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82575 Ethernet Controller Design Guide
32
5.0 Crystal Selection Parameters
All crystals used with Intel Ethernet controllers are described as “AT-cut,” which refers
to the angle at which the unit is sliced with respect to the long axis of the quartz stone.
Table 4 lists crystals which have been used successfully in other designs (however, no
particular product is recommended):
For information about crystal selection parameters, see the chart in the 82575 Ethernet
Controller Advance Information Datasheet. Contact your Intel representative to obtain
this document.
5.1 Vibrational Mode
Crystals in the above-referenced frequency range are available in both fundamental
and third overtone. Unless there is a special need for third overtone, use fundamental
mode crystals.
At any given operating frequency, third overtone crystals are thicker and more rugged
than fundamental mode crystals. Third overtone crystals are more suitable for use in
military or harsh industrial environments. Third overtone crystals require a trap circuit
(extra capacitor and inductor) in the load circuitry to suppress fundamental mode
oscillation as the circuit powers up. Selecting values for these components is beyond
the scope of this document.
5.2 Nominal Frequency
Intel Ethernet controllers use a crystal frequency of 25.000 MHz. The 25 MHz input is
used to generate a 125 MHz transmit clock for 100BASE-TX and 1000BASE-TX
operation – 10 MHz and 20 MHz transmit clocks, for 10BASE-T operation.
5.3 Frequency Tolerance
The frequency tolerance for an Ethernet Platform LAN Connect is dictated by the IEEE
802.3 specification as ±50 parts per million (ppm). This measurement is referenced to
a standard temperature of 25° C. Intel recommends a frequency tolerance of ±30 ppm.
5.4 Temperature Stability and Environmental Requirements
Temperature stability is a standard measure of how the oscillation frequency varies
over the full operational temperature range (and beyond). Several optional
temperature ranges are currently available, including -40° C to +85° C for industrial
environments. Some vendors separate operating temperatures from temperature
stability. Manufacturers may also list temperature stability as 50 ppm in their data
sheets.
Table 7. Crystal Manufacturers and Part Numbers
Manufacturer Part No.
RALTRON AS-25.000-20-F-SMD-T
CITIZEN AMERICA CORP HCM4925.000MBBKTR
NDK AMERICA INC 41CD25.0S11005020
TXC CORPORATION - USA 6C25000131