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Intel
®
820E Chipset
R
Design Guide 97
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 RTC
2
C3
1
VCC3_3SBY
1 k
Ω
Vbat_rtc
1 k
Ω
C1
0.047 uF
32768 Hz
Xtal
R1
10 M
Ω
R2
10 M
Ω
C2
1
1
µ
F
VCCRTC
3
RTCX2
4
RTCX1
5
VBIAS
6
VSS
7
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. VCC
RTC
: 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. V
SS
: 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 (C
LOAD
) 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):
C
LOAD
= (C2 × C3) / (C2 + C3) + C
PARASITIC
.
C3 can be chosen such that C3 > C2. Then C2 can be trimmed to obtain the 32.768 kHz.