General Rtc Layout Considerations; External Capacitors - Intel Quark SoC X1000 Design Manual

12.3.2

General RTC Layout Considerations

The Intel
default, the external RTC clock is required in cases where the SoC will be shutdown to
G3. In G3 the RTC is powered and the RTC clock is sourced from an 32kHz platform
crystal. The use of an external RTC clock source allows the use of a more precise 32kHz
clock.
Since the RTC circuit is very sensitive and requires highly accurate oscillation,
reasonable care must be taken during layout and routing of the RTC circuit. Some
recommendations are:
• Reduce trace capacitance by minimizing the RTC trace length. Intel SoC
recommends a trace length less than 1 inch on each branch (from crystal's terminal
to RTCXn ball). Keep routing on the RTC circuit basic to simplify the trace length
measurement and increase accuracy on calculating trace capacitances. Trace
capacitance depends on the trace width and dielectric constant of the board's
material. On FR4, a 5-mil trace has approximately 3.3 pF per inch.
• Using a ground guard plane is highly recommended.
The oscillator VCC should be clean; use a filter, such as an RC low-pass, or a ferrite
inductor.
12.3.3

External Capacitors

To maintain the RTC accuracy, the external capacitor values C
to provide the manufacturer's 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 = [(C
load
[(C + C
1 in1
Where:
C = Crystal's load capacitance. This value can be obtained from Crystal's
load
specification.
Cin1, Cin2 = input capacitances at RTCX1, RTCX2 balls of SoC. These values can be
obtained in the SoC data sheet.
C , C
trace1
RTCX2 balls. 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 mil wide trace
and a ½ ounce copper pour, is approximately equal to:
C = trace length • 4pF/inch
trace
C
parasitic
of 2 electrode plates and the dielectric constant of the crystal blank inside the Crystal
part. Refer to the crystal's specification to obtain this value.
Ideally, C1, C2 can be chosen such that C1 = C2. Using the equation of C
value of C1, C2 can be calculated to give the best accuracy (closest to 32.768 kHz) of
the RTC circuit at room temperature. However, C2 can be chosen such that C2 > C1.
Then C1 can be trimmed to obtain the 32.768 kHz.
In certain conditions, both C1, C2 values can be shifted away from the theoretical
values (calculated values from the above equation) to obtain the closest oscillation
frequency to 32.768 kHz. When C1, C2 value are smaller then the theoretical values,
the RTC oscillation frequency will be higher.
®
Intel Quark™ SoC X1000
PDG
88
®
Quark™ SoC X1000 operates in a internally source RTC clock mode by
+ C + C ) • (C
1 in1 trace1
+ C + C + C
trace1 2 in2
= Trace length capacitances measured from Crystal terminals to RTCX1,
trace2
= Crystal's parasitic capacitance. This capacitance is created by the existence
®
Intel Quark™ SoC X1000—RTC Design Guidelines
+ C + C )] /
2 in2 trace2
+ C )] + C
trace2 parasitic
and C should be chosen
1 2
) for the crystal when
load
above, the
load
June 2014
Order Number: 330258-002US