AN2400 Freescale Semiconductor / Motorola, AN2400 Datasheet - Page 43

• Manufacturer Part Number: AN2400
• Description: HCS12 NVM Guidelines
• Manufacturer: Freescale Semiconductor / Motorola

Normal Single Chip
Mode
Special Single Chip
Mode
Expanded Modes
MOTOROLA
NOTE:
Some older versions of HCS12 have only a single Backdoor Key Enable bit,
KEYEN, equivalent to KEYEN1. In this case, KEYEN = 0 means disabled and
KEYEN = 1 means enabled.
SEC[1:0] – Memory Security Bits
Secured operation takes effect on the next reset after programming the security
bits to a secure value. When enabled, secured operation has the following
effects on the microcontroller:
By these actions, unauthorised access to the EEPROM and Flash memory
contents can be prevented. However, it must be realised that the security of the
EEPROM and Flash memory contents also depends on the design of the
application program. For example, if the application has the capability of
downloading code through a serial port and then executing that code (e.g. an
application containing bootloader code), then this capability could potentially be
used to read the EEPROM and Flash memory contents even when the
microcontroller is in the secure state. In this example, the security of the
application could be enhanced by requiring a challenge/response
authentication before any code can be downloaded.
The SEC[1:0] bits define the security state of the microcontroller at reset as
shown in
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Freescale Semiconductor, Inc.
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Background Debug Module (BDM) operation is completely disabled.
Flash and EEPROM commands PROG, Mass Erase, Sector Erase,
Erase Verify and Sector Modify remain enabled.
BDM firmware commands are disabled.
BDM hardware commands are restricted to the register space.
Flash and EEPROM commands limited to MASS ERASE only.
BDM operation is completely disabled.
External access to internal Flash and EEPROM is disabled.
Internal visibility (IVIS) and CPU pipe (IPIPE) information is disabled.
Flash and EEPROM commands cannot be executed from external
memory in Normal Expanded mode.
Table 9. Security
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HCS12 NVM Guidelines
SEC[1:0]
00
01
10
11
Table 9. Security States
States.
Description
Unsecured
Secured
Secured
Secured
NVM Security
AN2400/D
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