XC886/888CLM
Memory Organization
User’s Manual
3-19
V1.3, 2010-02
Memory Organization, V 1.2
3.5.4.1
Bit Protection Scheme
The bit protection scheme prevents direct software writing of selected bits (i.e., protected
bits) using the PASSWD register. When the bit field MODE is 11
B
, writing 10011
B
to the
bit field PASS opens access to writing of all protected bits, and writing 10101
B
to the bit
field PASS closes access to writing of all protected bits. In both cases, the value of the
bit field MODE is not changed even if PASSWD register is written with 98
H
or A8
H
. It can
only be changed when bit field PASS is written with 11000
B
, for example, writing D0
H
to
PASSWD register disables the bit protection scheme.
Note that access is opened for maximum 32 CCLKs if the “close access” password is not
written. If “open access” password is written again before the end of 32 CCLK cycles,
there will be a recount of 32 CCLK cycles. The protected bits include the N- and K-
Divider bits, NDIV and KDIV; the Watchdog Timer enable bit, WDTEN; and the power-
down and slow-down enable bits, PD and SD.
PASSWD
Password Register
Reset Value: 07
H
7
6
5
4
3
2
1
0
PASS
PROTECT
_S
MODE
wh
rh
rw
Field
Bits
Type Description
MODE
[1:0]
rw
Bit Protection Scheme Control bits
00
Scheme disabled - direct access to the
protected bits is allowed.
11
Scheme enabled - the bit field PASS has to be
written with the passwords to open and close
the access to protected bits. (default)
Others: Scheme enabled
These two bits cannot be written directly. To change
the value between 11
B
and 00
B
, the bit field PASS
must be written with 11000
B
; only then, will the
MODE[1:0] be registered.
PROTECT_S
2
rh
Bit Protection Signal Status bit
This bit shows the status of the protection.
0
Software is able to write to all protected bits.
1
Software is unable to write to any protected
bits.
*