CS2000-CP_09 Datasheet, PDF(15/36 Page) Cirrus Logic – Fractional-N Clock Synthesizer & Clock Multiplier

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CS2000-CP_09 Datasheet, PDF (15/36 Pages) Cirrus Logic – Fractional-N Clock Synthesizer & Clock Multiplier

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5.1.2

CS2000-CP

Crystal Connections (XTI and XTO)

An external crystal may be used to generate RefClk. To accomplish this, a 20 pF fundamental mode par-

allel resonant crystal must be connected between the XTI and XTO pins as shown in Figure 12. As shown,

nothing other than the crystal and its load capacitors should be connected to XTI and XTO. Please refer

to the âAC Electrical Characteristicsâ on page 8 for the allowed crystal frequency range.

XTI

XTO

40 pF

Figure 12. External Component Requirements for Crystal Circuit

5.1.3

External Reference Clock (REF_CLK)

For operation with an externally generated REF_CLK signal, XTI/REF_CLK should be connected to the

reference clock source and XTO should be left unconnected or pulled low through a 47 kÎ© resistor to

GND.

5.2 Frequency Reference Clock Input, CLK_IN

The frequency reference clock input (CLK_IN) is used in Hybrid PLL Mode by the Digital PLL and Fractional-

N Logic block to dynamically generate a fractional-N value for the Frequency Synthesizer (see âHybrid An-

alog-Digital PLLâ on page 13). The Digital PLL first compares the CLK_IN frequency to the PLL output. The

Fractional-N logic block then translates the desired ratio based off of CLK_IN to one based off of the internal

timing reference clock (SysClk). This allows the low-jitter timing reference clock to be used as the clock

which the Frequency Synthesizer multiplies while maintaining synchronicity with the frequency reference

clock through the Digital PLL. The allowable frequency range for CLK_IN is found in the âAC Electrical Char-

acteristicsâ on page 8.

5.2.1

CLK_IN Skipping Mode

CLK_IN skipping mode allows the PLL to maintain lock even when the CLK_IN signal has missing pulses

for up to 20 ms (tCS) at a time (see âAC Electrical Characteristicsâ on page 8 for specifications). CLK_IN

skipping mode can only be used when the CLK_IN frequency is below 80 kHz and CLK_IN is reapplied

within 20 ms of being removed. The ClkSkipEn bit enables this function.

DS761F1

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