83905I Datasheet, PDF (13/21 Pages) Integrated Device Technology – LVCMOS/ LVTTL Fanout Buffer

83905I Datasheet
Applications Information
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 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.
VDD
VDD
R1
Ro
Rs
50Ω
0.1µf
XTAL_IN
Zo = Ro + Rs
R2
XTAL_OUT
Figure 2. General Diagram for LVCMOS Driver to XTAL Input Interface
©2016 Integrated Device Technology, Inc.
13
Revision C September 28, 2016