External Capacitors; External Circuitry For The Ich Rtc - Intel 810A3 Design Manual

Figure 4-26. External Circuitry for the ICH RTC
3.3V
VCCSUS
Vbatt
Note: Capacitor C2 and C3 values are crystal-dependent.
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):
C3 can be chosen such that C3 > C2; then, C2 can be trimmed to obtain the 32.768 KHz.
®
Intel 810A3 Chipset Design Guide
1 k Ω
1 k Ω
32768 Hz
C1
0.047 uF
Cload = (C2*C3)/(C2+C3) + Cparasitic
Layout and Routing Guidelines
1 µF
Xtal
C3
18 pF
C2
18 pF
V C C R T C
RTCX2
R1
10 M Ω
RTCX1
R2
10 M Ω
VBIAS
VSSRTC
4-25