Rainbow Electronics AT25256A - Manual (Page 7)

7
AT25128A/256A [Preliminary]
3404A–SEEPR–10/03
WRITE ENABLE (WREN): The device will power-up in the write disable state when V
CC
is applied. All programming instructions must therefore be preceded by a Write Enable
instruction.
WRITE DISABLE (WRDI): To protect the device against inadvertent writes, the Write
Disable instruction disables all programming modes. The WRDI instruction is indepen-
dent of the status of the WP pin.
READ STATUS REGISTER (RDSR): The Read Status Register instruction provides
access to the status register. The READY/BUSY and Write Enable status of the device
can be determined by the RDSR instruction. Similarly, the Block Write Protection
bits indicate the extent of protection employed. These bits are set by using the WRSR
instruction.
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 pro-
tection levels and corresponding status register control bits are shown in Table 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, t
WC
, RDSR).
Table 2. Status Register Format
Bit 7 Bit 6 Bit 5 Bit 4 Bit 3 Bit 2 Bit 1 Bit 0
WPEN X X X BP1 BP0 WEN RDY
Table 3. Read Status Register Bit Definition
Bit Definition
Bit 0 (RDY) Bit 0 = 0 (RDY) indicates the device is READY.
Bit 0 = 1 indicates the write cycle is in progress.
Bit 1 (WEN) Bit 1 = 0 indicates the device is not WRITE ENABLED. Bit 1 = 1 indicates
the device is WRITE ENABLED.
Bit 2 (BP0) See Table 4.
Bit 3 (BP1) See Table 4.
Bits 4 - 6 are 0s when device is not in an internal write cycle.
Bit 7 (WPEN) See Table 5.
Bits 0 - 7 are 1s during an internal write cycle.
Table 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