DS1345W Datasheet, PDF (2/12 Pages) Dallas Semiconductor – 3.3V 1024k Nonvolatile SRAM with Battery Monitor

DS1345W
READ MODE
The DS1345W 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 17 address inputs
(A₀ - A₁₆) defines which of the 131,072 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 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.
WRITE MODE
The DS1345W executes a write cycle whenever the WE and CE signals are in the active (low) state 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 DS1345W provides full functional capability for V_CC greater than 3.0 volts and write protects by 2.8
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 2.5 volts, the power switching circuit connects the lithium energy source to
RAM to retain data. During power-up, when V_CC rises above approximately 2.5 volts, 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 3.0 volts.
SYSTEM POWER MONITORING
The DS1345W has the ability to monitor the external V_CC power supply. When an out-of-tolerance power
supply condition is detected, the NV SRAM warns a processor-based system of impending power failure
by asserting RST . On power up, RST is held active for 200ms nominal to prevent system operation
during power-on transients and to allow t_REC to elapse. RST has an open-drain output driver.
BATTERY MONITORING
The DS1345W automatically performs periodic battery voltage monitoring on a 24-hour time interval.
Such monitoring begins within t_REC after V_CC rises above V_TP and is suspended when power failure
occurs.
After each 24-hour period has elapsed, the battery is connected to an internal 1MΩ test resistor for 1
second. During this 1 second, if battery voltage falls below the battery voltage trip point (2.6V), the
battery warning output BW is asserted. Once asserted, BW remains active until the module is replaced.
The battery is still retested after each V_CC power-up, however, even if BW is active. If the battery voltage
is found to be higher than 2.6V during such testing, BW is de-asserted and regular 24-hour testing
resumes. BW has an open-drain output driver.
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