8413S12I-100_16 Datasheet, PDF (18/33 Pages) Integrated Device Technology – Clock Generator for Cavium Processors

8413S12I-100 Data Sheet
Overdriving the XTAL Interface
The XTAL_IN input can be overdriven by an LVCMOS driver or by
one side of a differential driver through an AC coupling capacitor.
The XTAL_OUT pin can be left floating. The amplitude of the input
signal should be between 500mV and 1.8V and the slew rate
should not be less than 0.2V/nS. For 3.3V LVCMOS inputs, the
amplitude must be reduced from full swing to at least half the swing
in order to prevent signal interference with the power rail and to
reduce internal noise. Figure 3A shows an example of the interface
diagram for a high speed 3.3V LVCMOS driver. This configuration
requires that the sum of the output impedance of the driver (Ro)
and the series resistance (Rs) equals the transmission line
impedance. In addition, matched termination at the crystal input
will attenuate the signal in half. This can be done in one of two
ways. First, R1 and R2 in parallel should equal the transmission
line impedance. For most 50 applications, R1 and R2 can be
100. This can also be accomplished by removing R1 and
changing R2 to 50. The values of the resistors can be increased
to reduce the loading for a slower and weaker LVCMOS driver.
Figure 3B shows an example of the interface diagram for an
LVPECL driver. This is a standard LVPECL termination with one
side of the driver feeding the XTAL_IN input. It is recommended
that all components in the schematics be placed in the layout.
Though some components might not be used, they can be utilized
for debugging purposes. The datasheet specifications are
characterized and guaranteed by using a quartz crystal as the
input.
VCC
R1
100
Ro
Rs
Zo = 50 ohms
R2
Zo = Ro + Rs
100
LVCMOS Driver
XTAL_OUT
C1
XTAL_IN
.1uf
Figure 3A. General Diagram for LVCMOS Driver to XTAL Input Interface
XTAL_OUT
Zo = 50 ohms
Zo = 50 ohms
LVPECL Driver
R1
R2
50
50
R3
50
C2
XTAL_IN
.1uf
Figure 3B. General Diagram for LVPECL Driver to XTAL Input Interface
©2016 Integrated Device Technology, Inc.
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October 4, 2016