Intel 810A3, Design Manual

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Intel
®
810A3 Chipset Design Guide 4-25
Layout and Routing Guidelines
NOTES:
1. The exact capacitor value should be based on the crystal vendor’s recommendations.
2. VccRTC: Power for RTC Well.
3. RTCX2: Crystal Input 2 – Connected to the 32.768 KHz crystal.
4. RTCX1: Crystal Input 1 – Connected to the 32.768 KHz crystal.
5. 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.
6. Vss: Ground.
4.13.2 External Capacitors
To maintain the RTC accuracy, the external capacitor C1 should be set to 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. 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.
Figure 4-26. External Circuitry for the ICH RTC
C3
18 pF
3.3V
V C C S U S
1 k
Ω
Vbatt
1 k
Ω
C1
0.047 uF
32768 Hz
Xtal
R1
10 M
Ω
R2
10 M
Ω
C2
18 pF
1
µ
F
V C C R T C
R T C X 2
R T C X 1
VBIAS
V S S R T C
Note: Capacitor C2 and C3 values are crystal-dependent.