8©2016 Integrated Device Technology, Inc June 30, 2016
843441 Data Sheet
Applications Information
Recommendations for Unused Input Pins
Inputs:
LVCMOS Control Pins
All control pins have internal pullups and pulldowns; additional
resistance is not required but can be added for additional protection.
A 1k resistor can be used.
Overdriving the 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 1A. The XTAL_OUT pin can be left floating. The
maximum amplitude of the input signal should not exceed 2V and the
input edge rate can be as slow as 10ns. 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 50
applications, R1 and R2 can be 100. This can also be
accomplished by removing R1 and making R2 50
. By overdriving
the crystal oscillator, the device will be functional, but note, the device
performance is guaranteed by using a quartz crystal.
Figure 1A. General Diagram for LVCMOS Driver to XTAL Input Interface
VCC
XTAL_OUT
XTAL_IN
R1
100
R2
100
Zo = 50 ohmsRs
Ro
Zo = Ro + Rs
C1
.1uf
LVCMOS Driver
XTA L_ OU T
XTA L_ I N
Zo = 50 ohms
C2
.1uf
LVPECL Driver
Zo = 50 ohms
R1
50
R2
50
R3
50
