DS1685/DS1687 3V/5V Real-Time Clocks
Table 1. Crystal Specifications*
PARAMETER
SYMBOL
MIN
TYP
MAX
UNITS
Nominal Frequency
Series Resistance
Load Capacitance
fO
ESR
CL
32.768
kHz
kΩ
pF
50
6, 12.5
*The crystal, traces, and crystal input pins should be isolated from RF generating signals. Refer to Application Note 58: Crystal Considerations
for Dallas Real-Time Clocks for additional specifications.
CLOCK ACCURACY
The accuracy of the clock is dependent on the accuracy of the crystal and the accuracy of the match between the
capacitive load of the oscillator circuit and the capacitive load for which the crystal was trimmed. Additional error is
added by crystal frequency drift caused by temperature shifts. External circuit noise coupled into the oscillator
circuit can result in the clock running fast. Figure 3 shows a typical PC board layout for isolation of the crystal and
oscillator from noise. Refer to Application Note 58: Crystal Considerations with Dallas Real-Time Clocks for
detailed information.
The DS1685 can also be driven by an external 32.768 kHz oscillator. In this configuration, the X1 pin is connected
to the external oscillator signal and the X2 pin is floated. Refer to Application Note 58: Crystal Considerations with
Dallas Real-Time Clocks for detailed information about crystal selection and crystal layout.
Figure 2. Oscillator Circuit Showing Internal Bias Network
RTC
Countdown
Chain
CL1
CL2
RTC
Registers
X2
X1
Crystal
Figure 3. Typical Crystal Layout
Local ground plane (Layer 2)
X1
NOTE: AVOID ROUTING SIGNAL LINES IN
THE CROSSHATCHED AREA (UPPER
LEFT QUADRANT) OF THE PACKAGE
UNLESS THERE IS A GROUND PLANE
BETWEEN THE SIGNAL LINE AND THE
DEVICE PACKAGE.
crystal
X2
GND
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DALLAS [ DALLAS SEMICONDUCTOR ]
