ICS841664I Datasheet, PDF (9/16 Pages) Integrated Device Technology – FEMTOCLOCK™ CRYSTAL-TO-HCSL CLOCK GENERATOR

ICS841664I
FEMTOCLOCK™ CRYSTAL-TO-HCSL CLOCK GENERATOR
PRELIMINARY
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
V^VCDDC
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
RECOMMENDATIONS FOR UNUSED INPUT AND OUTPUT PINS
INPUTS:
CRYSTAL INPUTS
For applications not requiring the use of the crystal oscillator input,
both XTAL_IN and XTAL_OUT can be left floating. Though not
required, but for additional protection, a 1kΩ resistor can be tied
from XTAL_IN to ground.
REF_IN INPUT
For applications not requiring the use of the reference clock,
it can be left floating. Though not required, but for additional
protection, a 1kΩ resistor can be tied from the REF_IN to ground.
OUTPUTS:
HCSL OUTPUTS
All unused HCSL outputs can be left floating. We recommend
that there is no trace attached. Both sides of the differential output
pair should either be left floating or terminated.
LVCMOS OUTPUT
The unused LVCMOS output can be left floating. We recommend
that there is no trace attached.
LVCMOS CONTROL PINS
All control pins have internal pull-ups or pull-downs; additional
resistance is not required but can be added for additional
protection. A 1kΩ resistor can be used.
IDT^™ / ICS^™ HCSL CLOCK GENERATOR
9
ICS841664AGI REV. A JANUARY 30, 2009