DS1220Y
2 of 8
READ MODE
The DS1220Y executes a read cycle whenever WE (Write Enable) is inactive (high) and CE (Chip
Enable) and OE (Output Enable) are active (low). The unique address specified by the 11 address inputs
(A0-A10) defines which of the 2048 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 access times are also satisfied. If CE and OE access times are not satisfied, then data
access must be measured from the later-occurring signal and the limiting parameter is either t
CO
for CE or
t
OE
for OE rather than address access.
WRITE MODE
The DS1220Y executes a write cycle whenever the WE and CE signals are active (low) after address
inputs are stable. The later-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 drivers are enabled (
CE
and
OE
active)
then WE will disable the outputs in t
ODW
from its falling edge.
DATA RETENTION MODE
The DS1220Y provides full-functional capability for V
CC
greater than 4.5 volts and write protects at 4.25
nominal. Data is maintained in the absence of V
CC
without any additional support circuitry. The
DS1220Y constantly monitors V
CC
. Should the supply voltage decay, the NV SRAM automatically write
protects itself, 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 4.5 volts.
