S35720 Datasheet, PDF (15/21 Pages) Seiko Instruments Inc – FOR AUTOMOTIVE 125C OPERATION

FOR AUTOMOTIVE 125°C OPERATION WITH INTERRUPT TIME SETTING PIN CONVENIENCE TIMER
Rev.1.0_00
S-35720 Series
 Configuration of Oscillation Circuit
Since the S-35720 Series has built-in capacitors (C_g and C_d), adjustment of oscillation frequency is not necessary.
However, the crystal oscillation is sensitive to external noise and parasitic capacitance (C_P), these effects may become a
factor to worsen the clock accuracy. Therefore, the following steps are recommended for optimizing the configuration of
oscillation circuit.
• Locate the bypass capacitor adjacent to the power supply pin of the S-35720 Series.
• Place the S-35720 Series and the crystal oscillator as close to each other as possible, and shorten the substrate wiring.
• Increase the insulation resistance between pins and the substrate wiring patterns of XIN and XOUT.
• Do not place any signal or power lines close to the oscillation circuit.
• Locate the GND layer immediately below the oscillation circuit.
(In the case of a multi-layer board, only the layer farthest from the oscillation circuit should be located as the GND layer.
Do not locate a circuit pattern on the intermediate layers.)
Crystal oscillator: 32.768 kHz ^S-35720
C_L = 6 pF to 12.5 pF
XIN
C_g
Parasitic
capacitance (C_P)
C_d
XOUT
R_f
R_d
Internal constant standard values of oscillation circuit
R_f = 100 MΩ
R_d = 100 kΩ
Figure 17 Configuration of Oscillation Circuit
Locate the GND layer in the
layer immediately below
(In the case of a multi-layer board,
only the layer farthest from the oscillation
circuit should be located as the GND layer.
Do not locate a circuit pattern on the
intermediate layers.)
Crystal
oscill ator
XOUT
XIN
VSS
S-35720
Top view
Shield the perimeter with GND
Figure 18 Example of Recommended Connection Pattern Diagram
Caution 1. Oscillation characteristics are subject to the variation of each component such as substrate
parasitic capacitance, parasitic resistance, crystal oscillator, and external capacitor. When
configuring crystal oscillation circuit, pay sufficient attention for them.
2. When using the product in automobile equipment, select the components which can be
automobile carried for each component such as crystal oscillator, external capacitor, and
substrate.
Seiko Instruments Inc.
15