56F8335 datasheet(115/160 Pages) FREESCALE

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Operation with Security Enabled

56F8335 Technical Data, Rev. 1

Freescale Semiconductor

115

Preliminary

Part 7 Security Features

The 56F8335/56F8135 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 the core EOnCE debug capabilities. Normal

program execution is otherwise unaffected.

7.2 Flash Access Blocking Mechanisms

The 56F8335/56F8135 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:

•

Unsecured Mode

•

Secure Mode (EOnCE disabled)

When Flash security is enabled as described in the Flash Memory module specification, the device will

disable the EOnCE debug interface.

56F8335 Datasheet(PDF) 115 Page - Freescale Semiconductor, Inc