ICS83908I-02 Datasheet, PDF (11/16 Pages) Integrated Device Technology – LOW SKEW, 1-TO-8 CRYSTAL-TO-LVCMOS FANOUT BUFFER

ICS83908I-02
LOW SKEW, 1-TO-8 CRYSTAL-TO-LVCMOS FANOUT BUFFER
APPLICATION INFORMATION
PRELIMINARY
CRYSTAL INPUT INTERFACE
A crystal can be characterized for either series or parallel mode
operation. The ICS83908I-02 has a built-in crystal oscillator
circuit. This interface can accept either a series or parallel
crystal without additional components and generate fre-
quencies with accuracy suitable for most applications. Additional
accuracy can be achieved by adding two small capacitors C1
and C2 as shown in Figure 1. Typical results using parallel 18pF
crystals are shown in Table 5.
X1
18pF Parallel Crystal
XTAL_OUT
C1
15p
XTAL_IN
C2
15p
FIGURE 1. Crystal 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 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 50Ω applications, R1
and R2 can be 100Ω. This can also be accomplished by removing
R1 and making R2 50Ω.
VDD
VDD
R1
Ro
Rs
.1uf
Zo = 50
XTAL_IN
Zo = Ro + Rs
R2
XTAL_OUT
FIGURE 2. GENERAL DIAGRAM FOR LVCMOS DRIVER TO XTAL INPUT INTERFACE
IDT^™ / ICS^™ LVCMOS FANOUT BUFFER
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ICS83908AGI-02 REV. B JULY 24, 2007