82562EZ_08 Datasheet, PDF (17/66 Pages) Intel Corporation – Dual Footprint

82562EZ(EX)/82547GI(EI) Dual Footprint Design Guide
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 circuit is to establish resonance at a frequency higher than the
crystal’s inherent series resonant frequency.
Figure 4 illustrates a simplified schematic of the 82562EZ(EX) and the 82547GI(EI) controller’s
crystal circuit. 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.
LAN
Silicon
X1 or Xin
LAN
Silicon
X2 or Xout
C1 C2
Figure 4. Crystal Circuit
3.1.1.6 Load Capacitance
The formula for crystal load capacitance is as follows:
C_L = ₍-⁽--_C-^C---1-1----+-^â‹---^C-_C---2-2---⁾⁾- + C_stray
where C1 = C2 = 22 pF (as suggested in most Intel reference designs)
and C_stray = allowance for additional capacitance in pads, traces and the chip carrier
within the Ethernet controller package
An allowance of 3 pF to 7 pF accounts for lumped stray capacitance. The calculated load
capacitance is 16 pF with an estimated stray capacitance of about 5 pF.
Individual stray capacitance components can be estimated and added. For example, surface mount
pads for the load capacitors add approximately 2.5 pF in parallel to each capacitor. This technique
is especially useful if Y1, C1 and C2 must be placed farther than approximately one-half (0.5) inch
from the controller. It is worth noting that thin circuit boards generally have higher stray
capacitance than thick circuit boards.
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