X25040
Clock and Data Timing
Data input on the SI line is latched on the rising edge of
SCK. Data is output on the SO line by the falling edge of
SCK.
Read Sequence
When reading from the E
2
PROM memory array,
CS
is
first pulled LOW to select the device. The 8-bit READ
instruction is transmitted to the X25040, followed by the
8-bit address. Bit 3 of the Read Data instruction con-
tains address A
8
. This bit is used to select the upper or
lower half of the address. After the READ opcode and
address are sent, the data stored in the memory at the
selected address is shifted out on the SO line. The data
stored in memory at the next address can be read
sequentially by continuing to provide clock pulses. The
address is automatically incremented to the next higher
address after each byte of data is shifted out. When the
highest address is reached ($1FF) the address counter
rolls over to address $000 allowing the read cycle to be
continued indefinitely. The read operation is termi-
nated by taking
CS
HIGH. Refer to the read E
2
PROM
array operation sequence illustrated in Figure 1.
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 read status register opcode is
sent, the contents of the status register are shifted out
on the SO line. Figure 2 illustrates the read status
register sequence.
Write Sequence
Prior to any attempt to write data into the X25040, the
“write enable” latch must first be set by issuing the
WREN instruction (See Figure 3).
CS
is first taken LOW,
then the WREN instruction is clocked into the X25040.
After all eight bits of the instruction are transmitted,
CS
must then be taken HIGH. If the user continues the write
operation without taking
CS
HIGH after issuing the
WREN instruction, the write operation will be ignored.
To write data to the E
2
PROM memory array, the user
issues the WRITE instruction, followed by the address
and then the data to be written. This is minimally a
twenty-four clock operation.
CS
must go LOW and
remain LOW for the duration of the operation. The host
may continue to write up to 4 bytes of data to the X25040.
The only restriction is the 4 bytes must reside on the
same page. If the address counter reaches the end of
the page and the clock continues, the counter will “roll
over” to the first address of the page and overwrite any
data that may have been written.
For the write operation (byte or page write) to be
completed,
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
Figures 4 and 5 below for a detailed illustration of the
write sequences and time frames in which
CS
going
HIGH are valid.
To write to the status register, the WRSR instruction is
followed by the data to be written. Data bits 0, 1, 4, 5, 6
and 7 must be “0”. Figure 6 illustrates this 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.
Hold Operation
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 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.
The HOLD input may be tied HIGH either directly to V
CC
or tied to V
CC
through a resistor.
4
XICOR [ XICOR INC. ]
