DS1302 Trickle-Charge Timekeeping Chip
OSCILLATOR CIRCUIT
The DS1302 uses an external 32.768kHz crystal. The oscillator circuit does not require any external resistors or
capacitors to operate. Table 1 specifies several crystal parameters for the external crystal. Figure 2 shows a
functional schematic of the oscillator circuit. If using a crystal with the specified characteristics, the startup time is
usually less than one second.
CLOCK ACCURACY
The accuracy of the clock is dependent upon 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 will be added by crystal frequency drift caused by temperature shifts. External circuit noise coupled into the
oscillator circuit may result in the clock running fast. Figure 3 shows a typical PC board layout for isolating the
crystal and oscillator from noise. Refer to
Application Note 58: Crystal Considerations for Dallas Real-Time Clocks
for detailed information.
Table 1. Crystal Specifications*
PARAMETER
Nominal Frequency
Series Resistance
Load Capacitance
SYMBOL
f
O
ESR
C
L
6
MIN
TYP
32.768
45
MAX
UNITS
kHz
kΩ
pF
*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.
Figure 2. Oscillator Circuit Showing Internal Bias Network
RTC
COUNTDOWN
CHAIN
C
L
1
C
L
2
RTC
REGISTERS
X1
X2
CRYSTAL
Figure 3. Typical PC Board Layout for Crystal
LOCAL GROUND PLANE (LAYER 2)
X1
CRYSTAL
X2
NOTE:
AVOID ROUTING SIGNALS IN THE
CROSSHATCHED AREA (UPPER LEFT-
HAND QUADRANT) OF THE PACKAGE
UNLESS THERE IS A GROUND PLANE
BETWEEN THE SIGNAL LINE AND THE
PACKAGE.
GND
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DALLAS [ DALLAS SEMICONDUCTOR ]
