MC9S12XD256MAL Datasheet, PDF (1233/1348 Pages) Freescale Semiconductor, Inc – Freescale’s Scalable Controller Area Network

30.1.5 Unsecuring the Microcontroller
Unsecuring the microcontroller can be done by three different methods:
1. Backdoor key access
2. Reprogramming the security bits
3. Complete memory erase (special modes)
Chapter 30 Security (S12X9SECV2)
30.1.5.1 Unsecuring the MCU Using the Backdoor Key Access
In normal modes (single chip and expanded), security can be temporarily disabled using the backdoor key
access method. This method requires that:
• The backdoor key at 0xFF00–0xFF07 (= global addresses 0x7F_FF00–0x7F_FF07) has been
programmed to a valid value.
• The KEYEN[1:0] bits within the Flash options/security byte select ‘enabled’.
• In single chip mode, the application program programmed into the microcontroller must be
designed to have the capability to write to the backdoor key locations.
The backdoor key values themselves would not normally be stored within the application data, which
means the application program would have to be designed to receive the backdoor key values from an
external source (e.g. through a serial port). It is not possible to download the backdoor keys using
background debug mode.
The backdoor key access method allows debugging of a secured microcontroller without having to erase
the Flash. This is particularly useful for failure analysis.
NOTE
No word of the backdoor key is allowed to have the value 0x0000 or
0xFFFF.
30.1.5.2 Backdoor Key Access Sequence
These are the necessary steps for a successful backdoor key access sequence:
1. Set the KEYACC bit in the Flash configuration register FCNFG.
2. Write the first 16-bit word of the backdoor key to 0xFF00 (0x7F_FF00).
3. Write the second 16-bit word of the backdoor key to 0xFF02 (0x7F_FF02).
4. Write the third 16-bit word of the backdoor key to 0xFF04 (0x7F_FF04).
5. Write the fourth 16-bit word of the backdoor key to 0xFF06 (0x7F_FF06).
6. Clear the KEYACC bit in the Flash Configuration register FCNFG.
NOTE
Flash cannot be read while KEYACC is set. Therefore the code for the
backdoor key access sequence must execute from RAM.
Freescale Semiconductor
MC9S12XDP512 Data Sheet, Rev. 2.21
1235