The CE, OE, WE, and NE inputs control the X20C16
operation. The X20C16 byte-wide NOVRAM uses a
2-line control architecture to eliminate bus contention in
a system environment. The I/O bus will be in a high
impedance state when either OE or CE is HIGH, or
when NE is LOW.
RAM read and write operations are performed as they
would be with any static RAM. A read operation requires
CE and OE to be LOW with WE and NE HIGH. A write
operation requires CE and WE to be LOW with NE
HIGH. There is no limit to the number of read or write
operations performed to the RAM portion of the X20C16.
Memory Transfer Operations
There are two memory transfer operations: a recall
operation whereby the data stored in the E2PROM array
is transferred to the RAM array; and a store operation
which causes the entire contents of the RAM array to be
stored in the E2PROM array.
Recall operations are performed automatically upon
power-up and under host system control when NE, OE
and CE are LOW and WE is HIGH. The recall operation
takes a maximum of 5µs.
SDP (Software Data Protection)
There are two methods of initiating a store operation.
The first is the software store command. This command
takes the place of the hardware store employed on the
X20C04. This command is issued by entering into the
special command mode: NE, CE, and WE strobe LOW
while at the same time a specific address and data
combination is sent to the device. This is a three step
operation: the first address/data combination is 555[H]/
AA[H]; the second combination is 2AA[H]/55[H]; and the
final command combination is 555[H]/33[H]. This se-
quence of pseudo write operations will immediately
initiate a store operation. Refer to the software com-
mand timing diagrams for details on set and hold times
for the various signals.
The second method of storing data is with the
AUTOSTORE command. When enabled, data is auto-
matically stored from the RAM into the E2PROM array
whenever VCC falls below the preset Autostore thresh-
old. This feature is enabled by performing the first two
steps for the software store with the command combina-
tion being 555[H]/CC[H].
The AUTOSTORE feature is disabled by issuing the
three step command sequence with the command com-
bination being 555[H]/CD[H]. The AUTOSTORE feature
will also be reset if VCC falls below the power-up reset
threshold (approximately 3.5V) and is then raised back
into the operation range.
The X20C16 supports two methods of protecting the
—If after power-up the AUTOSTORE feature is not
enabled, no AUTOSTORE can occur.
—VCC Sense – All functions are inhibited when VCC is
≤ 3.0V typical.
The following symbol table provides a key to under-
standing the conventions used in the device timing
diagrams. The diagrams should be used in conjunction
with the device timing specifications to determine actual
device operation and performance, as well as device
suitability for user’s application.