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APPLICATION NOTE
A V A I LABLE
AN19 • AN38 • AN41 • AN61
64K
X25642
8K x 8 Bit
Advanced SPI Serial E2PROM with Block LockTM Protection
FEATURES
2MHz Clock Rate
Low Power CMOS
—<1µA Standby Current
—<5mA Active Current
2.7V To 5.5V Power Supply
SPI Modes (0,0 & 1,1)
8K X 8 Bits
—32 Byte Page Mode
Block Lock Protection
—Protect 1/4, 1/2 or all of E2PROM Array
Built-in Inadvertent Write Protection
—Power-Up/Down protection circuitry
—Write Enable Latch
—Write Protect Pin
Self-Timed Write Cycle
—5ms Write Cycle Time (Typical)
High Reliability
—Endurance: 100,000 cycles
—Data Retention: 100 Years
—ESD protection: 2000V on all pins
Packages
—8-Lead PDIP
—8-Lead SOIC
—14-Lead SOIC
—20-Lead TSSOP
DESCRIPTION
The X25642 is a CMOS 65,536-bit serial E2PROM,
internally organized as 8K x 8. The X25642 features a
Serial Peripheral Interface (SPI) and software protocol
allowing operation on a simple three-wire bus. The bus
signals are a clock input (SCK) plus separate data in
(SI) and data out (SO) lines. Access to the device is
controlled through a chip select (CS) input, allowing
any number of devices to share the same bus.
The X25642 also features two additional inputs that
provide the end user with added flexibility. By
asserting the HOLD input, the X25642 will ignore tran-
sitions on its inputs, thus allowing the host to service
higher priority interrupts. The WP input can be used as
a hardwire input to the X25642 disabling all write
attempts to the status register, thus providing a mech-
anism for limiting end user capability of altering 0, 1/4,
1/2 or all of the memory.
The X25642 utilizes Xicor’s proprietary Direct WriteTM
cell, providing a minimum endurance of 100,000
cycles and a minimum data retention of 100 years.
FUNCTIONAL DIAGRAM
SO
SI
SCK
CS
HOLD
STATUS
REGISTER
COMMAND
DECODE
AND
CONTROL
LOGIC
WRITE
PROTECT
LOGIC
X DECODE
LOGIC
64
64
128
8K BYTE
ARRAY
64 X 256
64 X 256
128 X 256
WRITE
CONTROL
AND
WP
TIMING
LOGIC
Direct Writeand Block LockProtection is a trademark of Xicor, Inc.
©Xicor, Inc. 1994, 1995, 1996 Patents Pending
3132-1.0 1/17/97 T5/C0/D1 SH
1
32 8
Y DECODE
DATA REGISTER
3132 ILL F01.1
Characteristics subject to change without notice

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X25642
PIN DESCRIPTIONS
Serial Output (SO)
SO is a push/pull serial data output pin. During a read
cycle, data is shifted out on this pin. Data is clocked
out by the falling edge of the serial clock.
Serial Input (SI)
SI is the serial data input pin. All opcodes, byte
addresses, and data to be written to the memory are
input on this pin. Data is latched by the rising edge of
the serial clock.
Serial Clock (SCK)
The Serial Clock controls the serial bus timing for data
input and output. Opcodes, addresses, or data present
on the SI pin are latched on the rising edge of the
clock input, while data on the SO pin change after the
falling edge of the clock input.
Chip Select (CS)
When CS is HIGH, the X25642 is deselected and the
SO output pin is at high impedance and unless an
internal write operation is underway, the X25642 will be
in the standby power mode. CS LOW enables the
X25642, placing it in the active power mode. It should
be noted that after power-up, a HIGH to LOW transition
on CS is required prior to the start of any operation.
Write Protect (WP)
When WP is LOW and the nonvolatile bit WPEN is “1”,
nonvolatile writes to the X25642 status register are
disabled, but the part otherwise functions normally.
When WP is held HIGH, all functions, including
nonvolatile writes operate normally. WP going LOW
while CS is still LOW will interrupt a write to the
PIN NAMES
Symbol
CS
SO
SI
SCK
WP
VSS
VCC
HOLD
NC
Description
Chip Select Input
Serial Output
Serial Input
Serial Clock Input
Write Protect Input
Ground
Supply Voltage
Hold Input
No Connect
7037 FRM T01
X25642 status register. If the internal write cycle has
already been initiated, WP going LOW will have no
affect on a write.
The WP pin function is blocked when the WPEN bit in
the status register is “0”. This allows the user to install
the X25642 in a system with WP pin grounded and still
be able to write to the status register. The WP pin func-
tions will be enabled when the WPEN bit is set “1”.
Hold (HOLD)
HOLD is used in conjunction with the CS pin to select
the device. Once the part is selected and a serial
sequence is underway, HOLD may be used to pause
PIN CONFIGURATION
SOIC/DIP
Not to Scale
CS
.197" SO
SOIC
Only
WP
VSS
18
27
X25642
36
45
VCC
HOLD
SCK
SI
.244"
SOIC
NC
CS*
CS*
.345"
SO
WP
VSS
NC
1 14
2 13
3 12
4 X25642 11
5 10
69
78
NC
NC
VCC
HOLD
SCK
SI
NC
.244"
NC
CS
NC
SO
NC
.300"
NC
WP
VSS
NC
NC
TSSOP
1 2200
2 1199
3 1188
4 1177
5 1166
6 X25642 1155
7 1144
8 1133
9 1122
10 1111
NC
VCC
NC
HOLD
NC
NC
SCK
SI
NC
NC
.252"
3132 ILL F02.5
* Pin 2 and Pin 3 are internally connected. Only one CS needs to
be connected externally.
2

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X25642
the serial communication with the controller without
resetting the serial sequence. To pause, HOLD must
be brought LOW while SCK is LOW. To resume
communication, HOLD is brought HIGH, again while
SCK is LOW. If the pause feature is not used, HOLD
should be held HIGH at all times.
PRINCIPLES OF OPERATION
The X25642 is a 8K x 8 E2PROM designed to interface
directly with the synchronous serial peripheral inter-
face (SPI) of many popular microcontroller families.
The X25642 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
transferred 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
X25642 into a “PAUSE” condition. After releasing
HOLD, the X25642 will resume operation from the
point when HOLD was first asserted.
Write Enable Latch
The X25642 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
formatted as follows:
7 654 3 2 1 0
WPEN X X X BP1 BP0 WEL WIP
7037 FRM T02
WPEN, BP0 and BP1 are set by the WRSR instruc-
tion. WEL and WIP are read-only and automatically set
by other operations.
The Write-In-Process (WIP) bit indicates whether the
X25642 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
protection. The X25642 is divided into four 16384-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
00
01
10
11
Array Addresses
Protected
None
$1800–$1FFF
$1000–$1FFF
$0000–$1FFF
7037 FRM T03
Table 1. Instruction Set
Instruction Name Instruction Format*
Operation
WREN
0000 0110
Set the Write Enable Latch (Enable Write Operations)
WRDI
0000 0100
Reset the Write Enable Latch (Disable Write Operations)
RDSR
0000 0101
Read Status Register
WRSR
0000 0001
Write Status Register
READ
0000 0011
Read Data from Memory Array beginning at selected address
WRITE
0000 0010
Write Data to Memory Array beginning at Selected Address (1 to 32
Bytes)
*Instructions are shown MSB in leftmost position. Instructions are transferred MSB first.
7037 FRM T04
3

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X25642
Write-Protect Enable
The Write-Protect-Enable (WPEN) is available for the
X25642 as a nonvolatile enable bit for the WP pin.
Protected Unprotected Status
WPEN WP WEL Blocks
Blocks Register
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
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 hardware write
protected, nonvolatile writes are disabled to the Status
Register, including the Block Protect 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 written.
Note: Since the WPEN bit is write protected, it cannot
be changed back to a “0”, as long as the WP pin
is held LOW.
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 E2PROM memory array, CS is
first pulled LOW to select the device. The 8-bit READ
instruction is transmitted to the X25642, 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.
Write Sequence
Prior to any attempt to write data into the X25642, 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
X25642. 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
X25642. 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 over-
write 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.
Hold Operation
The HOLD input should be HIGH (at VIH) 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
VCC or tied to VCC through a resistor.
4

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X25642
Operational Notes
The X25642 powers-up in the following state:
• The device is in the low power standby state.
• A HIGH to LOW transition on CS is required to enter
an active state and receive an instruction.
• SO pin is high impedance.
• The “write enable” latch is reset.
Data Protection
The following circuitry has been included to prevent in-
advertent writes:
• The “write enable” latch is reset upon power-up.
• A WREN instruction must be issued to set the “write
enable” latch.
• CS must come HIGH at the proper clock count in or-
der to start a write cycle.
Figure 1. Read E2PROM Array Operation Sequence
CS
SCK
0 1 2 3 4 5 6 7 8 9 10
20 21 22 23 24 25 26 27 28 29 30
INSTRUCTION
16 BIT ADDRESS
SI
15 14 13
3210
HIGH IMPEDANCE
SO
DATA OUT
76543210
MSB
3132 ILL F03.1
Figure 2. Read Status Register Operation Sequence
CS
SCK
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14
INSTRUCTION
SI
HIGH IMPEDANCE
SO
76
MSB
DATA OUT
5432
10
3132 ILL F04
5