82801BA Datasheet, PDF (52/498 Pages) Intel Corporation – Intel 82801BA I/O Controller Hub 2 (ICH2) and Intel 82801BAM I/O Controller Hub 2 Mobile

Signal Description
2.20.3 External RTC Circuitry
To reduce RTC well power consumption, the ICH2 implements an internal oscillator circuit that is
sensitive to step voltage changes in VccRTC and VBIAS. Figure 2-1 shows a schematic diagram of
the circuitry required to condition these voltages to ensure correct operation of the ICH2 RTC.
Figure 2-1. Required External RTC Circuit
3.3V
VCCSUS
1 kΩ
1 µF
VCCRTC
Vbatt
1 kΩ
32768 Hz
Xtal
C1
0.047 uF
C3
12.5 pF
C2
12.5 pF
Note: Capacitor C2 and C3 values are crystal-dependent.
RTCX2
R1
10 MΩ
RTCX1
R2
10 MΩ
VBIAS
VSSRTC
2.20.4 V5REF / Vcc3_3 Sequencing Requirements
V5REF and V5REF_Sus are the reference voltages for 5V tolerance on inputs to the ICH2. V5REF
and V5REF_Sus must power up before or simultaneous to Vcc3_3 and VccSus3_3 respectively,
and must power down after or simultaneous to Vcc3_3 and VccSus3_3 respectively. Refer to
Figure 2-2 for an example circuit schematic that may be used to ensure proper V5REF sequencing.
Note that separate circuits must be implemented for both the Core and Suspend well supplies.
Figure 2-2. Example V5REF Sequencing Circuit
VCC Supply
(3.3V)
5V Supply
1k
Schottky
Diode
1 uF
To System
5VREF
To System
2-16
82801BA ICH2 and 82801BAM ICH2-M Datasheet