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R
9.8.1.
RTC Crystal
The Intel 82801DBM ICH4-M RTC module requires an external oscillating source of 32.768 kHz
connected on the RTCX1 and RTCX2 balls. Figure 114 documents the external circuitry that comprises
the oscillator of the ICH4-M RTC.
Figure 114. External Circuitry for the Intel 82801DBM ICH4-M RTC
3.3V Sus
1kΩ
Vbatt
Notes
Reference Designators Arbitrarily Assigned
3.3V Sus is Active Whenever System Plugged In
Vbatt is Voltage Provided By Battery
NOTES:
The exact capacitor value needs to be based on what the crystal maker recommends. (Typical values for C1 and
1.
C2 are 18 pF, based on crystal load of 12.5 pF.)
V
2.
CCRTC
RTCX2: Crystal Input 2 – Connected to the 32.7 68 kHz crystal
3.
RTCX1: Crystal Input 1 – Connected to the 32.7 68 kHz crystal
4.
V
: RTC BIAS Voltage – This ball is used to provide a reference voltage, and this DC voltage sets a current,
5.
BIAS
which is mirrored throughout the oscillator and buffer circuitry.
V
: Ground
6.
SS
Table 56. RTC Routing Summary
RTC Routing
Requirements
5 mil trace width
(results in ~2 pF per
inch)
®
Intel
855PM Chipset Platform Design Guide
C3
0.047uF
: Power for RTC Well
Maximum Trace
Length To Crystal
1 inch
1uF
32.768 kHz
Xtal
C1
18pF
VBIAS, VCCRTC, RTCX1, and RTCX2 are ICH4 pins
VBIAS is used to bias the ICH4 Internal Oscillator
VCCRTC powers the RTC well of the ICH4
RTCX1 is the Input to the Internal Oscillator
RTCX2 is the feedback for the external crystal
Signal
R1, R2, C1, and C2
Length
tolerances
Matching
NA
R1 = R2 = 10 M ± 5%
C1 = C2 = (NPO class)
See Section 9.8.2 for
calculating a specific
capacitance value for C1
and C2
I/O Subsystem
VCCRTC
RTCX2
R1
10MΩ
RTCX1
R2
C2
10MΩ
18pF
VBIAS
Signal
Referencing
Ground
207
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