SH7721 datasheet(646/1524 Pages) RENESAS

646 / 1524 page

Section 17 Realtime Clock (RTC)

Rev. 3.00 Jan. 18, 2008 Page 584 of 1458

REJ09B0033-0300

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)

osc

out

32.768 kHz

10 to 22 pF

This LSI

EXTAL_RTC

XTAL_RTC

XTAL

Cin

Cout

Notes: 1. Select either the Cin or Cout side for frequency adjustment variable capacitor

according to requirements such as frequency range, degree of stability, etc.

2. Built-in resistance value Rf (Typ value) = 10 MΩ, RD (Typ value) = 400 kΩ

3. Cin and Cout 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 Cin and Cout 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

SH7721 Datasheet(PDF) 646 Page - Renesas Technology Corp