82583V Datasheet, PDF (328/374 Pages) Intel Corporation – Intel® 82583V GbE Controller

82583V GbE Controller—Design Considerations
11.3.1.5
11.3.1.6
Crystal Oscillation Mode
The terms series-resonant and parallel-resonant are often used to describe crystal
oscillator circuits. Specifying parallel mode is critical to determining how the crystal
frequency is calibrated at the factory.
A crystal specified and tested as series resonant oscillates without problem in a
parallel-resonant circuit, but the frequency is higher than nominal by several hundred
parts per million. The purpose of adding load capacitors to a crystal oscillator circuit is
to establish resonance at a frequency higher than the crystal’s inherent series resonant
frequency.
Figure 51 shows the recommended placement and layout of an internal oscillator
circuit. Note that pin X1 and X2 refers to XTAL1 and XTAL2 in the Ethernet device,
respectively. The crystal and the capacitors form a feedback element for the internal
inverting amplifier. This combination is called parallel-resonant, because it has positive
reactance at the selected frequency. In other words, the crystal behaves like an
inductor in a parallel LC circuit. Oscillators with piezoelectric feedback elements are
also known as “Pierce” oscillators.
Load Capacitance and Discrete Capacitors
The formula for crystal load capacitance is as follows:
Note:
Note:
where:
C_L = ₍-⁽-_C-^C--₁-¹---₊^â‹---^CC---²-₂-⁾-₎ + C_stray
cCr^Lyisstathl ^{ecirrcauteitdcaCplo}a^ac^ditoofrtsh. ^{e crystal component and C1 and C2 are discrete}
C82_{st5ra8y3aVllopwacskfaogrea.dIdnidtiiovnidaulaclasptaracyitacnacpeacfritoamncseocldoemrppoandesn,ttsraccaensbaendestthime ated}
and
pF.
added
as
parallel
capacitances.
Note
that
total
C_stray
is
typically
3
pF
to
7
Solve for the discrete capacitor values as follows:
C1 = C2 = 2 * [C_load - C_stray]
For example:
If total C_stray = 4.0 pF and if the C_load rating is 18 pF, then the calculated C1
and C2 = 2 * [18 pF - 4.0 pF] = 28 pF.
Because 28 pF is not a standard value, use 27 pF capacitors for C1 and C2, which is the
closest standard value.
The oscillator frequency should be measured with a precision frequency counter where
possible. The values of C1 and C2 should be fine tuned for the design. As the actual
capacitive load increases, the oscillator frequency decreases.
Intel recommends COG or NPO capacitors with a tolerance of ±5% (approximately
±1 pF) or smaller.
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