mc9s12e256 Freescale Semiconductor, Inc, mc9s12e256 Datasheet - Page 73

• Manufacturer Part Number: mc9s12e256
• Description: Hcs12 Microcontrollers 16-bit Microcontroller
• Manufacturer: Freescale Semiconductor, Inc

1.7
The device will make available a security feature preventing the unauthorized read and write of the
memory contents. This feature allows:
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.
1.7.1
Once the user has programmed the FLASH, 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
configuration.
1.7.2
1.7.2.1
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.
1.7.2.2
The user may wish to execute from external space with a secured microcontroller. This is accomplished
by resetting directly into expanded mode. The internal FLASH will be disabled. BDM operations will be
blocked.
1.7.3
In order to unsecure the microcontroller, the internal FLASH must be erased. This can be done through an
external program in expanded mode.
Once the user has erased the FLASH, the part can be reset into special single chip mode. This invokes a
program that verifies the erasure of the internal FLASH. Once this program completes, the user can erase
and program the FLASH security bits to the unsecured state. This is generally done through the BDM, but
the user could also change to expanded mode (by writing the mode bits through the BDM) and jumping to
Freescale Semiconductor
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Chapter 2, “256 Kbyte Flash Module (FTS256K2V1)”
Protection of the contents of FLASH,
Operation in single-chip mode,
Operation from external memory with internal FLASH disabled.
Security
Securing the Microcontroller
Operation of the Secured Microcontroller
Unsecuring the Microcontroller
Normal Single Chip Mode
Executing from External Memory
MC9S12E256 Data Sheet, Rev. 1.08
Chapter 1 MC9S12E256 Device Overview (MC9S12E256DGV1)
for more details on the security
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