S-35392A Datasheet, PDF (39/47 Pages) Seiko Instruments Inc – 2-WIRE REAL-TIME CLOCK

Rev.1.3_00
2-WIRE REAL-TIME CLOCK
S-35392A
3. Adjustment of oscillation frequency
(1) Adjustment by setting C_g
Matching of the crystal oscillator with the nominal frequency must be performed with the stray capacitance on the
board included. Select a crystal oscillator and optimize the C_g value in accordance with the flowchart below.
START
Select a crystal
oscillator^*1
Variable
capacitance
NO
Fixed capacitor
Set Cg
YES
Trimmer capacitor
Set to center
of variable
capacitance^*3
NO
Cg in
specification
YES
Change Cg
Optimal ^NO
value^*2
YES
NO
Frequency
YES
Make fine adjustment
of frequency using
variable capacitance
END
*1. Request a crystal manufacturer for matching evaluation between the IC and a crystal. The recommended
crystal characteristic values are, C_L value (load capacitance) = 6 pF, R₁ value (equivalent serial resistance) = 50
kΩ max.
*2. The C_g value must be selected on the actual PCB since it is affected by stray capacitance. Select the external C_g
value in a range of 0 pF to 9.1 pF.
*3. Adjust the rotation angle of the variable capacitance so that the capacitance value is slightly smaller than the
center, and confirm the oscillation frequency and the center value of the variable capacitance. This is done in
order to make the capacitance of the center value smaller than one half of the actual capacitance value because a
smaller capacitance value increases the frequency variation.
Figure 48 Crystal Oscillator Setting Flow
Caution
1. The oscillation frequency varies depending on the ambient temperature and power supply
voltage. Refer to “„ Characteristics (Typical Data)”.
2. The 32.768 kHz crystal oscillator operates more slowly at an operating temperature than higher
or lower 20 to 25°C. Therefore, it is recommended to set the oscillator to operate slightly faster
at normal temperature.
Seiko Instruments Inc.
39