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X25021
2K
X25021
256 x 8 Bit
SPI Serial E2PROM with Block LockTM Protection
FEATURES
1MHz Clock Rate
SPI Modes (0,1 & 1,0)
256 X 8 Bits
—4 Byte Page Mode
Low Power CMOS
—10µA Standby Current
—3mA Active Current
2.7V To 5.5V Power Supply
Block Lock Protection
—Protect 1/4, 1/2 or all of E2PROM Array
Built-in Inadvertent Write Protection
—Power-Up/Power-Down protection circuitry
—Write Latch
—Write Protect Pin
Self-Timed Write Cycle
—5ms Write Cycle Time (Typical)
High Reliability
—Endurance: 100,000 cycles per byte
—Data Retention: 100 Years
—ESD protection: 2000V on all pins
8-Lead PDlP Package
8-Lead SOIC Package
DESCRIPTION
The X25021 is a CMOS 2048-bit serial E2PROM, inter-
nally organized as 256 x 8. The X25021 features a Serial
Peripheral Interface (SPI) and software protocol allow-
ing 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 con-
trolled through a chip select (CS) input, allowing any
number of devices to share the same bus.
The X25021 also features two additional inputs that
provide the end user with added flexibility. By asserting
the HOLD input, the X25021 will ignore transitions on its
inputs, thus allowing the host to service higher priority
interrupts. The WP input can be used as a hardwire input
to the X25021 disabling all write attempts, thus providing
a mechanism for limiting end user capability of altering
the memory.
The X25021 utilizes Xicor’s proprietary Direct Write™
cell, providing a minimum endurance of 100,000 cycles
per byte and a minimum data retention of 100 years.
FUNCTIONAL DIAGRAM
STATUS
REGISTER
SO
SI
SCK
CS
HOLD
COMMAND
DECODE
AND
CONTROL
LOGIC
WRITE
PROTECT
LOGIC
WRITE
CONTROL
AND
WP
TIMING
LOGIC
Direct Write™ and Block Lock™ Protection is a trademark of Xicor, Inc.
©Xicor, Inc. 1995, 1996 Patents Pending
6615-1.5 6/10/96 T3/C1/D0 NS
1
X DECODE
LOGIC
16
16
32
256 BYTE
ARRAY
16 X 32
16 X 32
32 X 32
48
Y DECODE
DATA REGISTER
6615 FHD F01
Characteristics subject to change without notice

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X25021
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 rising edge of the serial clock.
Serial Input (SI)
SI is the serial data input pin. All opcodes, byte ad-
dresses, and data to be written to the memory are input
on this pin. Data is latched by the falling 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 falling edge of the clock
input, while data on the SO pin change after the rising
edge of the clock input.
Chip Select (CS)
When CS is HIGH, the X25021 is deselected and the SO
output pin is at high impedance and unless an internal
write operation is underway, the X25021 will be in the
standby power mode. CS LOW enables the X25021,
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, nonvolatile writes to the X25021 are
disabled, but the part otherwise functions normally.
When WP is held HIGH, all functions, including nonvola-
tile writes operate normally. WP going LOW while CS is
still LOW will interrupt a write to the X25021. If the
internal write cycle has already been initiated, WP going
LOW will have no affect on a write.
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 the serial
communication with the controller without resetting the
serial sequence. To pause, HOLD must be brought LOW
while SCK is HIGH. To resume communication, HOLD is
brought HIGH, again while SCK is HIGH. If the pause
feature is not used, HOLD should be held HIGH at all
times.
PIN CONFIGURATION
CS
SO
WP
VSS
DIP/SOIC
18
27
X25021
36
45
VCC
HOLD
SCK
SI
6615 FHD F02
PIN NAMES
Symbol
CS
SO
SI
SCK
WP
VSS
VCC
HOLD
Description
Chip Select Input
Serial Output
Serial Input
Serial Clock Input
Write Protect Input
Ground
Supply Voltage
Hold Input
6615 PGM T01
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X25021
PRINCIPLES OF OPERATION
The X25021 is a 256 x 8 E2PROM designed to interface
directly with the synchronous serial peripheral interface
(SPI) of many popular microcontroller families.
The X25021 contains an 8-bit instruction register. It is
accessed via the SI input, with data being clocked in on
the falling SCK. CS must be LOW during the entire
operation.
Table 1 contains a list of the instructions and their codes.
All instructions, addresses and data are transferred
MSB first.
Data input is sampled on the first falling 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
X25021 into a “PAUSE” condition. After releasing HOLD,
the X25021 will resume operation from the point when
HOLD was first asserted.
Write Enable Latch
The X25021 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:
7 654 3 2 1 0
X X X X BP1 BP0 WEL WIP
6615 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
X25021 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 X25021 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
00
01
10
11
Array Addresses
Protected
None
$C0–$FF
$80–$FF
$00–$FF
6615 PGM 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.
6615 PGM T04
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X25021
Clock and Data Timing
Data input on the SI line is latched on the falling edge of
SCK. Data is output on the SO line by the rising 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 X25021, followed by the
8-bit 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 ($FF) the address counter rolls
over to address $00 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 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 X25021, 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 X25021.
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 E2PROM 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 X25021.
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 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
HIGH when HOLD is first pulled LOW and SCK must
also be HIGH 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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X25021
Operational Notes
The X25021 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
inadvertent 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
order 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 11 12 13 14 15 16 17 18 19 20 21 22
INSTRUCTION
BYTE ADDRESS
SI 7 6 5 4 3 2 1 0
HIGH IMPEDANCE
SO
DATA OUT
76543210
MSB
6615 FHD F14
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
6615 ILL F13
5