SH7720 Datasheet, PDF (646/1524 Pages) Renesas Technology Corp – Renesas 32-Bit RISC Microcomputer SuperHTM RISC engine Family / SH7700 Series

Section 17 Realtime Clock (RTC)
17.5.4 Crystal Oscillator Circuit
Crystal oscillator circuit constants (recommended values) are shown in table 17.2, and the RTC
crystal oscillator circuit in figure 17.5.
Table 17.2 Recommended Oscillator Circuit Constants (Recommended Values)
f_osc
32.768 kHz
C_in
10 to 22 pF
C_out
10 to 22 pF
This LSI
R_f
EXTAL_RTC
XTAL
C_in
R_D
XTAL_RTC
C_out
Notes: 1. Select either the C_in or C_out side for 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,
stray 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
EXTAL_RTC and XTAL_RTC pins.)
6. Ensure that the crystal resonator connection pin (EXTAL_RTC, XTAL_RTC)
wiring is routed as far away as possible from other power lines (except GND) and
signal lines.
Figure 17.6 Example of Crystal Oscillator Circuit Connection
Rev. 3.00 Jan. 18, 2008 Page 584 of 1458
REJ09B0033-0300