841S012DI Datasheet

©2016 Integrated Device Technology, Inc

January 4, 2016

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FIGURE 3. GENERAL DIAGRAM FOR LVCMOS DRIVER TO XTAL 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 3. The XTAL_OUT pin can be left floating. The

input edge rate can be as slow as 10ns. For LVCMOS signals, 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

Ω applications, R1 and

R2 can be 100

Ω. This can also be accomplished by removing R1

and making R2 50

Ω. By overdriving the crystal oscillator, the device

will be functional, but note the device performance is guaranteed

by using a quartz crystal.

R2

Zo = 50

VDD

Ro

Zo = Ro + Rs

R1

VDD

XTAL_IN

XTAL_OUT

.1uf

Rs

CRYSTAL INPUT INTERFACE

The 841S012DI has been characterized with 18pF parallel

resonant crystals. The capacitor values shown in Figure 2 below

were determined using a 25MHz, 18pF parallel resonant crystal

FIGURE 2. CRYSTAL INPUT INTERFACE

and were chosen to minimize the ppm error. NOTE: External tuning

capacitors must be used for proper operations.

XTAL_IN

XTAL_OUT

X1

18pF Parallel Crystal

C1

15pF

C2

22pF