DS17285/DS17287
DS17285 ONLY
X1, X2 –
Connections for a standard 32.768kHz quartz crystal. For greatest accuracy, the DS17285 must
be used with a crystal that has a specified load capacitance of either 6pF or 12.5pF. The crystal select
(CS) bit in extended-control register 4B is used to select operation with a 6pF or 12.5pF crystal. The
crystal is attached 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.” The DS17285 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
BAT
–
Battery input 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 in conjunction with a lithium battery. See “Conditions of Acceptability” at
POWER-DOWN/POWER-UP CONSIDERATIONS
The RTC function continues to operate and all of the RAM, time, calendar, and alarm memory locations
remain nonvolatile regardless of the level of the V
CC
input. When V
CC
is applied to the
DS17285/DS17287 and reaches a level of greater than V
PF
(power-fail trip point), the device becomes
accessible after t
REC
, provided that the oscillator is running and the oscillator countdown chain is not in
reset (Register A). This time period allows the system to stabilize after power is applied.
The DS17285/DS17287 is available in either a 3V or a 5V device.
The 5V device is fully accessible and data can be written and read only when V
CC
is greater than 4.5V.
When V
CC
is below 4.5V, read and writes are inhibited. However, the timekeeping function continues
unaffected by the lower input voltage. As V
CC
falls below the greater of V
BAT
and V
BAUX
, the RAM and
timekeeper are switched over to a lithium battery connected either to the V
BAT
pin or V
BAUX
pin.
The 3V device is fully accessible and data can be written or read only when V
CC
is greater than 2.7V.
When V
CC
falls below V
PF
, access to the device is inhibited. If V
PF
is less than V
BAT
and V
BAUX
, the
power supply is switched from V
CC
to the backup supply (the greater of
VBAT
and V
BAUX
) when V
CC
drops
below V
PF
. If V
PF
is greater than V
BAT
and V
BAUX
, the power supply is switched from V
CC
to the backup
supply when V
CC
drops below the larger of V
BAT
and V
BAUX
.
When V
CC
falls below V
PF
, the chip is write-protected. With the possible exception of the
KS
,
PWR
,
RCLR
, and SQW pins, all inputs are ignored and all outputs are in a high-impedance state.
6 of 38
DALLAS [ DALLAS SEMICONDUCTOR ]
