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8593C–SEEPR–8/09
AT25128B/256B [Preliminary]
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 AT25128B/256B via the SO pin requires the following 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 (Table 2-6). Upon completion, 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 automatically incremented and data will continue to be shifted out. When the highest address is
reached, the address counter will roll over to the lowest address allowing the entire memory to be read in
one continuous read cycle.
WRITE SEQUENCE (WRITE): In order to program the AT25128B/256B, two separate instructions must be
executed. First, the device must be write enabled via the Write Enable (WREN) Instruction. Then a Write
instruction may be executed. Also, the address of the memory location(s) to be programmed must be
outside the protected address field location selected by the Block Write Protection Level. During an internal
write cycle, all commands will be ignored except the RDSR instruction.
A Write Instruction requires the following sequence. After the CS line is pulled low to select the
device, the Write op-code is transmitted via the SI line followed by the byte address and the data
Table 2-4. Block Write Protect Bits
Level Status Register Bits Array Addresses Protected
BP1 BP0 AT25128B AT25256B
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 2-5. 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