X25256 – Preliminary Information
can be read sequentially by continuing to provide clock
pulses. The address is automatically incremented to the
To read the status register the CS line is first pulled
LOW to select the device followed by the 8-bit RDSR
instruction. After the RDSR opcode is sent, the contents of
the status register are shifted out on the SO line.
next higher address after each byte of data is shifted out.
When the highest address is reached ($7FFF) the
Figure 3 illustrates the read status register sequence.
address counter rolls over to address $0000 allowing the
read cycle to be continued indefinitely. The read operation
is terminated by taking CS HIGH. Refer to the read E
2PROM array operation sequence illustrated in Figure 2.
Memory Array
Block Protected
Block Lock
Bits
Memory Array Not
Block Protected
WP
HIGH
LOW
LOW
WPEN
WPEN Bit
Writable
Protection
Software
X
0
1
Writable
Writable
Writable
Blocked
Writable
Blocked
Writable
Writable
Software
Blocked
Writes Blocked
Writes Blocked
Hardware
Write Sequence
While the write is in progress following a status register or
E
2
PROM write sequence, the status register may be
read to check the WIP bit. During this time the WIP bit will
be HIGH.
Prior to any attempt to write data into the X25256, the
“write enable” latch must first be set by issuing the
WREN instruction (See Figure 4). CS is first taken
LOW, then the WREN instruction is clocked into the
Hold Operation
X25256. After all eight bits of the instruction are trans-
mitted, CS must then be taken HIGH. If the user con-
The HOLD input should be HIGH (at V
IH
) under normal
operation. If a data transfer is to be interrupted HOLD
can be pulled LOW to suspend the transfer until it can
tinues the write operation without taking CS HIGH after
issuing the WREN instruction, the write operation will
be ignored.
be resumed. The only restriction is the SCK input must be
LOW when HOLD is first pulled LOW and SCK must
also be LOW when HOLD is released.
To write data to the 2E
PROM memory array, the user
issues the WRITE instruction, followed by the address
and then the data to be written. This is minimally a thirty-
The HOLD input may be tied HIGH either directly to V
CC or tied to VCC through a resistor.
two clock operation. CS must go LOW and remain LOW for
the duration of the operation. The host may continue
Operational Notes
to write up to 64 bytes of data to the X25256. The only
restriction is the 64 bytes must reside on the same
The X25256 powers-up in the following state:
page. If the address counter reaches the end of the
page and the clock continues, the counter will “roll over”
– The device is in the low power standby state.
– A HIGH to LOW transition on CS is required to enter
to the first address of the page and overwrite any data
that may have been written.
an active state and receive an instruction. –
SO pin is high impedance.
– The “write enable” latch is reset.
For the write operation (byte or page write) to be com-
pleted, CS can only be brought HIGH after bit 0 of data
byte N is clocked in. If it is brought HIGH at any other
time the write operation will not be completed. Refer to
Data Protection
Figures 5 and 6 below for a detailed illustration of the write
sequences and time frames in which CS going
The following circuitry has been included to prevent
inadvertent writes:
HIGH are valid.
– The “write enable” latch is reset upon power-up.
– A WREN instruction must be issued to set the “write
To write to the status register, the WRSR instruction is
followed by the data to be written. Data bits 0, 1, 5, and
6 are “don’t care”. Figure 7 illustrates this sequence.
enable” latch.
– CS must come HIGH at the proper clock count in
order to start a write cycle.
Characteristics subject to change without notice. 5 of 17
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