M95256-RBN6T ST Microelectronics, M95256-RBN6T Datasheet - Page 7

• Manufacturer Part Number: M95256-RBN6T
• Description: 256/128 Kbit Serial SPI Bus EEPROM With High Speed Clock
• Manufacturer: ST Microelectronics

Table 6. Write Protected Block Size
The size of the write-protection area applies
equally in SPM and HPM. The BP1 and BP0 bits
of the status register have the appropriate value
(see Table 6) written into them after the contents
of the protected area of the EEPROM have been
written.
The initial delivery state of the BP1 and BP0 bits is
00, indicating a write-protection size of 0.
Software Protected Mode (SPM)
The act of writing a non-zero value to the BP1 and
BP0 bits causes the Software Protected Mode
(SPM) to be started. All attempts to write a byte or
page in the protected area are ignored, even if the
Write Enable Latch is set. However, writing is still
allowed in the unprotected area of the memory
array and to the SRWD, BP1 and BP0 bits of the
status register, provided that the WEL bit is first
set.
Hardware Protected Mode (HPM)
The Hardware Protected Mode (HPM) offers a
higher level of protection, and can be selected by
setting the SRWD bit after pulling down the W pin
or by pulling down the W pin after setting the
SRWD bit. The SRWD is set by the WSR
instruction, provided that the WEL bit is first set.
Figure 8. WRSR: Write Status Register Sequence
Status Register Bits
BP1
0
0
1
1
BP0
S
C
D
Q
0
1
0
1
0
Protected Block
Whole memory
Upper quarter
1
Upper half
HIGH IMPEDANCE
INSTRUCTION
none
2
3
4
5
6
7
MSB
7
8
6
The setting of the SRWD bit can be made
independently of, or at the same time as, writing a
new value to the BP1 and BP0 bits.
Once the device is in the Hardware Protected
Mode, the data bytes in the protected area of the
memory array, and the content of the status
register, are write-protected. The only way to re-
enable writing new values to the status register is
to pull the W pin high. This cause the device to
leave the Hardware Protected Mode, and to revert
to being in the Software Protected Mode. (The
value in the BP1 and BP0 bits will not have been
changed).
Further details of the operation of the Write Protect
pin (W) is given earlier, on page 3.
Typical Use of HPM and SPM
The W pin can be dynamically driven by an output
port of a microcontroller. It is also possible,
though, to connect it permanently to V
solder connection, or through a pull-down
resistor). The manufacturer of such a printed
circuit board can take the memory device, still in its
initial delivery state, and can solder it directly on to
the board. After power on, the microcontroller can
be instructed to write the protected data into the
9 10 11 12 13 14 15
STATUS REG.
5
6000h - 7FFFh
4000h - 7FFFh
0000h - 7FFFh
4
M95256
none
3
Array Addresses Protected
2
1
0
AI02282
3000h - 3FFFh
2000h - 3FFFh
0000h - 3FFFh
M95256, M95128
M95128
none
SS
(by a
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