Rainbow Electronics DS1243Y - Manual (Page 2)

DS1243Y
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The Phantom Clock provides timekeeping information including hundredths of seconds, seconds,
minutes, hours, day, date, month, and year information. The date at the end of the month is automatically
adjusted for months with less than 31 days, including correction for leap years. The Phantom Clock
operates in either 24–hour or 12–hour format with an AM/PM indicator.
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 t
ACC
(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
t
CO
for CE or t
OE
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
(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 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 V
CC
greater than V
TP
and write protects by 4.25
volts. Data is maintained in the absence of V
CC
without any additional support circuitry. The nonvolatile
static RAM constantly monitors V
CC
. 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 V
CC
falls below approximately 3.0 volts, the 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 the RAM and disconnects the lithium energy source. Normal
RAM operation can resume after V
CC
exceeds 4.5 volts.
FRESHNESS SEAL
Each DS1243Y is shipped from Dallas Semiconductor with its lithium energy source disconnected,
insuring 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.
PHANTOM CLOCK OPERATION
Communication with the Phantom Clock is established by pattern recognition on a serial bit stream of 64
bits which must be matched by executing 64 consecutive write cycles containing the proper data on DQ0.
All accesses which occur prior to recognition of the 64–bit pattern are directed to memory.
After recognition is established, the next 64 read or write cycles either extract or update data in the
Phantom Clock, and memory access is inhibited.