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10

20

30

40

50

60

70

80

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100

5

10

15

20

25

30

35

40

45

50

55

C=18pF

Frequency − MHz

21 W

ESR=60

ESR=50

ESR=40

ESR=30

ESR=25

ESR=15

XO

or

2LVCMOS

or

Differential

input

CLK_IN0

CLK_IN1

Input Source Select

(From EEPROM)

CDCE906

SCAS814H – NOVEMBER 2005 – REVISED DECEMBER 2007

In order to ensure a stable oscillating, a certain drive power must be applied. The CDCE906 features an input

oscillator with adaptive gain control which relieves the user to manually program the gain. The drive level is the

amount of power dissipated by the oscillating crystal unit and is usually specified in terms of power dissipated by

the resonator (equivalent series resistance (ESR)). Figure 16 gives the resulting drive level vs crystal frequency

and ESR.

Figure 16. Crystal Drive Power

For example, if a 27-MHz crystal with ESR of 50

Ω is used and 2

× C

level should be held to a minimum to avoid over driving the crystal. The maximum power dissipation is specified

for each type of crystal in the oscillator specifications, i.e., 100

µW for the example above.

Single-Ended LVCMOS Clock Inputs

When selecting the LVCMOS clock mode, CLK_IN0 and CLK_IN1 act as regular clock inputs pins and can be

driven up to 167 MHz. Both clock inputs circuitry are equal in design and can be used independently to each

other (see Figure 17). The internal clock select bit, Byte 10, Bit [4], selects one of the two input clocks. CLK_IN0

is the default selection. There is also the option to program the external control pin S0/A0/CLK_SEL as clock

select pin, Byte 10, Bit [1:0].

The two clock inputs can be used for redundancy switching, i.e. to switch between a primary clock and

secondary clock. Note a phase difference between the clock inputs may require PLL correction. Also in case of

different frequencies between the primary and secondary clock, the PLL has to re-lock to the new frequency.

A.

CLK_SEL is optional and can be configured by EEPROM setting.

Figure 17. LVCMOS Clock Input Circuitry

Copyright © 2005–2007, Texas Instruments Incorporated

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Product Folder Link(s): CDCE906