FS6377

For example, a fixed divide-by-eight could be used in the feedback divider. Unfortunately, a divide-by-eight would limit the effective

modulus of the entire feedback divider to multiples of eight. This limitation would restrict the ability of the PLL to achieve a desired input-

frequency-to-output frequency ratio without making both the reference and feedback divider values comparatively large.

A large feedback modulus means that the divided VCO frequency is relatively low, requiring a wide loop bandwidth to permit the low

frequencies. A narrow loop bandwidth tuned to high frequencies is essential to minimizing jitter; therefore, divider moduli should always

be as small as possible.

To understand the operation, refer to Figure 4. The M-counter (with a modulus always equal to M) is cascaded with the dual-modulus

prescaler. The A-counter controls the modulus of the prescaler. If the value programmed into the A-counter is A, the prescaler will be

set to divide by N+1 for A prescaler outputs. Thereafter, the prescaler divides by N until the M-counter output resets the A-counter, and

the cycle begins again. Note that N=8 and A and M are binary numbers.

Suppose that the A-counter is programmed to zero. The modulus of the prescaler will always be fixed at N; and the entire modulus of

the feedback divider becomes MxN.

Next, suppose that the A-counter is programmed to a one. This causes the prescaler to switch to a divide-by-N+1 for its first divide

cycle and then revert to a divide-by-N. In effect, the A-counter absorbs (or "swallows") one extra clock during the entire cycle of the

feedback divider. The overall modulus is now seen to be equal to MxN+1.

This example can be extended to show that the feedback divider modulus is equal to MxN+A, where A<M.

Figure 4: Feedback Divider

3.1.3. Feedback Divider Programming

For proper operation of the feedback divider, the A-counter must be programmed only for values that are less than or equal to the M-

counter. Therefore, not all divider moduli below 56 are available for use. The selection of divider values is listed in Table 2.

Above a modulus of 56, the feedback divider can be programmed to any value up to 2047.

Table 2: Feedback Divider Modulus Under 56

M-Counter:

FBKDIV[10:3]

A-Counter: FBKDIV[2:0]

000

001

010

011

100

101

110

111

00000001

8

9

00000010

16

17

18

00000011

24

25

26

27

00000100

32

33

34

35

36

00000101

40

41

42

43

44

45

00000110

48

49

50

51

52

53

54

00000111

56

57

58

59

60

61

62

63

Feedback Divider Modulus

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