Data Sheet

Design Considerations—Ethernet Controller I210
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12.2.2.2 Fixed Crystal Oscillator
A packaged fixed crystal oscillator comprises an inverter, a quartz crystal, and passive components
conveniently packaged together. The device renders a strong, consistent square wave output.
Oscillators used with microprocessors are supplied in many configurations and tolerances.
Crystal oscillators should be restricted to use in special situations, such as shared clocking among
devices or multiple controllers. As clock routing can be difficult to accomplish, it is preferable to provide
a separate crystal for each device.
12.2.2.3 Programmable Crystal Oscillators
A programmable oscillator can be configured to operate at many frequencies. The device contains a
crystal frequency reference and a phase lock loop (PLL) clock generator. The frequency multipliers and
divisors are controlled by programmable fuses.
A programmable oscillator’s accuracy depends heavily on the Ethernet device’s differential transmit
lines. The Physical Layer (PHY) uses the clock input from the device to drive a differential Manchester
(for 10 Mb/s operation), an MLT-3 (for 100 Mbps operation) or a PAM-5 (for 1000 Mb/s operation)
encoded analog signal across the twisted pair cable. These signals are referred to as self-clocking,
which means the clock must be recovered at the receiving link partner. Clock recovery is performed
with another PLL that locks onto the signal at the other end.
PLLs are prone to exhibit frequency jitter. The transmitted signal can also have considerable jitter even
with the programmable oscillator working within its specified frequency tolerance. PLLs must be
designed carefully to lock onto signals over a reasonable frequency range. If the transmitted signal has
high jitter and the receiver’s PLL loses its lock, then bit errors or link loss can occur.
PHY devices are deployed for many different communication applications. Some PHYs contain PLLs with
marginal lock range and cannot tolerate the jitter inherent in data transmission clocked with a
programmable oscillator. The American National Standards Institute (ANSI) X3.263-1995 standard test
method for transmit jitter is not stringent enough to predict PLL-to-PLL lock failures, therefore, the use
of programmable oscillators is not recommended.
12.2.2.4 Ceramic Resonator
Similar to a quartz crystal, a ceramic resonator is a piezoelectric device. A ceramic resonator typically
carries a frequency tolerance of ±0.5%, – inadequate for use with Intel Ethernet controllers, and
therefore, should not be utilized.