AN2400 Freescale Semiconductor / Motorola, AN2400 Datasheet - Page 42

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

AN2400/D
NVM Security
Secured Operation
42
$FF0F
HCS12 microcontrollers offer a memory security feature. This security feature
is designed to prevent unauthorised access to the non-volatile memory. Note
that memory security is not the same as memory protection, which is designed
to prevent accidental modification of the NVM and is discussed in section
Memory
The memory contents are secured by programming the security bits within the
Flash Options/Security byte at address $FF0F. The Flash Options/Security
byte is located within the Flash memory at address $FF0F and is erased and
programmed like any other Flash location. On devices that have a memory
page window, the Flash Options/Security byte is also available at address
$BF0F by selecting page $3F with the PPAGE register. The contents of this
byte are copied into the Flash Security Register (FSEC) during a reset
sequence.
The Flash sector $FE00 to $FFFF must be erased before the Flash
Options/Security byte is programmed. The Flash is programmed by aligned
word only, so address $FF0E must be written as the word address to be
programmed, to program the Flash Options/Security byte. The Flash
Options/Security byte can only be erased or programmed when this sector is
not protected (see
KEYEN[1:0] – Backdoor Key Enable Bits
The KEYEN[1:0] bits define the enabling of the Backdoor Key access, as
shown in
Freescale Semiconductor, Inc.
KEYEN1 KEYEN0
For More Information On This Product,
7
Protection.
Table 8. Backdoor Key
Go to: www.freescale.com
HCS12 NVM Guidelines
6
Figure 23. Flash Options/Security Byte
Flash Memory
KEYEN[1:0]
Table 8. Backdoor Key States
NV5
5
00
01
10
11
NV4
4
Protection).
States.
Description
NV3
Disabled
Disabled
Disabled
Enabled
3
NV2
2
SEC1
1
MOTOROLA
SEC0
Flash
0