
Chapter 4 Memory
MC9S08AC16 Series Data Sheet, Rev. 9
58
Freescale Semiconductor
4.6.2
FLASH Options Register (FOPT and NVOPT)
During reset, the contents of the nonvolatile location NVOPT are copied from FLASH into FOPT. Bits 5
through 2 are not used and always read 0. This register may be read at any time, but writes have no meaning
or effect. To change the value in this register, erase and reprogram the NVOPT location in FLASH memory
as usual and then issue a new MCU reset.
Table 4-7. FLASH Clock Divider Settings
fBus
PRDIV8
(Binary)
DIV5:DIV0
(Decimal)
fFCLK
Program/Erase Timing Pulse
(5
s Min, 6.7s Max)
20 MHz
1
12
192.3 kHz
5.2
s
10 MHz
0
49
200 kHz
5
s
8 MHz
0
39
200 kHz
5
s
4 MHz
0
19
200 kHz
5
s
2 MHz
0
9
200 kHz
5
s
1 MHz
0
4
200 kHz
5
s
200 kHz
0
200 kHz
5
s
150 kHz
0
150 kHz
6.7
s
76
54
321
0
R
KEYEN
FNORED
0
SEC01
SEC00
W
Reset
This register is loaded from nonvolatile location NVOPT during reset.
= Unimplemented or Reserved
Figure 4-6. FLASH Options Register (FOPT)
Table 4-8. FOPT Register Field Descriptions
Field
Description
7
KEYEN
Backdoor Key Mechanism Enable — When this bit is 0, the backdoor key mechanism cannot be used to
disengage security. The backdoor key mechanism is accessible only from user (secured) firmware. BDM
commands cannot be used to write key comparison values that would unlock the backdoor key. For more detailed
0 No backdoor key access allowed.
1 If user firmware writes an 8-byte value that matches the nonvolatile backdoor key (NVBACKKEY through
NVBACKKEY+7 in that order), security is temporarily disengaged until the next MCU reset.
6
FNORED
Vector Redirection Disable — When this bit is 1, then vector redirection is disabled.
0 Vector redirection enabled.
1 Vector redirection disabled.
1:0
SEC0[1:0]
Security State Code — This 2-bit field determines the security state of the MCU as shown in
Table 4-9. When
the MCU is secure, the contents of RAM and FLASH memory cannot be accessed by instructions from any
unsecured source including the background debug interface. For more detailed information about security, refer