SH7261 datasheet(698/1348 Pages) RENESAS

698 / 1348 page

Section 15 Realtime Clock (RTC)

Rev. 2.00 Sep. 07, 2007 Page 666 of 1312

REJ09B0320-0200

15.5.3

Transition to Standby Mode after Setting Register

When a transition to standby mode is made after registers in the RTC are set, sometimes counting

is not performed correctly. In case the registers are set, be sure to make a transition to standby

mode after waiting for two RTC clocks or more.

15.5.4

Crystal Oscillator Circuit for RTC

Crystal oscillator circuit constants (recommended values) for the RTC are shown in table 15.3, and

the RTC crystal oscillator circuit in figure 15.6.

Table 15.3

Crystal Oscillator Circuit Constants (Recommended Values)

osc

out

32.768 kHz

10 to 22 pF

This LSI

RTC_X1

RTC_X2

XTAL

out

Notes: 1. Select either the C

in or Cout 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, 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 stabilization time may differ depending on the mounted circuit component constants,

stray capacitance, and so forth, so a suitable value should be determined in consultation with the resonator

manufacturer.

5. Place the crystal resonator and load capacitors C

in and Cout as close as possible to the chip. Make wiring

length as short as possible. Do not allocate signal lines close to oscillation circuit.

(Correct oscillation may not be possible if there is externally induced noise in the RTC_X1 and RTC_X2 pins.)

6. Ensure that the wiring of the crystal oscillator connection pins (RTC_X1 and RTC_X2) is routed as far away as

possible from the power lines (except GND) and signal lines.

7. When crystal oscillation circuit for RTC is not used, connect the RTC_X1 pin to GND and leave the RTC_X2

pin open

Figure 15.6 Example of Connecting Crystal Oscillator Circuit for RTC

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