236
Intel
®
855GME Chipset and Intel
®
6300ESB ICH Embedded Platform Design Guide
Intel
®
6300ESB Design Guidelines
In certain conditions, both C
1
, C
2
values may be shifted away from the theoretical values
(calculated values from the above equation) to obtain the closest oscillation frequency to
32.768 kHz. When C
1
, C
2
value are smaller then the theoretical values, the RTC oscillation
frequency is higher.
The following example illustrates the use of the practical values C
1
, C
2
in the case that theoretical
values may not ensure the accuracy of the RTC in low temperature condition:
Example:
According to a required 12 pF load capacitance of a typical crystal that is used with the ICHn, the
calculated values of C
1
= C
2
is 10 pF at room temperature (25
°
C) to yield an 32.768 kHz
oscillation.
At 0° C the frequency stability of crystal gives –23 ppm (assumed that the circuit has 0 ppm at
25
°
C). This makes the RTC circuit oscillate at 32.767246 kHz instead of 32.768 kHz.
When the values of C
1
, C
2
are chosen to be 6.8 pF instead of 10 pF, the RTC oscillates at higher
frequency at room temperature (+23 ppm), but this configuration of C
1
/ C
2
makes the circuit
oscillate closer to 32.768 kHz at 0
°
C. The 6.8 pF value of C1 and 2 is the practical value.
Note:
The temperature dependency of a crystal frequency is a parabolic relationship (ppm/degree
square). The effect of changing crystal’s frequency when operating at 0
°
C (25
°
below room
temperature) is the same when operating at 50
°
C (25
°
C above room temperature).
9.11.3
RTC Layout Considerations
Since the RTC circuit is very sensitive and requires a 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. The 6300ESB requires a trace
length less than one inch on each branch (from crystal’s terminal to RTCXn ball). Routing of
the RTC circuit should be kept simple to simplify the trace length measurement and increase
accuracy on calculating trace capacitances. Trace capacitance depends on the trace width and
dielectric constant of board’s material. On FR406, a 5 mil trace has approximately 2 pF per
inch.
•
Trace signal coupling must be importantly reduced, by avoiding routing of adjacent PCI
signals close to RTCX1, RTCX2, and VBIAS.
•
Ground guard plane is highly recommended.
•
The oscillator V
CC
should be clean; use a filter, such as an RC low-pass, or a ferrite inductor.
9.11.4
RTC External Battery Connection
The RTC requires an external battery connection to maintain its functionality and its RAM while
the 6300ESB is not powered by the system.
Example batteries are: Duracell* 2032, 2025, or 2016 (or equivalent). Batteries are rated by storage
capacity. The battery life, measured in years, may be calculated by dividing the capacity by the
average current required. For example, when the battery storage capacity is 220 mAh
(approximate) and the average current required is 8.5 µA, the battery life will be at least:
~220,000 µAh / 8.5 µA
≅
26,000h
≅
3 years
Summary of Contents for 6300ESB ICH
Page 24: ...24 Intel 855GME Chipset and Intel 6300ESB ICH Embedded Platform Design Guide Introduction...
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Page 190: ...190 Intel 855GME Chipset and Intel 6300ESB ICH Embedded Platform Design Guide Hub Interface...
Page 318: ...318 Intel 855GME Chipset and Intel 6300ESB ICH Embedded Platform Design Guide Layout Checklist...
