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DS1337S+C01 开发手册 - Maxim Integrated（美信）
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Maxim Integrated（美信）
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实时时钟芯片
封装:
SOIC-8
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DS1337S+C01数据手册
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Crystal Parameters
Figure 3 shows the equivalent circuit for a crystal. Near the resonate frequency the circuit consists of a series circuit including motional inductance L
1
, motional resistance
R
1
, and motional capacitance C
1
. The parallel component C
O
is the shunt capacitance of the crystal.
Figure 3. Crystal equivalent circuit.
The load capacitance C
L
is the capacitive load of the oscillating circuit as seen from the pins of the crystal. Figure 4 shows C
L
as a capacitance in parallel with the crystal.
The load capacitors used in an oscillator circuit, C
L
1 and C
L
2, plus any stray capacitance in the circuit, combine to create the overall load capacitance. All Maxim RTCs
have integrated C
L
1 and C
L
2 capacitors. Care should be taken to minimize stray capacitance in the printed circuit board (PCB) layout. The following formula shows the
relationship between C
L
and load capacitor values:
C
L
= [(C
L
1 × C
L
2)/(C
L
1 + C
L
2) + C
STRAY
]
Figure 4. Crystal load capacitors and equivalent parallel load.
Most crystals allow a maximum drive level of 1µW. All Maxim RTCs run under 1µW. Drive level can determined using the following formula:
P = 2R
1
× [π × 32,768(C
O
+ C
L
)V
RMS
]²
where V
RMS
is the RMS value of the voltage across the crystal.
Oscillator Startup Time
Oscillator startup times are highly dependent upon crystal characteristics, PCB leakage, and layout. High ESR and excessive capacitive loads are the major contributors to
long startup times. A circuit using a crystal with the recommended characteristics and proper layout usually starts within one second.
Table 1. Crystal Specifications
Parameter Symbol Min Typ Max Units
Nominal Frequency f
O
32.768 kHz
Frequency Tolerance delta f/f
O
±20 ppm
Load Capacitance C
L
6 pF
Temperature Turnover Point T
0
20 25 30 °C
Parabolic Curvature Constant k
0.042 ppm/°C
Quality Factor Q 40,000 70,000
Series Resistance ESR 45 kΩ
Shunt Capacitance C
0
1.1 1.8 pF
Capacitance Ratio C
0
/C
1
430 600
Drive Level D
L
1 µW
Note 1: Some devices allow higher ESR values, check the datasheet for specific requirements.
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