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Rev. 5.00, 09/03, page 425 of 760
13.3.5 Crystal Oscillator Circuit
Crystal oscillator circuit constants (recommended values) are shown in table 13.5, and the RTC
crystal oscillator circuit in figure 13.5.
Table 13.5 Recommended Oscillator Circuit Constants (Recommended Values)
fosc Cin Cout
32.768 kHz 10 to 22 pF 10 to 22 pF
SH7709S
EXTAL2
XTAL2
XTAL
C
in
C
out
R
f
R
D
Notes: 1. Select either the C
in
or C
out
side for the frequency adjustment variable capacitor
according to requirements such as frequency range, degree of stability, etc.
2. Built-in resistance value R
f
(Typ value) = 10 MΩ, R
D
(Typ value) = 400 kΩ
3. C
in
and C
out
values include floating capacitance due to the wiring. Take care
when using a ground plane.
4. The crystal oscillation settling time depends on the mounted circuit constants,
floating capacitance, etc., and should be decided after consultation with the
crystal resonator manufacturer.
5. Place the crystal resonator and load capacitors C
in
and C
out
as close as possible
to the chip.
(Correct oscillation may not be possible if there is externally induced noise in the
EXTAL2 and XTAL2 pins.)
6. Ensure that the crystal resonator connection pin (EXTAL2, XTAL2) wiring is
routed as far away as possible from other power lines (except GND) and signal
lines.
Figure 13.5 Example of Crystal Oscillator Circuit Connection