82575 Ethernet Controller Design Guide

4.1.3Programmable 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 Mbps operation), an MLT-3 (for 100 Mbps operation) or a PAM-5 (for 1000 Mbps 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 generally not recommended.

4.1.4Ceramic 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.

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Intel 317698-001 manual Programmable Crystal Oscillators, Ceramic Resonator
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317698-001 specifications

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