Data Sheet
Design Considerations—Ethernet Controller I210
811
appropriate capacitive load). A crystal with zero or near zero ppm deviation will be a good reference
crystal to use in subsequent frequency tests to determine the best values for C1 and C2.
• 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.
12.3.1.11.2 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.
12.3.1.11.3 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.
12.3.2 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.