The DS1270 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 21 address inputs
(A0 - A20) defines which of the 2,097,152 bytes of data is accessed. Valid data will be available to the
eight data output drivers within tACC (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 tCO for CE or tOE for OE rather than tACC.
The DS1270 devices execute a write cycle whenever 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 (tWR)
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 tODW from its falling edge.
DATA RETENTION MODE
The DS1270AB provides full-functional capability for VCC greater than 4.75 volts and write protects by
4.5 volts. The DS1270Y provides full-functional capability for VCC greater than 4.5 volts and write
protects by 4.25 volts. Data is maintained in the absence of VCC without any additional support circuitry.
The nonvolatile static RAMs constantly monitor VCC. 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 VCC falls below approximately 3.0 volts, a power switching circuit connects the lithium
energy source to RAM to retain data. During power-up, when VCC rises above approximately 3.0 volts,
the power switching circuit connects external VCC to RAM and disconnects the lithium energy source.
Normal RAM operation can resume after VCC exceeds 4.75 volts for the DS1270AB and 4.5 volts for the
Each DS1270 device is shipped from Maxim with its lithium energy source disconnected, guaranteeing
full energy capacity. When VCC is first applied at a level greater than VTP, the lithium energy source is
enabled for battery backup operation.
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