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X28C256
256K
X28C256
5 Volt, Byte Alterable E2PROM
32K x 8 Bit
FEATURES
Access Time: 150ns
Simple Byte and Page Write
— Single 5V Supply
—No External High Voltages or VPP Control
Circuits
— Self-Timed
—No Erase Before Write
—No Complex Programming Algorithms
—No Overerase Problem
Low Power CMOS:
—Active: 60mA
—Standby: 200µA
Software Data Protection
— Protects Data Against System Level
Inadvertent Writes
High Speed Page Write Capability
Highly Reliable Direct WriteCell
— Endurance: 100,000 Write Cycles
— Data Retention: 100 Years
Early End of Write Detection
DATA Polling
—Toggle Bit Polling
PIN CONFIGURATION
DESCRIPTION
The X28C256 is an 32K x 8 E2PROM, fabricated with
Xicor’s proprietary, high performance, floating gate
CMOS technology. Like all Xicor programmable non-
volatile memories the X28C256 is a 5V only device. The
X28C256 features the JEDEC approved pinout for byte-
wide memories, compatible with industry standard RAMs.
The X28C256 supports a 64-byte page write operation,
effectively providing a 78µs/byte write cycle and en-
abling the entire memory to be typically written in less
than 2.5 seconds. The X28C256 also features DATA
and Toggle Bit Polling, a system software support
scheme used to indicate the early completion of a write
cycle. In addition, the X28C256 includes a user-optional
software data protection mode that further enhances
Xicor’s hardware write protect capability.
Xicor E2PROMs are designed and tested for applica-
tions requiring extended endurance. Inherent data re-
tention is greater than 100 years.
PLASTIC DIP
CERDIP
FLAT PACK
SOIC
LCC
PLCC
A14
A12
A7
A6
A5
A4
A3
A2
A1
A0
I/O0
I/O1
I/O2
VSS
1 28
2 27
3 26
4 25
5 24
6 23
7 22
X28C256
8 21
9 20
10 19
11 18
12 17
13 16
14 15
VCC
WE
A13
A8
A9
A11
OE
A10
CE
I/O7
I/O6
I/O5
I/04
I/O3
3855 FHD F02
4 3 2 1 32 31 30
A6 5
29 A8
A5 6
28 A9
A4 7
27 A11
A3 8
A2 9
X28C256
26 NC
25 OE
A1 10
24 A10
A0 11
23 CE
NC 12
22 I/O7
I/O0
13 21
14 15 16 17 18 19 20
I/O6
3855 FHD F03
A2
A1
A0
I/O0
I/O1
I/O2
NC
VSS
NC
I/O3
I/O4
I/O5
I/O6
I/O7
CE
A10
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
TSOP
X28C256
32 A3
31 A4
30 A5
29 A6
28 A7
27 A12
26 A14
25 NC
24 VCC
23 NC
22 WE
21 A13
20 A8
19 A9
18 A11
17 OE
3855 ILL F23
© Xicor, Inc. 1991, 1995 Patents Pending
3855-2.0 8/19/98 T3/C0/D0 RZ
1 Characteristics subject to change without notice

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X28C256
PIN DESCRIPTIONS
Addresses (A0–A14)
The Address inputs select an 8-bit memory location
during a read or write operation.
Chip Enable (CE)
The Chip Enable input must be LOW to enable all read/
write operations. When CE is HIGH, power consumption
is reduced.
Output Enable (OE)
The Output Enable input controls the data output buffers
and is used to initiate read operations.
Data In/Data Out (I/O0–I/O7)
Data is written to or read from the X28C256 through the
I/O pins.
Write Enable (WE)
The Write Enable input controls the writing of data to the
X28C256.
FUNCTIONAL DIAGRAM
PIN NAMES
Symbol
A0–A14
I/O0–I/O7
WE
CE
OE
VCC
VSS
NC
Description
Address Inputs
Data Input/Output
Write Enable
Chip Enable
Output Enable
+5V
Ground
No Connect
3855 PGM T01
PIN CONFIGURATION
PGA
I/O1 I/O2 I/O3 I/O5 I/O6
12 13 15 17 18
I/O0 A0
VSS I/O4 I/O7
11 10 14 16 19
A1 A2
98
CE A10
20 21
X28C256
A3 A4
OE A11
76
22 23
A5 A12 VCC A9 A8
5 2 28 24 25
A6 A7 A14 WE A13
4 3 1 27 26
3855 FHD F04
BOTTOM VIEW
A0–A14
ADDRESS
INPUTS
X BUFFERS
LATCHES AND
DECODER
Y BUFFERS
LATCHES AND
DECODER
256K-BIT
E2PROM
ARRAY
I/O BUFFERS
AND LATCHES
CE
OE
WE
VCC
VSS
CONTROL
LOGIC AND
TIMING
I/O0–I/O7
DATA INPUTS/OUTPUTS
3855 FHD F01
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X28C256
DEVICE OPERATION
Read
Read operations are initiated by both OE and CE LOW.
The read operation is terminated by either CE or OE
returning HIGH. This two line control architecture elimi-
nates bus contention in a system environment. The data
bus will be in a high impedance state when either OE or
CE is HIGH.
Write
Write operations are initiated when both CE and WE are
LOW and OE is HIGH. The X28C256 supports both a CE
and WE controlled write cycle. That is, the address is
latched by the falling edge of either CE or WE, whichever
occurs last. Similarly, the data is latched internally by the
rising edge of either CE or WE, whichever occurs first.
A byte write operation, once initiated, will automatically
continue to completion, typically within 5ms.
Page Write Operation
The page write feature of the X28C256 allows the entire
memory to be written in 2.5 seconds. Page write allows
two to sixty-four bytes of data to be consecutively written
to the X28C256 prior to the commencement of the
internal programming cycle. The host can fetch data
from another device within the system during a page
write operation (change the source address), but the
page address (A6 through A14) for each subsequent
valid write cycle to the part during this operation must be
the same as the initial page address.
The page write mode can be initiated during any write
operation. Following the initial byte write cycle, the host
can write an additional one to sixty-three bytes in the
same manner as the first byte was written. Each succes-
sive byte load cycle, started by the WE HIGH to LOW
transition, must begin within 100µs of the falling edge of
the preceding WE. If a subsequent WE HIGH to LOW
transition is not detected within 100µs, the internal
automatic programming cycle will commence. There is
no page write window limitation. Effectively the page
write window is infinitely wide, so long as the host
continues to access the device within the byte load cycle
time of 100µs.
Write Operation Status Bits
The X28C256 provides the user two write operation
status bits. These can be used to optimize a system
write cycle time. The status bits are mapped onto the
I/O bus as shown in Figure 1.
Figure 1. Status Bit Assignment
I/O DP TB 5 4 3 2 1 0
RESERVED
TOGGLE BIT
DATA POLLING
3855 FHD F11
DATA Polling (I/O7)
The X28C256 features DATA Polling as a method to
indicate to the host system that the byte write or page
write cycle has completed. DATA Polling allows a simple
bit test operation to determine the status of the X28C256,
eliminating additional interrupt inputs or external hard-
ware. During the internal programming cycle, any at-
tempt to read the last byte written will produce the
complement of that data on I/O7 (i.e. write data = 0xxx
xxxx, read data = 1xxx xxxx). Once the programming
cycle is complete, I/O7 will reflect true data. Note: If the
X28C256 is in the protected state and an illegal write
operation is attempted DATA Polling will not operate.
Toggle Bit (I/O6)
The X28C256 also provides another method for deter-
mining when the internal write cycle is complete. During
the internal programming cycle I/O6 will toggle from
HIGH to LOW and LOW to HIGH on subsequent
attempts to read the device. When the internal cycle is
complete the toggling will cease and the device will be
accessible for additional read or write operations.
3

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X28C256
DATA POLLING I/O7
Figure 2. DATA Polling Bus Sequence
LAST
WE WRITE
CE
OE
VIH
I/O7
A0–A14
An
HIGH Z
VOL
An An
An
An
VOH
X28C256
READY
An An
3855 FHD F12
Figure 3. DATA Polling Software Flow
WRITE DATA
DATA Polling can effectively halve the time for writing to
the X28C256. The timing diagram in Figure 2 illustrates
the sequence of events on the bus. The software flow
diagram in Figure 3 illustrates one method of implement-
ing the routine.
WRITES
COMPLETE?
NO
YES
SAVE LAST DATA
AND ADDRESS
READ LAST
ADDRESS
IO7
COMPARE?
YES
NO
X28C256
READY
3855 FHD F13
4

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X28C256
THE TOGGLE BIT I/O6
Figure 4. Toggle Bit Bus Sequence
LAST
WE WRITE
CE
OE
I/O6
VOH
*
VOL
* Beginning and ending state of I/O6 will vary.
Figure 5. Toggle Bit Software Flow
LAST WRITE
LOAD ACCUM
FROM ADDR n
HIGH Z
*
X28C256
READY
3855 FHD F14
The Toggle Bit can eliminate the software housekeeping
chore of saving and fetching the last address and data
written to a device in order to implement DATA Polling.
This can be especially helpful in an array comprised of
multiple X28C256 memories that is frequently updated.
The timing diagram in Figure 4 illustrates the sequence
of events on the bus. The software flow diagram in
Figure 5 illustrates a method for polling the Toggle Bit.
COMPARE
ACCUM WITH
ADDR n
COMPARE
OK?
YES
X28C256
READY
NO
3855 FHD F15
5