82583V Datasheet, PDF (330/374 Pages) Intel Corporation – Intel® 82583V GbE Controller

82583V GbE Controller—Design Considerations
• If a crystal analyzer is not available, then the selection of a reference crystal can be
done by measuring a statistically valid sample population of crystals, which has
units from multiple lots and approved vendors. The crystal, which has an oscillation
frequency closest to the center of the distribution, should be the reference crystal
used during testing to determine the best values for C1 and C2.
• It may also be possible to ask the approved crystal vendors or manufacturers to
provide a reference crystal with zero or nearly zero deviation from the specified
frequency when it has the specified CLoad capacitance.
When choosing a crystal, customers must keep in mind that to comply with IEEE
specifications for 10/100 and 10/100/1000Base-T Ethernet LAN, the transmitter
reference frequency must be precise within ±50 ppm. Intel® recommends customers to
use a transmitter reference frequency that is accurate to within ±30 ppm to account for
variations in crystal accuracy due to crystal manufacturing tolerance.
11.3.1.11.2 Circuit Board
Since the dielectric layers of the circuit board are allowed some reasonable variation in
thickness, the stray capacitance from the printed board (to the crystal circuit) will also
vary. If the thickness tolerance for the outer layers of dielectric are controlled within
±17 percent of nominal, then the circuit board should not cause more than ±2 pF
variation to the stray capacitance at the crystal. When tuning crystal frequency, it is
recommended that at least three circuit boards are tested for frequency. These boards
should be from different production lots of bare circuit boards.
Alternatively, a larger sample population of circuit boards can be used. A larger
population will increase the probability of obtaining the full range of possible variations
in dielectric thickness and the full range of variation in stray capacitance.
Next, the exact same crystal and discrete load capacitors (C1 and C2) must be soldered
onto each board, and the LAN reference frequency should be measured on each circuit
board.
The circuit board, which has a LAN reference frequency closest to the center of the
frequency distribution, should be used while performing the frequency measurements
to select the appropriate value for C1 and C2.
11.3.1.11.3 Temperature Changes
Temperature changes can cause the crystal frequency to shift. Therefore, frequency
measurements should be done in the final system chassis across the system’s rated
operating temperature range.
11.3.2
Crystal Placement and Layout Recommendations
Crystal clock sources should not be placed near I/O ports or board edges. Radiation
from these devices can be coupled into the I/O ports and radiate beyond the system
chassis. Crystals should also be kept away from the Ethernet magnetics module to
prevent interference.
Note:
Failure to follow these guidelines could result in the 25 MHz clock failing to start.
When designing the layout for the crystal circuit, the following rules must be used:
• Place load capacitors as close as possible (within design-for-manufacturability
rules) to the crystal solder pads. They should be no more than 90 mils away from
crystal pads.
• The two load capacitors, crystal component, the Ethernet controller device, and the
crystal circuit traces must all be located on the same side of the circuit board
(maximum of one via-to-ground load capacitor on each XTAL trace).
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