WG82574L S LBA8 Intel, WG82574L S LBA8 Datasheet - Page 424

• Manufacturer Part Number: WG82574L S LBA8
• Manufacturer: Intel

Specifications of WG82574L S LBA8

• Operating Supply Voltage (typ): 3.3V
• Operating Temp Range: 0C to 85C
• Operating Temperature Classification: Commercial
• Mounting: Surface Mount
• Pin Count: 64
• Lead Free Status / RoHS Status: Compliant

13.3.1.5
13.3.1.6
Note:
13.3.1.7
424
Calibration 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 64
circuit. Note that 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.
Load Capacitance
The formula for crystal load capacitance is as follows:
An allowance of 3 pF to 7 pF accounts for lumped stray capacitance. The calculated load
capacitance is 16 pF with an estimated stray capacitance of about 5 pF.
Individual stray capacitance components can be estimated and added. For example,
surface mount pads for the load capacitors add approximately 2.5 pF in parallel to each
capacitor. This technique is especially useful if Y1, C1 and C2 must be placed farther
than approximately one-half (0.5) inch from the device. It is worth noting that thin
circuit boards generally have higher stray capacitance than thick circuit boards. Consult
the PCIe Design Guide for more information.
The oscillator frequency should be measured with a precision frequency counter where
possible. The load specification or values of C1 and C2 should be fine tuned for the
design. As the actual capacitance load increases, the oscillator frequency decreases.
C1 and C2 may vary by as much as 5% (approximately 1 pF) from their nominal
values.
Shunt Capacitance
The shunt capacitance parameter is relatively unimportant compared to load
capacitance. Shunt capacitance represents the effect of the crystal’s mechanical holder
and contacts. The shunt capacitance should equal a maximum of 6 pF.
where C1 = C2 = 27 pF
and C
carrier within the Ethernet device package
shows the recommended placement and layout of an internal oscillator
stray
= allowance for additional capacitance in pads, traces and the chip
C
L
=
- - - - - - - - - - - - - - - - - - -


C1
C1 C2
+

C2


82574 GbE Controller—Design Considerations
+
C
stray