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3368J–SEEPR–06/07
AT25128A_256A
WRITE STATUS REGISTER (WRSR): The WRSR instruction allows the user to select one of
four levels of protection. The AT25128A/256A is divided into four array segments. Top quarter
(1/4), top half (1/2), or all of the memory segments can be protected. Any of the data within any
selected segment will therefore be read only. The block write protection levels and correspond-
ing status register control bits are shown in Table 3-4.
The three bits, BP0, BP1, and WPEN are nonvolatile cells that have the same properties and
functions as the regular memory cells (e.g. WREN, tWC, RDSR).
The WRSR instruction also allows the user to enable or disable the write protect (WP) pin
through the use of the write protect enable (WPEN) bit. Hardware write protection is enabled
when the WP pin is low and the WPEN bit is “1”. Hardware write protection is disabled when
either the WP pin is high or the WPEN bit is “0”. When the device is hardware write protected,
writes to the Status Register, including the Block Protect bits and the WPEN bit, and the block-
protected sections in the memory array are disabled. Writes are only allowed to sections of the
memory which are not block-protected.
NOTE: When the WPEN bit is hardware write protected, it cannot be changed back to “0”, as
long as the WP pin is held low.
READ SEQUENCE (READ): Reading the AT25128A/256A via the SO pin requires the follow-
ing sequence. After the CS line is pulled low to select a device, the Read op-code is transmitted
via the SI line followed by the byte address to be read (see Table 3-6 on page 11). Upon com-
pletion, any data on the SI line will be ignored. The data (D7 - D0) at the specified address is
then shifted out onto the SO line. If only one byte is to be read, the CS line should be driven high
after the data comes out. The read sequence can be continued since the byte address is auto-
matically incremented and data will continue to be shifted out. When the highest address is
Table 3-4. Block Write Protect Bits
Level
Status Register Bits Array Addresses Protected
BP1 BP0 AT25128A AT25256A
0 0 0 None None
1(1/4) 0 1 3000 – 3FFF 6000 – 7FFF
2(1/2) 1 0 2000 – 3FFF 4000 – 7FFF
3(All) 1 1 0000 – 3FFF 0000 – 7FFF
Table 3-5. WPEN Operation
WPEN WP WEN
Protected
Blocks
Unprotected
Blocks
Status
Register
0 X 0 Protected Protected Protected
0 X 1 Protected Writable Writable
1 Low 0 Protected Protected Protected
1 Low 1 Protected Writable Protected
X High 0 Protected Protected Protected
X High 1 Protected Writable Writable