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DS1243Y
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RAM READ MODE
The DS1243Y executes a read cycle whenever WE (Write Enable) is inactive (high) and CE (Chip
Enable) is active (low). The unique address specified by the 13 address inputs (A0–A12) defines which of
the 8192 bytes of data is to be 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 and states 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 address access.
RAM WRITE MODE
The DS1243Y is in the write mode whenever the WE and CE signals are in the active (low) state after
address inputs are stable. The latter 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 bus has been enabled ( CE and OE active)
then WE will disable the outputs in t ODW from its falling edge.
DATA RETENTION MODE
The DS1243Y provides full functional capability for VCC greater than VTP and write protects by 4.25V.
Data is maintained in the absence of VCC without any additional support circuitry. The nonvolatile static
RAM constantly monitors VCC. Should the supply voltage decay, the RAM automatically write protects
itself. All inputs to the RAM become “don’t care” and all outputs are high impedance. As VCC falls below
approximately 3.0V, the power switching circuit connects the lithium energy source to RAM to retain
data. During power-up, when VCC rises above approximately 3.0V, the power switching circuit connects
external VCC to the RAM and disconnects the lithium energy source. Normal RAM operation can resume
after VCC exceeds 4.5V.
FRESHNESS SEAL
Each DS1243Y is shipped from Dallas Semiconductor with its lithium energy source disconnected,
insuring full energy capacity. When VCC is first applied at a level greater than VTP, the lithium energy
source is enabled for battery backup operation.