256K
X28HC256
5 Volt, Byte Alterable EEPROM
DESCRIPTION
32K x 8 Bit
FEATURES
• Access time: 70ns
• Simple byte and page write
—Single 5V supply
—No external high voltages or V
PP
control circuits
—Self-timed
—No erase before write
—No complex programming algorithms
—No overerase problem
• Low power CMOS
—Active: 60mA
—Standby: 500µA
• Software data protection
—Protects data against system level inadvertent
writes
• High speed page write capability
• Highly reliable Direct Write
™
cell
—Endurance: 1,000,000 cycles
—Data retention: 100 years
• Early end of write detection
—DATA polling
—Toggle bit polling
The X28HC256 is a second generation high perfor-
mance CMOS 32K x 8 EEPROM. It is fabricated with
Xicor’s proprietary, textured poly floating gate technol-
ogy, providing a highly reliable 5 Volt only nonvolatile
memory.
The X28HC256 supports a 128-byte page write opera-
tion, effectively providing a 24µs/byte write cycle, and
enabling the entire memory to be typically rewritten in
less than 0.8 seconds. The X28HC256 also features
DATA Polling and Toggle Bit Polling, two methods of
providing early end of write detection. The X28HC256
also supports the JEDEC standard Software Data Pro-
tection feature for protecting against inadvertent writes
during power-up and power-down.
Endurance for the X28HC256 is specified as a mini-
mum 1,000,000 write cycles per byte and an inherent
data retention of 100 years.
BLOCK DIAGRAM
256Kbit
EEPROM
Array
X Buffers
Latches and
Decoder
A
0
–A
14
Address
Inputs
Y Buffers
Latches and
DECODER
I/O Buffers
and Latches
CE
OE
WE
V
CC
V
SS
Control
Logic and
Timing
I/O
0
–I/O
7
Data Inputs/Outputs
REV 1.1 2/1/01
www.xicor.com
Characteristics subject to change without notice.
1 of 23
X28HC256
PIN CONFIGURATION
Plastic DIP
CERDIP
Flat Plastic
SOIC
A
14
A
12
A
7
A
6
A
5
A
4
A
3
A
2
A
1
A
0
I/O
0
I/O
1
I/O
2
V
SS
1
2
3
4
5
6
7
8
9
10
11
12
13
14
X28HC256
28
27
26
25
24
23
22
21
20
19
18
17
16
15
V
CC
WE
A
13
A
8
A
9
A
11
OE
A
10
CE
I/O
7
I/O
6
I/O
5
I/O
4
I/O
3
A
6
A
5
A
4
A
3
A
2
A
1
A
0
NC
I/O
0
5
6
7
8
9
10
11
12
29
28
27
26
A
8
A
9
A
11
NC
OE
A
10
CE
I/O
7
I/O
6
I/O
1
12
I/O
0
11
A
1
9
A
3
7
A
5
I/O
2
13
A
0
10
TSOP
LCC
PLCC
V
CC
A
12
A
14
A
13
WE
NC
A
7
A2
A1
A0
I/O 0
I/O 1
I/O 2
NC
VSS
NC
I/O 3
I/O 4
I/O 5
I/O 6
I/O 7
CE
A10
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
32
31
30
29
28
27
26
25
24
23
22
21
20
19
18
17
A3
A4
A5
A6
A7
A 12
A 14
NC
VCC
NC
WE
A13
A8
A9
A 11
OE
4
3
2
1 32 31 30
X28HC256
X28HC256
(Top View)
25
24
23
22
13
21
14 15 16 17 18 19 20
I/O
1
I/O
2
I/O
3
I/O
4
V
SS
I/O
5
NC
PGA
I/O
3
15
V
SS
14
I/O
5
17
I/O
4
16
I/O
6
18
I/O
7
19
A
10
21
A
11
23
A
8
25
A
13
26
A
2
CE
8
20
X28HC256
A
4
OE
6
22
A
12
2
A
7
3
V
CC
28
A
14
A
9
24
WE
27
5
A
6
4
1
(Bottom View)
PIN DESCRIPTIONS
Addresses (A
0
–A
14
)
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 consump-
tion is reduced.
Output Enable (OE)
The Output Enable input controls the data output buff-
ers, and is used to initiate read operations.
Data In/Data Out (I/O
0
–I/O
7
)
Data is written to or read from the X28HC256 through
the I/O pins.
Write Enable (WE)
The Write Enable input controls the writing of data to
the X28HC256.
PIN NAMES
Symbol
A
0
–A
14
I/O
0
–I/O
7
WE
CE
OE
V
CC
V
SS
NC
Description
Address Inputs
Data Input/Output
Write Enable
Chip Enable
Output Enable
+5V
Ground
No Connect
REV 1.1 2/1/01
www.xicor.com
Characteristics subject to change without notice.
2 of 23
X28HC256
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 X28HC256 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 comple-
tion, typically within 3ms.
Page Write Operation
The page write feature of the X28HC256 allows the
entire memory to be written in typically 0.8 seconds.
Page write allows up to one hundred twenty-eight
bytes of data to be consecutively written to the
X28HC256, 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 (A
7
through A
14
) 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 one hundred twenty-
seven bytes in the same manner as the first byte was
written. Each successive 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 sub-
sequent 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 X28HC256 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
DATA Polling (I/O
7
)
The X28HC256 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 sim-
ple bit test operation to determine the status of the
X28HC256. This eliminates additional interrupt inputs
or external hardware. During the internal programming
cycle, any attempt to read the last byte written will pro-
duce the complement of that data on I/O
7
(i.e., write
data = 0xxx xxxx, read data = 1xxx xxxx). Once the
programming cycle is complete, I/O
7
will reflect true
data.
Toggle Bit (I/O
6
)
The X28HC256 also provides another method for
determining when the internal write cycle is complete.
During the internal programming cycle I/O
6
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 and write operations.
REV 1.1 2/1/01
www.xicor.com
Characteristics subject to change without notice.
3 of 23
X28HC256
DATA POLLING I/O
7
Figure 2. DATA Polling Bus Sequence
WE
Last
Write
CE
OE
V
IH
I/O
7
HIGH Z
V
OL
A
0
–A
14
An
An
An
An
An
An
An
V
OH
X28HC256
Ready
Figure 3. DATA Polling Software Flow
Write Data
DATA Polling can effectively halve the time for writing to
the X28HC256. The timing diagram in Figure 2 illus-
trates the sequence of events on the bus. The software
flow diagram in Figure 3 illustrates one method of
implementing the routine.
Writes
Complete?
Yes
Save Last Data
and Address
No
Read Last
Address
IO
7
Compare?
Yes
No
X28HC256
Ready
REV 1.1 2/1/01
www.xicor.com
Characteristics subject to change without notice.
4 of 23
X28HC256
THE TOGGLE BIT I/O
6
Figure 4. Toggle Bit Bus Sequence
Last
WE Write
CE
OE
I/O
6
V
OH
*
V
OL
HIGH Z
*
X28C512/513
Ready
* I/O
6
Beginning and ending state of I/O
6
will vary.
Figure 5. Toggle Bit Software Flow
¬
HARDWARE DATA PROTECTION
The X28HC256 provides two hardware features that
protect nonvolatile data from inadvertent writes.
– Default V
CC
Sense—All write functions are inhibited
when V
CC
is 3.5V typically.
– Write Inhibit—Holding either OE LOW, WE HIGH, or
CE HIGH will prevent an inadvertent write cycle during
power-up and power-down, maintaining data integrity.
SOFTWARE DATA PROTECTION
Last Write
Yes
Load Accum
From Addr n
Compare
Accum with
Addr n
Compare
ok?
Yes
No
The X28HC256 offers a software-controlled data pro-
tection feature. The X28HC256 is shipped from Xicor
with the software data protection NOT ENABLED; that
is, the device will be in the standard operating mode. In
this mode data should be protected during power-up/
down operations through the use of external circuits.
The host would then have open read and write access
of the device once V
CC
was stable.
The X28HC256 can be automatically protected during
power-up and power-down (without the need for exter-
nal circuits) by employing the software data protection
feature. The internal software data protection circuit is
enabled after the first write operation, utilizing the soft-
ware algorithm. This circuit is nonvolatile, and will
remain set for the life of the device unless the reset
command is issued.
Once the software protection is enabled, the X28HC256 is
also protected from inadvertent and accidental writes in the
powered-up state. That is, the software algorithm must be
issued prior to writing additional data to the device.
Characteristics subject to change without notice.
X28C256
Ready
The Toggle Bit can eliminate the chore of saving and
fetching the last address and data in order to implement
DATA Polling. This can be especially helpful in an array
comprised of multiple X28HC256 memories that is
frequently updated. The timing diagram in Figure 4
illustrates the sequence of events on the bus. The soft-
ware flow diagram in Figure 5 illustrates a method for
polling the Toggle Bit.
REV 1.1 2/1/01
www.xicor.com
5 of 23
stm32/stm8
