The Write-Protect-Enable (WPEN) bit is available for
the X25650 as a nonvolatile enable bit for the WP pin.
Protected Unprotected Status
WPEN WP WEL Blocks
0 X 0 Protected Protected Protected
0 X 1 Protected Writable Writable
1 LOW 0 Protected Protected Protected
1 LOW 1 Protected Writable Protected
X HIGH 0 Protected Protected Protected
X HIGH 1 Protected Writable Writable
7037 FRM T05
Programmable Hardware Write Protection
The Write Protect (WP) pin and the nonvolatile Write
Protect Enable (WPEN) bit in the Status Register
control the Programmable Hardware Write Protect
feature. Hardware Write Protection is enabled when
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 chip is hard-
ware write protected, nonvolatile writes are disabled to
the Status Register, including the Block Lock bits and
the WPEN bit itself, as well as the block-protected
sections in the memory array. Only the sections of the
memory array that are not block-protected can be
In Circuit Programmable ROM Mode
Note that since the WPEN bit is write protected, it
cannot be changed back to a LOW state; so write
protection is enabled as long as the WP pin is held
LOW. Thus an In Circuit Programmable ROM function
can be emplemented by hardwiring the WP pin to Vss,
writing to and Block Locking the desired portion of the
array to be ROM, and then programming the WPEN bit
HIGH. The table above deﬁnes the program protect
status for each combination of WPEN and WP.
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.
When reading from the E2PROM memory array, CS is
ﬁrst pulled LOW to select the device. The 8-bit READ
instruction is transmitted to the X25650, followed by
the 16-bit address of which the last 13 are used. 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 ($1FFF) the address counter rolls
over to address $0000 allowing the read cycle to be
continued indeﬁnitely. The read operation is termi-
nated by taking CS HIGH. Refer to the read E2PROM
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 RDSR opcode is sent, the contents
of the status register are shifted out on the SO line.
Figure 2 illustrates the read status register sequence.
Prior to any attempt to write data into the X25650, 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
X25650. After all eight bits of the instruction are trans-
mitted, 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 E2PROM memory array, the user
issues the WRITE instruction, followed by the address
and then the data to be written. This is minimally a
thirty-two clock operation. CS must go LOW and remain
LOW for the duration of the operation. The host may
continue to write up to 32 bytes of data to the X25650.
The only restriction is the 32 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 illustra-
tion 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
and 6 must be “0”. Figure 6 illustrates this sequence.
While the write is in progress following a status
register or E2PROM write sequence, the status
register may be read to check the WIP bit. During this
time the WIP bit will be HIGH.