AN2153 Freescale Semiconductor / Motorola, AN2153 Datasheet - Page 8

• Manufacturer Part Number: AN2153
• Description: A Serial Bootloader for Reprogramming the MC9S12DP256 FLASH Memory
• Manufacturer: Freescale Semiconductor / Motorola

Freescale Semiconductor, Inc.
Application Note
allowed a customer’s firmware to disable the BDM interface (BDM
lockout) once the part had been placed in the circuit and programmed.
While this prevents the FLASH and EEPROM from being easily
accessed in-circuit, it does not prevent a D Family part from being
removed from the circuit and placed in expanded mode so the FLASH
and EEPROM can be read.
The security features of the MC9S12DP256 have been greatly
enhanced. While no security feature can be 100 percent guaranteed to
prevent access to an MCU’s internal resources, the MC9S12DP256’s
security mechanism makes it extremely difficult to access the FLASH or
EEPROM contents. Once the security mechanism has been enabled,
access to the FLASH and EEPROM either through the BDM or the
expanded bus is inhibited. Gaining access to either of these resources
may be accomplished only by erasing the contents of the FLASH and
EEPROM or through a built-in back door mechanism. While having a
back door mechanism may seem to be a weakness of the security
mechanism, the target application must specifically support this feature
for it to operate.
Erasing the FLASH or EEPROM can be accomplished using one of two
methods. The first method requires resetting the target MCU in special
single-chip mode and using the BDM interface. When a secured device
is reset in special single-chip mode, a special BDM security ROM
becomes active. The program in this small ROM performs a blank check
of the FLASH and EEPROM memories. If both memory spaces are
erased, the BDM firmware temporarily disables device security, allowing
full BDM functionally. However, if the FLASH or EEPROM are not blank,
security remains active and only the BDM hardware commands remain
functional. In this mode, the BDM commands are restricted to reading
and writing the I/O register space. Because all other BDM commands
and on-chip resources are disabled, the contents of the FLASH and
EEPROM remain protected. This functionality is adequate to manipulate
the FLASH and EEPROM control registers to erase their contents.
NOTE:
Use of the BDM interface to erase the FLASH and EEPROM memories
is not present in the initial mask set (0K36N) of the MC9S12DP256.
Great care must be exercised to ensure that the microcontroller is not
programmed in a secure state unless the back door mechanism is
supported by the target firmware.
AN2153
8
MOTOROLA
For More Information On This Product,
Go to: www.freescale.com