DS1339 Datasheet, PDF (9/18 Pages) Dallas Semiconductor – Serial Real-Time Clock

DS1339 I²C Serial Real-Time Clock
Table 1. Crystal Specifications*
PARAMETER
Nominal Frequency
Series Resistance
Load Capacitance
SYMBOL MIN
TYP
MAX
f_O
32.768
ESR
45
C_L
6
UNITS
kHz
kW
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 4. Oscillator Circuit Showing Internal Bias Network
RTC
COUNTDOWN
CHAIN
C_L1
C_L2
RTC
REGISTERS
X1
X2
CRYSTAL
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 is 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 5 shows a typical PC board layout for isolating the
crystal and oscillator from noise. Refer to Application Note 58: Crystal Considerations with Dallas Real-Time
Clocks for detailed information
DS1339C ONLY
The DS1339C integrates a standard 32,768Hz crystal in the package. Typical accuracy at nominal V_CC and +25°C
is approximately 10ppm. Refer to Application Note 58 for information about crystal accuracy vs. temperature.
Figure 5. 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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