X25040
PRINCIPLES OF OPERATION
The X25040 is a 512 x 8 E
2
PROM designed to interface
directly with the synchronous serial peripheral interface
(SPI) of many popular microcontroller families.
The X25040 contains an 8-bit instruction register. It is
accessed via the SI input, with data being clocked in on
the rising SCK.
CS
must be LOW and the
HOLD
and
WP
inputs must be HIGH during the entire operation.
Table 1 contains a list of the instructions and their
opcodes. All instructions, addresses and data are trans-
ferred MSB first.
Data input is sampled on the first rising edge of SCK after
CS
goes LOW. SCK is static, allowing the user to stop
the clock and then resume operations. If the clock line is
shared with other peripheral devices on the SPI bus, the
user can assert the
HOLD
input to place the X25040 into
a “PAUSE” condition. After releasing
HOLD,
the X25040
will resume operation from the point when
HOLD
was
first asserted.
Write Enable Latch
The X25040 contains a “write enable” latch. This latch
must be SET before a write operation will be completed
internally. The WREN instruction will set the latch and
the WRDI instruction will reset the latch. This latch is
automatically reset upon a power-up condition and after
the completion of a byte, page, or status register write
cycle.
Status Register
The RDSR instruction provides access to the status
register. The status register may be read at any time,
even during a write cycle. The status register is format-
ted as follows:
7
X
6
X
5
X
4
X
3
BP1
2
BP0
1
WEL
0
WIP
6451 PGM T02
BP0 and BP1 are set by the WRSR instruction. WEL
and WIP are read-only and automatically set by other
operations.
The Write-In-Process (WIP) bit indicates whether the
X25040 is busy with a write operation. When set to a “1”,
a write is in progress, when set to a “0”, no write is in
progress. During a write, all other bits are set to “1”.
The Write Enable Latch (WEL) bit indicates the status of
the “write enable” latch. When set to a “1”, the latch is set,
when set to a “0”, the latch is reset.
The Block Protect (BP0 and BP1) bits are nonvolatile
and allow the user to select one of four levels of protec-
tion. The X25040 is divided into four 1024-bit segments.
One, two, or all four of the segments may be protected.
That is, the user may read the segments but will be
unable to alter (write) data within the selected segments.
The partitioning is controlled as illustrated below.
Status Register Bits
BP1
BP0
0
0
1
1
0
1
0
1
Array Addresses
Protected
None
$180–$1FF
$100–$1FF
$000–$1FF
6451 PGM T03
Table 1. Instruction Set
Instruction Name
WREN
WRDI
RDSR
WRSR
READ
WRITE
Instruction Format*
0000 0110
0000 0100
0000 0101
0000 0001
0000 A
8
011
0000 A
8
010
Operation
Set the Write Enable Latch (Enable Write Operations)
Reset the Write Enable Latch (Disable Write Operations)
Read Status Register
Write Status Register
Read Data from Memory Array beginning at selected address
Write Data to Memory Array beginning at Selected Address
(1 to 32 Bytes)
6451 PGM T04.2
*Instructions are shown MSB in leftmost position. Instructions are transferred MSB first.
3
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