87608AYI
www.idt.com
REV. C OCTOBER 13, 2010
10
ICS87608I
LOW VOLTAGE/LOW SKEW, 1:8 PCI/PCI-X
ZERO DELAY CLOCK GENERATOR
CRYSTAL INPUT INTERFACE
The ICS87608I has been characterized with 18pF parallel
resonant crystals. The capacitor values, C1 and C2, shown in
Figure 2 below were determined using a 25MHz, 18pF parallel
Figure 2. CRYSTAL INPUt INTERFACE
resonant crystal and were chosen to minimize the frequency
ppm error. The optimum C1 and C2 values can be slightly ad-
justed for optimum frequency accuracy.
C1
22p
X1
18pF Parallel Cry stal
C2
22p
XTAL2
XTAL1
LVCMOS TO XTAL INTERFACE
The XTAL_IN input can accept a single-ended LVCMOS signal
through an AC coupling capacitor. A general interface diagram
is shown in
Figure 3. The XTAL_OUT pin can be left floating.
The input edge rate can be as slow as 10ns. For LVCMOS
inputs, it is recommended that the amplitude be reduced from
full swing to half swing in order to prevent signal interference
with the power rail and to reduce noise. This configuration
requires that the output impedance of the driver (Ro) plus the
FIGURE 3. GENERAL DIAGRAM FOR LVCMOS DRIVER TO XTAL INPUT INTERFACE
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
Ω applications, R1
and R2 can be 100
Ω. This can also be accomplished by
removing R1 and making R2 50
Ω.
R2
Zo = 50
VDD
Ro
Zo = Ro + Rs
R1
VDD
XTAL_IN
XTAL_OUT
.1uf
Rs