DS1249W_10 Datasheet, PDF (2/8 Pages) Dallas Semiconductor – 3.3V 2048kb Nonvolatile SRAM

DS1249W
READ MODE
The DS1249 devices execute 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 18 address inputs
(A₀ – A₁₇) defines which of the 262,144 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
CE and OE access times 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 t_ACC.
WRITE MODE
The DS1249 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 DS1249W provides full functional capability for V_CC greater than 3.0 volts and write protects by
2.8V. 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 protects themselves, all inputs become “don’t care,” and all outputs become high impedance. As
V_CC falls below approximately 2.5V, a power-switching circuit connects the lithium energy source to
RAM to retain data. During power-up, when V_CC rises above approximately 2.5V, 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 3.0V.
FRESHNESS SEAL
Each DS1249 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.
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