ICS841S02BGILF datasheet(11/17 Pages) IDT

11 / 17 page

IDT™ / ICS™ PCI EXPRESS CLOCK GENERATOR

ICS841S02BGI REV. C NOVEMBER 1, 2007

ICS841S02I

PCI EXPRESS™ CLOCK GENERATOR

PRELIMINARY

APPLICATION INFORMATION

POWER SUPPLY FILTERING TECHNIQUES

As in any high speed analog circuitry, the power supply pins are

vulnerable to random noise. To achieve optimum jitter

performance, power supply isolation is required. The ICS841S02I

provides separate power supplies to isolate any high switching

noise from the outputs to the internal PLL. V

DD_SRC, VDDA and VDD_REF

should be individually connected to the power supply

plane through vias, and 0.01µF bypass capacitors should be used

for each pin. Figure 1 illustrates this for a generic V

DD_SRC pin and

also shows that V

DDA requires that an additional10Ω resistor

along with a 10µF bypass capacitor be connected to the V

DDA pin.

FIGURE 1. POWER SUPPLY FILTERING

DDA

μF

.01

μF

3.3V

.01

μF

USING THE ON-BOARD CRYSTAL OSCILLATOR

The ICS841S02I features a fully integrated Pierce oscillator to

minimize system implementation costs. The recommended

operation of the ICS841S02I is with a 25MHz, 18pF parallel

resonant crystal. See Table 7 for complete crystal specifications.

For proper operation, a minimum of 10pF capacitance on each

crystal pin is required. The capacitor values shown in Figure 2

TABLE 7. RECOMMENDED CRYSTAL SPECIFICATIONS

(

L)F

(

O)F

)

(

FIGURE 2. CRYSTAL OSCILLATOR WITH TRIM CAPACITOR

XT AL_IN

XTAL_OUT

25MHz

TBD

33pF

18pF

are typical values for the recommended crystal as show in

Table 7. The specific values may be adjusted to trim the frequency

for the individual board layouts if desired.

The crystal and optional trim capacitors should be located as

close to the ICS841S02I XTAL_IN and XTAL_OUT pins as

possible to minimize board level parasitics.

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