CS2000-OTP

12

DS758F1

example of how to determine the range of RefClk frequencies around 12 MHz to be used in order to

achieve the lowest jitter PLL output at a frequency of 12.288 MHz is as follows:

where:

and

5.2.2

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 10. As shown,

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

to the “AC Electrical Characteristics” on page 7 for the allowed crystal frequency range.

5.2.3

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 terminated through a 47 k

Ω resistor to

GND.

5.3

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 “Hybrid An-

alog-Digital PLL” on page 10). 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

Referenced Control

Parameter Definition

RefClkDiv[1:0] ....................... “Reference Clock Input Divider (RefClkDiv[1:0])” on page 24

-80

-60

-40

-20

0

20

40

60

80

20

40

60

80

100

120

140

160

180

Normalized REF__CLK Frequency (kHz)

CLK__OUT Jitter

-15 kHz

+15 kHz

CLK__OUT

f

*32/N

Figure 9. REF_CLK Frequency vs a Fixed CLK_OUT

f

L

f

RefClk

f

H

≤≤

f

L

f

CLK_OUT

31

32

------15kHz

+

×

=

12.288MHz

0.96875

15kHz

+

×

=

11.919MHz

=

f

H

f

CLK_OUT

32

32

------15kHz

–

×

=

12.288MHz

115kHz

+

×

=

12.273MHz

=

XTI

XTO

40 pF

40 pF

Figure 10. External Component Requirements for Crystal Circuit