Data Sheet

Design Considerations—Ethernet Controller I210
809
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.
12.3.1.5 Crystal Oscillation 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 12-10 shows the recommended placement and layout of an internal oscillator 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.
12.3.1.6 Load Capacitance and Discrete Capacitors
The formula for crystal load capacitance is as follows:
where:
C
L
is the rated C
load
of the crystal component and C1 and C2 are discrete crystal circuit
capacitors.
C
stray
allows for additional capacitance from solder pads, traces and the I210 package.
Individual stray capacitance components can be estimated and added as parallel capacitances.
Note that total C
stray
is typically 3 pF to 7 pF.
Solve for the discrete capacitor values as follows:
C1 = C2 = 2 * [C
load
- C
stray
]
For example:
If total C
stray
= 4.0 pF and if the C
load
rating is 18 pF, then the calculated C1 and C2 = 2 * [18
pF - 4.0 pF] = 28 pF.
Note: Because 28 pF is not a standard value, use 27 pF capacitors for C1 and C2, which is the
closest standard value.
The oscillator frequency should be measured with a precision frequency counter where possible. The
values of C1 and C2 should be fine tuned for the design. As the actual capacitive load increases, the
oscillator frequency decreases.
C
L
C1 C2
C1 C2+
-------------------
C
stray
+=