Datasheet

Memory
MC9S08GT16A/GT8A Data Sheet, Rev. 1
Freescale Semiconductor 57
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
f
Bus
PRDIV8
(Binary)
DIV5:DIV0
(Decimal)
f
FCLK
Program/Erase Timing Pulse
(5 µs Min, 6.7 µs Max)
20 MHz 1 12 192.3 kHz 5.2 µs
10 MHz 0 49 200 kHz 5 µs
8 MHz 0 39 200 kHz 5 µs
4 MHz 0 19 200 kHz 5 µs
2 MHz 0 9 200 kHz 5 µs
1 MHz 0 4 200 kHz 5 µs
200 kHz 0 0 200 kHz 5 µs
150 kHz 0 0 150 kHz 6.7 µs
76543210
R
KEYEN FNORED
0000SEC01 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 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
information about the backdoor key mechanism, refer to Section 4.5, “Security.”
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.