Rainbow Electronics DS1558Y - Manual (Page 4)

DS1558
4 of 18
RST – This pin is an output used to signal that V
CC
is out of tolerance. On power-up, RST is held low for
a period of time to allow the system to stabilize. The RTC and SRAM are not accessible while RST is
active. This pin is open drain and requires an external pullup resistor.
CE – Chip-enable input that is used to access the RTC and the external SRAM.
CER – Chip-enable RAM output. CE is passed through to CER , with an added propagation delay. When
the signals on A0–A18 match an RTC address, CER is held high, disabling the SRAM. If OE is also low,
the RTC outputs data on DQ0–DQ7.
OE – Output-enable input that is used to access the RTC and the external SRAM.
OER – Output-enable RAM output. OE is passed through to OER , with an added propagation delay.
When the signals on A0–A18 match an RTC address, CER is held high, disabling the SRAM. If CE is
also low, the RTC outputs data on DQ0–DQ7.
WE – Write-enable input that is used to write data to the RTC registers.
V
CC
, GND – DC power is provided to the device on these pins. V
CC
is the +5V input. When 5V (or 3.3V
for the 3.3V version) is applied within normal limits, the device is fully accessible and data can be written
and read. Reads and writes are inhibited when a 3V battery is connected to the device and V
CC
is V
TP
.
However, the timekeeping function continues unaffected by the lower input voltage. As V
CC
falls below
V
BAT
, the RAM and RTC are switched over to the external power supply (nominal 3.0V DC) at V
BAT
.
V
CCO
– V
CC
output to RAM. While V
CC
is above V
BAT
, the external SRAM is powered by V
CC
. When
V
CC
is below the battery level, the SRAM is powered by one of the V
BAT
inputs.
N.C. – No internal connection.
X1, X2 – Connections for a standard 32.768kHz quartz crystal. The internal oscillator circuitry is
designed for operation with a crystal having a specified load capacitance (C
L
) of 6pF. For more
information about crystal selection and crystal layout considerations, refer to Application Note 58
“Crystal Considerations with Dallas Real-Time Clocks.” The DS1558 can also be driven by an external
32.768kHz oscillator. In this configuration, the X1 pin is connected to the external oscillator signal and
the X2 pin is floated.
V
BAT1
, V
BAT2
– Battery inputs for any standard 3V lithium cell or other energy source. Battery voltage
must be held between 2.5V and 3.7V for proper operation. UL recognized to ensure against reverse
charging current when used with a lithium battery. If only one battery is used, it should be attached to
V
BAT1
, and V
BAT2
should be grounded.
See “Conditions of Acceptability” at http://www.maxim-ic.com/TechSupport/QA/ntrl.htm .