Motorola MC9S12GC-Family - Manual (Page 67)

Device User Guide — 9S12C128DGV1/D V01.05
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of this is the user downloads a key through the SCI which allows access to a programming routine that
updates parameters.
4.3.1 Securing the Microcontroller
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 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.
4.3.2.2 Executing from External Memory
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.
4.3.3 Unsecuring the Microcontroller
In order to unsecure the microcontroller, the internal FLASH must be erased. This can be done through an
external program in expanded mode or via a sequence of BDM commands. Unsecuring is also possible via
the Backdoor Key Access. Refer to Flash Block Guide for details.
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
an external program (again through BDM commands). Note that if the part goes through a reset before the
security bits are reprogrammed to the unsecure state, the part will be secured again.
4.4 Low Power Modes
The microcontroller features three main low power modes. Consult the respective Block User Guide for
information on the module behavior in Stop, Pseudo Stop, and Wait Mode. An important source of
information about the clock system is the Clock and Reset Generator User Guide (CRG).