ICS83905AGLF View Datasheet(PDF) - Integrated Device Technology
Part Name
Description
MFG CO.
'ICS83905AGLF' PDF : 19 Pages View PDF
ICS83905
LOW SKEW, 1:6 CRYSTAL-TO-LVCMOS/LVTTL FANOUT BUFFER
APPLICATION INFORMATION
CRYSTAL INPUT INTERFACE
Figure 2 shows an example of ICS83905 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 2. CRYSTAL OSCILLATOR INTERFACE
LVCMOS TO XTAL INTERFACE
The XTAL_IN input can accept a single-ended LVCMOS
signal through an AC couple 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 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.
VVCDDC
VVCDDC
R1
Ro
Rs
.1uf
Zo = 50
XTAL_IN
Zo = Ro + Rs
R2
XTAL_OUT
FIGURE 3. GENERAL DIAGRAM FOR LVCMOS DRIVER TO XTAL INPUT INTERFACE
IDT™ / ICS™ LVCMOS/LVTTL FANOUT BUFFER
11
ICS83905AM REV. B JULY 9, 2007
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