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56F8356 Technical Data
Preliminary
Part 7 Security Features
The 56F8356 offers security features intended to prevent unauthorized users from reading the
contents of the Flash memory (FM) array. The 56F8356’s 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 56F8356 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 56F8356
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 has 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
At boot time, the SIM determines in which functional modes the 56F8356 will operate. These are:
Forced Operating Mode Selection
Internal Boot Mode
External Boot Mode
Secure Mode
When Flash security is enabled as described in the Flash Memory module specification, the
56F8356 will boot in internal boot mode, disable all access to external P-space, and start executing
code from the Boot Flash at address 0x02_0000.
This security affords protection only to applications in which the 56F8356 operates in internal
Flash security mode. Therefore, the security feature cannot be used unless all executing code
resides on-chip.
When security is enabled, any attempt to override the default internal operating mode by asserting
the EXTBOOT pin in conjunction with reset will be ignored.
F
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