DS12885 Datasheet, PDF (3/4 Pages) Dallas Semiconductor – Real-Time Clock

DS12885/DS12885Q/DS12885T
PIN DESCRIPTION
AD0–AD7 – Multiplexed Address/Data Bus
N.C.
– No Connection
MOT
– Bus Type Selection
CS
– Chip Select
AS
– Address Strobe
R/ W
– Read/Write Input
DS
– Data Strobe
RESET
– Reset Input
IRQ
SQW
V_CC
GND
X1, X2
V_BAT
RCLR
– Interrupt Request Output
(Open Drain)
– Square-Wave Output
– +5V Supply
– Ground
– 32.768kHz Crystal
Connections
– +3V Battery Input
– RAM Clear
PIN DESCRIPTION
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. The crystal is
connected directly to the X1 and X2 pins. There is no need for external capacitors or resistors. Note: X1
and X2 are very high- impedance nodes. It is recommended that they and the crystal be guard-ringed with
ground and that high- frequency signals be kept away from the crystal area. For more information about
crystal selection and crystal layout considerations, refer to Application Note 58 “Crystal Considerations
with Dallas Real Time Clocks.”
Oscillator startup times are highly dependent upon crystal characteristics and layout. High ESR and
escessive capactitive loads are the major contributors to long startup times. A circuit using a crystal with
the recommended characteristics and following the recommended layout usually start within one second.
V_BAT – Battery input for any standard 3V lithium cell or other energy source. Battery voltage must be
held between 2.5V and 4V for proper operation. A maximum load of 0.5µA at +25°C in the absence of
power should be used to size the external energy source. Maximum load is measured using a
recommended crystal type connected to X1 and X2.
The battery should be connected directly to the V_BAT pin. A diode must not be placed in series with the
battery to the V_BAT pin. Furthermore, a diode is not necessary because reverse charging-current protection
circuitry is provided internally to the device and has passed the requirements of Underwriters
Laboratories for UL listing.
See “Conditions of Acceptability” at http://www.maxim- ic.com/TechSupport/QA/ntrl.htm.
RCLR – The RCLR pin is used to clear (set to logic 1) all 114 bytes of general purpose RAM but does
not affect the RAM associated with the real-time clock. In order to clear the RAM, RCLR must be forced
to an input logic “0” (-0.3V to +0.8V) during battery-backup mode when V_CC is not applied. The RCLR
function is designed to be used by human interface (shorting to ground manually or by switch) and not to
be driven with external buffers. This pin is internally pulled up. Do not use an external pullup resistor on
this pin.
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