840004AGI-01

11

REV. A OCTOBER 22, 2007

ICS840004I-01

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 1k

Ω

resistor can be tied from XTAL_IN to ground.

For applications not requiring the use of the reference clock,

it can be left floating. Though not required, but for additional

protection, a 1k

Ω resistor can be tied from the REF_CLK to

ground.

All control pins have internal pull-ups or pull-downs; additional

resistance is not required but can be added for additional

protection. A 1k

Ω resistor can be used.

All unused LVCMOS output can be left floating. We

recommend that there is no trace attached.

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 50

Ω applications, R1 and R2 can be

100

Ω. This can also be accomplished by removing R1 and

making R2 50

Ω.

R2

Zo = 50

VDD

Ro

Zo = Ro + Rs

R1

VDD

XTAL_IN

XTAL_OUT

.1uf

Rs