FS6131

Dual-

Modulus

Prescaler

A

Counter

M

Counter

f

vco

Figure 3: Feedback Divider

To understand the operation, refer to Error! Reference source not found.. The M-counter (with a modulus of M) is cascaded with the

dual-modulus pre-scaler. If the prescaler modulus were fixed at N, the overall modulus of the feedback divider chain would be MXN.

However, the A-counter causes the pre-scaler modulus to be altered to N+1 for the first A outputs of the pre-scaler. The A-counter then

causes the dual-modulus prescaler to revert to a modulus of N until the M-counter reaches its terminal state and resets the entire

divider. The overall modulus can be expressed as

)

(

)

1

(

A

M

N

N

A

−

+

+

where M

≥ A, which simplifies to

A

N

M

+

×

4.1.3. Feedback Divider Programming

The requirement that M

≥ A means that the feedback divider can only be programmed for certain values below a divider modulus of 56.

The selection of divider values is listed in Table 2.

If the desired feedback divider is less than 56, find the divider value in the table. Follow the column up to find the A-counter program

value. Follow the row to the left to find the M-counter value.

Above a modulus of 56, the feedback divider can be programmed to any value up to 16383. See both Table 3 and Table 8 for additional

programming information.

Table 2: Feedback Modulus Below 56

A-counter: FBKDIV[2:0]

M-Counter:

FBKDIV[13:3]

000

001

010

011

100

101

110

111

00000000001

8

9

-

-

-

-

-

-

00000000010

16

17

18

-

-

-

-

-

00000000011

24

25

26

27

-

-

-

-

00000000100

32

33

34

35

36

-

-

-

00000000101

40

41

42

43

44

45

-

-

00000000110

48

49

50

51

52

53

54

-

00000000111

56

57

58

59

60

61

62

63

Feedback Divider Modulus

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