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Real Time Clock (RTC) Interface—Intel
Figure 130. External Circuitry for the RTC
VCCPSUS
1 kΩ
Vbatt
+
Notes:
1.
The exact capacitor values for C1 and C2 must be based on the crystal maker recommendations.
Typical values for C1 and C2 are 15 pF, based on crystal load of 12.5 pF.
2.
Reference designators are arbitrarily assigned.
3.
VCCPSUS is active whenever the system is plugged in.
4.
Vbatt is voltage provided by the battery.
5.
VCCPRTC, RTCX1, and RTCX2 are EP80579 pins.
6.
VCCPRTC powers the EP80579 RTC well.
7.
RTCX1 is the input to the internal oscillator.
8.
RTCX2 is the feedback for the external crystal.
Table 71.
RTC Routing Summary
Trace
Impedance
50 Ω ±10%
15.1.2
External Capacitors
To maintain the RTC accuracy, the external capacitor values C1 and C2 must be chosen
to provide the manufacturer-specified load capacitance (C
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:
C
=
[
(
load
where:
• C
load
specification.
• C
, C
in1
values can be obtained from the Intel
Datasheet.
• C
trace1
the RTCX1 and RTCX2 signals. These values depend on the characteristics of board
material, the width of signal traces and the length of the traces. A typical value,
based on a 5 mils wide trace and a ½ ounce copper pour, is approximately equal to:
®
Intel
EP80579 Integrated Processor Product Line
Order Number: 320068-005US
®
EP80579 Integrated Processor Product Line
Schottky
Diodes
RTC Routing
Maximum Trace
Requirements
Length To Crystal
5 mil trace width
1.2 in. (the total
(results in ~2 pF/
net length including
in.)
caps is 1.5 inches)
C
+
C
+
C
)
²
(
C
+
C
1
in1
2
in2
trace1
= crystal's load capacitance. This value can be obtained from the crystal
= input capacitances at the RTCX1 and RTCX2 signals of EP80579. These
in2
, C
= trace length capacitances measured from the crystal terminals to
trace2
1μF
32.768 kHz
Xtal
C1
Signal
Length
Matching
N/A
⁄
+
C
)
]
[
C
+
C
+
C
trace2
1
in1
trace1
®
EP80579 Integrated Processor Product Line
VCCPRTC
RTCX2
R1
10 MΩ
RTCX1
C2
R1, C1, and C2
Signal
Tolerances
Referencing
R1 = 10 MΩ ±5%
C1 = C2 = (NPO
class)
Ground
See
Section 15.1.2
for calculating a
specific capacitance
value for C1 and C2
) for the crystal when
load
+
C
+
C
+
C
]
+
C
2
in2
trace2
parasitic
May 2010
194
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