Intel 820E manual RTC Crystal, External Capacitors

Intel® 820E Chipset

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2.19.1.RTC Crystal

The ICH2 RTC module requires an external 32.768 kHz oscillating source connected on the RTCX1 and RTCX2 pins. The following figure shows the external circuitry that comprises the oscillator of the ICH2 RTC.

Figure 59. External Circuitry for the ICH RTC2

VCC3_3SBY

Vbat_rtc

1kΩ

1kΩ

C1

0.047 uF

1 ∝ F

32768 Hz

Xtal

C31

C21

VCCRTC3

RTCX24

R1

10 MΩ

RTCX15

R2

10 MΩ

VBIAS6

VSS7

rtc_cir

NOTES:

1.The exact capacitor value must be based on the crystal maker’s recommendation.

2.This circuit is not the same as the one used for PIIX4.

3.VCCRTC: Power for RTC well

4.RTCX2: Crystal input 2 – Connected to the 32.768 kHz crystal

5.RTCX1: Crystal input 1 – Connected to the 32.768 kHz crystal

6.VBIAS: RTC bias voltage – This pin is used to provide a reference voltage, and this DC voltage sets a current that is mirrored throughout the oscillator and buffer circuitry.

7.VSS: Ground

2.19.2.External Capacitors

To maintain RTC accuracy, the external capacitor C1 must have a capacitance of 0.047 µF, and the external capacitor values (C2 and C3) should be chosen to provide the manufacturer’s specified load capacitance (CLOAD) for the crystal, when combined with the parasitic capacitance of the trace, socket (if used), and package. When the external capacitor values are combined with the capacitance of the trace, socket, and package, the closer the capacitor value can be matched to the actual load capacitance of the crystal used, the more accurate the RTC will be.

The following equation can be used to choose the external capacitance values (C2 and C3):

CLOAD = (C2 ⋅ C3) / (C2 + C3) + CPARASITIC.

C3 can be chosen such that C3 > C2. Then C2 can be trimmed to obtain the 32.768 kHz.

Design Guide 97