3-WIRE REAL-TIME CLOCK
S-35190A
Rev.2.4_00
3. Adjustment of oscillation frequency
(1) Adjustment by setting Cg
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 Cg value in accordance with the flowchart below.
START
Select a crystal
oscillator*1
Variable
capacitance
YES
Trimmer capacitor
NO
Set to center
Fixed capacitor
of variable
capacitance*3
Set C
g
NO
Frequency
YES
NO
C in
g
specification
YES
Change C
g
Make fine adjustment
of frequency using
variable capacitance
NO
Optimal
value*2
YES
END
*1. Request a crystal manufacturer for matching evaluation between the IC and a crystal. The recommended
crystal characteristic values are, CL value (load capacitance) = 6 pF, R1 value (equivalent serial resistance) = 50
kΩ max.
*2. The Cg value must be selected on the actual PCB since it is affected by stray capacitance. Select the external Cg
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 47 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.
38
Seiko Instruments Inc.
SII [ SEIKO INSTRUMENTS INC ]
