DS1270Y/AB
2 of 8
READ MODE
The DS1270 devices execute a read cycle whenever
(Write Enable) is inactive (high) and
(Chip
Enable) and
(Output Enable) are active (low). The unique address specified by the 21 address inputs
(A
0
- A
20
) defines which of the 2,097,152 bytes of data is 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
and
(Output Enable) access times are also satisfied. If
and
access times are not
satisfied, then data access must be measured from the later-occurring signal (
or
) and the limiting
parameter is either t
CO
for
or t
OE
for
rather than t
ACC
.
WRITE MODE
The DS1270 devices execute a write cycle whenever
and
signals are active (low) after address
inputs are stable. The later-occurring falling edge of
or
will determine the start of the write cycle.
The write cycle is terminated by the earlier rising edge of
or
. All address inputs must be kept
valid throughout the write cycle.
must return to the high state for a minimum recovery time (t
WR
)
before another cycle can be initiated. The
control signal should be kept inactive (high) during write
cycles to avoid bus contention. However, if the output drivers are enabled (
and
active) then
will disable the outputs in t
ODW
from its falling edge.
DATA RETENTION MODE
The DS1270AB provides full-functional capability for V
CC
greater than 4.75 volts and write protects by
4.5 volts. The DS1270Y provides full-functional capability for V
CC
greater than 4.5 volts and write
protects by 4.25 volts. Data is maintained in the absence of V
CC
without any additional support circuitry.
The nonvolatile static RAMs constantly monitor V
CC
. Should the supply voltage decay, the NV SRAMs
automatically write protect themselves, all inputs become don’t care, and all outputs become high-
impedance. As V
CC
falls below approximately 3.0 volts, a power switching circuit connects the lithium
energy source to RAM to retain data. During power-up, when V
CC
rises above approximately 3.0 volts,
the power switching circuit connects external V
CC
to RAM and disconnects the lithium energy source.
Normal RAM operation can resume after V
CC
exceeds 4.75 volts for the DS1270AB and 4.5 volts for the
DS1270Y.
FRESHNESS SEAL
Each DS1270 device is shipped from Maxim with its lithium energy source disconnected, guaranteeing
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.
