8

AT25128A/256A

3368H–SEEPR–8/05

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 protec-

tion levels and corresponding status register control bits are shown in Table 8.

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, t

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 hard-

ware 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

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 (see Table 10 on

page 9). 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.

Table 8. 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 9. WPEN Operation

WPEN

WP

WEN

Protected

Blocks

Unprotected

Blocks

Status

Register

0

X

0

Protected

Protected

Protected

0X

1

Protected

Writable

Writable

1

Low

0

Protected

Protected

Protected

1Low

1

Protected

Writable

Protected

X

High

0

Protected

Protected

Protected

X

High

1

Protected

Writable

Writable