X28C256D Xicor, X28C256D Datasheet - Page 6

• Manufacturer Part Number: X28C256D
• Description: 5 Volt/ Byte Alterable E2PROM
• Manufacturer: Xicor

X28C256
HARDWARE DATA PROTECTION
The X28C256 provides three hardware features (com-
patible with X28C64) that protect nonvolatile data from
inadvertent writes.
• Noise Protection—A WE pulse typically less than
• Default V
• Write Inhibit—Holding either OE LOW, WE HIGH,
SOFTWARE DATA PROTECTION
The X28C256 offers a software controlled data protec-
tion feature. The X28C256 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
20ns will not initiate a write cycle.
when V
or CE HIGH will prevent an inadvertent write cycle
during power-up and power-down, maintaining data
integrity.
CC
CC
is 3.5V typically.
CC
Sense—All write functions are inhibited
was stable.
6
The X28C256 can be automatically protected during
power-up and power-down without the need for external
circuits by employing the software data protection fea-
ture. 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 X28C256
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.
Software Algorithm
Selecting the software data protection mode requires
the host system to precede data write operations by a
series of three write operations to three specific ad-
dresses. Refer to Figure 6 and 7 for the sequence. The
three-byte sequence opens the page write window
enabling the host to write from one to sixty-four bytes of
data.* Once the page load cycle has been completed,
the device will automatically be returned to the data
protected state.
*Note: Once the three-byte sequence is issued it
must be followed by a valid byte or page write
operation.