82575 Ethernet Controller Design Guide

Note: Crystals also carry other specifications for storage temperature, shock resistance, and reflow solder conditions. Crystal vendors should be consulted early in the design cycle to discuss the application and its environmental requirements.

5.5Calibration Mode

The terms “series-resonant” and “parallel-resonant” are often used to describe crystal oscillator circuits. Specifying parallel mode is critical to determining how the crystal frequency is calibrated at the factory.

A crystal specified and tested as series resonant oscillates without problem in a parallel-resonant circuit, but the frequency is higher than nominal by several hundred parts per million. The purpose of adding load capacitors to a crystal oscillator circuit is to establish resonance at a frequency higher than the crystal’s inherent series resonant frequency.

Figure 3 illustrates a simplified schematic of the internal oscillator circuit. Pin X1 and X2 refers to XTAL1 and XTAL2 in the Ethernet device, respectively. The crystal and the capacitors form a feedback element for the internal inverting amplifier. This combination is called parallel-resonant, because it has positive reactance at the selected frequency. In other words, the crystal behaves like an inductor in a parallel LC circuit. Oscillators with piezoelectric feedback elements are also known as “Pierce” oscillators.

Pin X1

 

Pin X2

C1

Y1

C2

27 pF

25 MHz

27 pF

5%

 

5%

Figure 9. Internal Oscillator Circuit

5.6Load Capacitance

The formula for crystal load capacitance is as follows:

(C1 ⋅ C2)

CL = ------------------+ Cstray

(C1 + C2)

where C1 = C2 = 27 pF

and Cstray = allowance for additional capacitance in pads, traces and the chip carrier within the Ethernet device package

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Intel 317698-001 manual Calibration Mode, Load Capacitance
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317698-001 specifications

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