Datasheet

DS1243Y
3 of 14
RAM READ MODE
The DS1243Y executes a read cycle whenever
WE
(Write Enable) is inactive (high) and
CE
(Chip
Enable) is active (low). The unique address specified by the 13 address inputs (A0–A12) defines which of
the 8192 bytes of data is to be accessed. Valid data will be available to the eight data output drivers
within t
ACC
(Access Time) after the last address input signal is stable, providing that
CE
and
OE
(Output
Enable) access times and states are also satisfied. If
OE
and
CE
access times are not satisfied, then data
access must be measured from the later occurring signal (
CE
or
OE
) and the limiting parameter is either
t
CO
for
CE
or t
OE
for
OE
rather than address access.
RAM WRITE MODE
The DS1243Y is in the write mode whenever the
WE
and
CE
signals are in the active (low) state after
address inputs are stable. The latter occurring falling edge of
CE
or
WE
will determine the start of the
write cycle. The write cycle is terminated by the earlier rising edge of
CE
or
WE
. All address inputs must
be kept valid throughout the write cycle.
WE
must return to the high state for a minimum recovery time
(t
WR
) before another cycle can be initiated. The
OE
control signal should be kept inactive (high) during
write cycles to avoid bus contention. However, if the output bus has been enabled (
CE
and
OE
active)
then
WE
will disable the outputs in t ODW from its falling edge.
DATA RETENTION MODE
The DS1243Y provides full functional capability for V
CC
greater than V
TP
and write protects by 4.25V.
Data is maintained in the absence of V
CC
without any additional support circuitry. The nonvolatile static
RAM constantly monitors V
CC
. Should the supply voltage decay, the RAM automatically write protects
itself. All inputs to the RAM become “don’t care” and all outputs are high impedance. As V
CC
falls below
approximately 3.0V, the power switching circuit connects the lithium energy source to RAM to retain
data. During power-up, when V
CC
rises above approximately 3.0V, the power switching circuit connects
external V
CC
to the RAM and disconnects the lithium energy source. Normal RAM operation can resume
after V
CC
exceeds 4.5V.
See “Conditions of Acceptability” at www.maxim-ic.com/TechSupport/QA/ntrl.htm
FRESHNESS SEAL
Each DS1243Y is shipped from Maxim with its lithium energy source disconnected, insuring full energy
capacity. When V
CC
is first applied at a level greater than V
TP
, the lithium energy source is enabled for
battery backup operation.