ICS83904-02 Datasheet, PDF (11/15 Pages) Integrated Circuit Systems – LOW SKEW, 1-TO-4 CRYSTAL-TO-LVCMOS/LVTTL FANOUT BUFFER

ICS83904-02
LOW SKEW, 1-TO-4, CRYSTAL-TO-LVCMOS/LVTTL FANOUT BUFFER
APPLICATION INFORMATION
CRYSTAL INPUT INTERFACE
Figure 1 shows an example of ICS83904-02 crystal interface with
a parallel resonant crystal. The frequency accuracy can be fine
tuned by adjusting the C1 and C2 values. For a parallel crystal
with loading capacitance CL = 18pF, we suggest C1 = 15pF and
C2 = 15pF to start with. These values may be slightly fine tuned
further to optimize the frequency accuracy for different board
layouts. Slightly increasing the C1 and C2 values will slightly reduce
the frequency. Slightly decreasing the C1 and C2 values will slightly
increase the frequency. For the oscillator circuit below, R1 can be
used, but is not required. For new designs, it is recommended
that R1 not be used.
C1
15p
X1
18pF Parallel Cry stal
0
C2
R1 (optional)
15p
XTAL_IN
XTAL_OUT
FIGURE 1. Crystal Input Interface
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 2. 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 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 50Ω applications, R1
and R2 can be 100Ω. This can also be accomplished by removing
R1 and making R2 50Ω.
VDD
VDD
R1
Ro
Rs
.1uf
Zo = 50
XTAL_IN
Zo = Ro + Rs
R2
XTAL_OUT
FIGURE 2. GENERAL DIAGRAM FOR LVCMOS DRIVER TO XTAL INPUT INTERFACE
IDT^™ / ICS^™ LVCMOS/LVTTL FANOUT BUFFER
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ICS83904AG-02 REV. A SEPTEMBER 12, 2007