Datasheet

Operation with Security Enabled
56F8356 Technical Data, Rev. 13
Freescale Semiconductor 127
Preliminary
then followed by a 32 clock window in which peripherals are released to implement Flash security, and,
finally, followed by a 32 clock window in which the core is initialized. After completion of the described
reset sequence, application code will begin execution.
Resets may be asserted asynchronously, but are always released internally on a rising edge of the system
clock.
Part 7 Security Features
The 56F8356/56F8156 offer security features intended to prevent unauthorized users from reading the
contents of the Flash memory (FM) array. The Flash security consists of several hardware interlocks that
block the means by which an unauthorized user could gain access to the Flash array.
However, 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, as this code would defeat the purpose of security. At the same
time, the user may also wish to put a “backdoor” in his program. As an example, the user downloads a
security key through the SCI, allowing access to a programming routine that updates parameters stored in
another section of the Flash.
7.1 Operation with Security Enabled
Once the user has programmed the Flash with his application code, the device can be secured by
programming the security bytes located in the FM configuration field, which occupies a portion of the FM
array. These non-volatile bytes will keep the part secured through reset and through power-down of the
device. Only two bytes within this field are used to enable or disable security. Refer to the Flash Memory
section in the 56F8300 Peripheral User Manual for the state of the security bytes and the resulting state
of security. When Flash security mode is enabled in accordance with the method described in the Flash
Memory module specification, the device will disable external P-space accesses restricting code execution
to internal memory, disable EXTBOOT = 1 mode, and disable the core EOnCE debug capabilities. Normal
program execution is otherwise unaffected.
7.2 Flash Access Blocking Mechanisms
The 56F8356/56F8156 have several operating functional and test modes. Effective Flash security must
address operating mode selection and anticipate modes in which the on-chip Flash can be compromised
and read without explicit user permission. Methods to block these are outlined in the next subsections.
7.2.1 Forced Operating Mode Selection
At boot time, the SIM determines in which functional modes the device will operate. These are:
• Internal Boot Mode
• External Boot Mode