Rainbow Electronics DS1337 - Manual (Page 7)

DS1337 I 2 C Serial Real-Time Clock
7 of 15
Figure 1. Oscillator Circuit Showing Internal Bias Network
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. Crystal
frequency drift caused by temperature shifts creates additional error. External circuit noise coupled into the
oscillator circuit can result in the clock running fast. Figure 2 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.
Figure 2. Typical PC Board Layout for Crystal
DS1337C ONLY
The DS1337C integrates a standard 32,768Hz crystal in the package. Typical accuracy at nominal V
CC
and +25°C
is approximately 35ppm. Refer to Application Note 58 for information about crystal accuracy vs. temperature.
LOCAL GROUND PLANE (LAYER 2)
CRYSTAL
X1
X2
GND
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.
COUNTDOWN
CHAIN
DS1337
X1 X2
CRYSTAL
RTC
REGISTERS
C
L
2 C
L
1