User's Manual

Layout/Routing Guidelines
2-68 Intel
®
820 Chipset Design Guide
2.20.1 RTC Crystal
The ICH RTC module requires an external oscillating source of 32.768 KHz connected on the
RTCX1 and RTCX2 pins. Figure 2-56 documents the external circuitry that comprises the
oscillator of the ICH RTC.
NOTES:
1. The exact capacitor value needs to based on what the crystal maker recommends.
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 which is mirrored throughout the oscillator and buffer circuitry.
7. VSS: Ground
2.20.2 External Capacitors
To maintain the RTC accuracy, the external capacitor C1 needs to be 0.047 uF, 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.
Equation 2-4 can be used to choose the external capacitance values (C2 and C3):
Equation 2-4. External Capacitance Calculation
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.
Figure 2-56. 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