
Chapter 1 MC9S12Q Device Overview (MC9S12Q128-Family)
58
MC9S12Q128
Freescale Semiconductor
Rev 1.10
For further explanation on the modes refer to the S12_MEBI block guide.
1.5.2
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,
Operation in single-chip mode,
Operation from external memory with internal FLASH 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.
1.5.2.1
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.
Table 1-7. Mode Selection
BKGD =
MODC
PE6 =
MODB
PE5 =
MODA
PP6 =
ROMCTL
ROMON
Bit
Mode Description
00
0
X
1
Special Single Chip, BDM allowed and ACTIVE. BDM is allowed in
all other modes but a serial command is required to make BDM
active.
00
1
0
1
Emulation Expanded Narrow, BDM allowed
10
0
1
0
X
0
Special Test (Expanded Wide), BDM allowed
01
1
0
1
Emulation Expanded Wide, BDM allowed
10
1
0
X
1
Normal Single Chip, BDM allowed
10
1
0
Normal Expanded Narrow, BDM allowed
11
0
X
1
Peripheral; BDM allowed but bus operations would cause bus
conicts (must not be used)
11
1
0
Normal Expanded Wide, BDM allowed
11
Table 1-8. Clock Selection Based on PE7
PE7 = XCLKS
Description
1
Colpitts Oscillator selected
0
Pierce Oscillator/external clock selected