcontroller without having to erase
the Flash. This is particularly useful for failure analysis.
NOTE
No word backdoor key word is allowed to have the value 0x0000 or
0xFFFF.
1.10.5
Reprogramming the Security Bits
In normal single chip mode, security can also be disabled by erasing and reprogramming the security bits
within Flash options/security byte to the unsecured value. Because the erase operation will erase the entire
sector from (0xFF_FE00–0xFF_FFFF), the backdoor key and the interrupt vectors will also be erased; this
method is not recommended for normal single chip mode. The application software can only erase and
program the Flash options/security byte if the Flash sector containing the Flash options/security byte is not
protected (see Flash protection). Thus Flash protection is a useful means of preventing this method. The
microcontroller will enter the unsecured state after the next reset following the programming of the
security bits to the unsecured value.
This method requires that:
•
The application software previously programmed into the microcontroller has been designed to
have the capability to erase and program the Flash options/security byte.
•
The Flash sector containing the Flash options/security byte is not protected.
1.10.6
Complete Memory Erase
The microcontroller can be unsecured by erasing the entire EEPROM and Flash memory contents. If
ERASE_FLASH is successfully completed, then the Flash unsecures the device and programs the security
byte automatically.
1.11
Resets and Interrupts
1.11.1
Resets
Table 1-10. lists all reset sources and the vector locations. Resets are explained in detail in the
“S12 Clock, Reset and Power Management Unit (S12CPMU_UHV_V5)
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