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CHAPTER 35 ROM/FLASH SECURITY
■ ROM security register
Table 35.2-2 ROM security register FMSEC
Bit name
bit 0 -
bit 7
• The FMSEC register is reset to 00
• The FMSEC register is programmed by the Boot ROM program after release of the reset. After release of
Software, Watchdog or Clock Stop Detection reset, the code security sequence of the Boot ROM program
is skipped.
• After setting FMSEC the bootrom sets the MCR:MPDD bit which locks the write access to the FMSEC
register. This is necessary to ensure that the user program can not accidentally lock the ROM later.
• The initial value of the register after main reset does not allow access to the ROM.
• Only setting FMSEC to 55
• As long as the ROM is locked the following limitations apply
_ 0000
transferred to the output only through a "window" read access to the FMSEC register. After setting
FMSEC 00
_ Storing output data from ROM in the read buffers is disabled
900
Figure 35.2-2 ROM security register
7
6
5
4
FMSEC7 FMSEC6 FMSEC5 FMSEC4 FMSEC3
R/W R/W R/W R/W R/W
FMSEC0 to
•
These bits control the code security (read protection) of the ROM.
FMSEC7
•
When these bits are set to 55
•
When these bits are set to any other value, any reading of the ROM
returns 0000
•
The bits can only be reset by power on or external reset.
•
A CPU write command can only set bits from '0' to '1'.
•
Writing to the register is only possible as long as MCR:MPDD is '0'
to prevent the user program from accidentally lock the ROM by wild
running software but still offer the Boot ROM program full control over
the register.
•
The return value of a read access depends on the register contents
itself:
•
for FMSEC 00
•
for FMSEC = 00
put. This is used for the ROM security fetch. A ROM read ac-
cess is initiated automatically when reading the FMSEC
register, independently of the FMSEC register contents.
by a power on or external reset.
H
allows access (unlocked).
H
is driven to the read data output during a ROM CPU read access. ROM output data can be
H
this window is closed.
H
MB96300 Super Series Hardware Manual
3
2
1
0
Initial value
00000000
FMSEC2 FMSEC1 FMSEC0
:
Address
R/W R/W R/W
0003F0
H
Function
, full access to ROM is possible.
H
.
H
, the value of the FMSEC register is returned.
H
, the value of LSByte of the data read is out-
H
B
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