NXP Semiconductors
FXTH87E
FXTH87E, Family of Tire Pressure Monitor Sensors
FXTH87ERM
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Reference manual
Rev. 5.0 — 4 February 2019
32 / 183
f
BUS
PRDIV8
(Binary)
DIV5:DIV0
(Decimal)
f
FCLK
Program/Erase Timing Pulse
(5 μs Min, 6.7 μs Max)
150 kHz
0
0
150 kHz
6.7 μs
6.9.2 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 17. FLASH options register (FOPT) (address $1821)
Bit
7
6
5
4
3
2
1
0
R
KEYEN
FNORED
0
0
0
0
SEC1
SEC0
W
Reset
This register is loaded from nonvolatile location NVOPT during reset.
= Reserved
Table 18. 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 information
about the backdoor key mechanism, refer to
Section 6.8 "Security"
.
0 No backdoor key access allowed
1 If user firmware writes an 8-byte value that matches the nonvolatile backdoor key (NVBACKKEY through
NV7 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
SEC[1:0]
Security State Code — This 2-bit field determines the security state of the MCU as shown in
Table 19
.
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 to
Section 6.8 "Security"
. SEC[1:0] changes to 1 0 after successful backdoor key entry
or a successful blank check of FLASH.
Table 19. Security states
SEC[1:0]
Description
0 0
secure
0 1
secure
1 0
unsecured
1 1
secure
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