MC9S12DT256_06 Datasheet, PDF (72/132 Pages) Freescale Semiconductor, Inc – Device User Guide V03.07

MC9S12DT256 Device User Guide — V03.07
Table 4-2 Clock Selection Based on PE7
PE7 = XCLKS
Description
0
Pierce Oscillator/external clock selected
Table 4-3 Voltage Regulator VREGEN
VREGEN
Description
1
Internal Voltage Regulator enabled
Internal Voltage Regulator disabled, VDD1,2 and
0
VDDPLL must be supplied externally with 2.5V
4.3 Security
The device will make available a security feature preventing the unauthorized read and write of the
memory contents. This feature allows:
• Protection of the contents of FLASH,
• Protection of the contents of EEPROM,
• Operation in single-chip mode,
• Operation from external memory with internal FLASH and EEPROM disabled.
The user must be reminded that part of the security must lie with the user’s code. An extreme example
would be user’s code that dumps the contents of the internal program. This code would defeat the purpose
of security. At the same time the user may also wish to put a back door in the user’s program. An example
of this is the user downloads a key through the SCI which allows access to a programming routine that
updates parameters stored in EEPROM.
4.3.1 Securing the Microcontroller
Once the user has programmed the FLASH and EEPROM (if desired), the part can be secured by
programming the security bits located in the FLASH module. These non-volatile bits will keep the part
secured through resetting the part and through powering down the part.
The security byte resides in a portion of the Flash array.
Check the Flash Block User Guide for more details on the security configuration.
4.3.2 Operation of the Secured Microcontroller
4.3.2.1 Normal Single Chip Mode
This will be the most common usage of the secured part. Everything will appear the same as if the part was
not secured with the exception of BDM operation. The BDM operation will be blocked.
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