S e t t i n g t h e P L L f r e q u e n c y

The PLL frequency can be changed by writing to the PLLCNT field in the PLL Control register. At bootup, the default value in the PLLCNT field has no affect on the PLL frequency. This is because the PLL frequency set on bootup is based on the state of address lines A[8:0]. The value in the PLLCNT field must be rewritten to change the PLL frequency. The NS7520 resets whenever the PLLCNT field is changed, then starts running at the new frequency dictated by the PLLCNT value. The readback value is not valid until software has performed a write to this register.

When using a 18.432MHz crystal, the 4-bit value in the PLLCNT field produces frequencies of 23MHz to 92.2MHz, in increments of 4.6MHz, by changing the IS and ND fields in the PLL Settings register.

The FS field remains the same as configured on bootup.

The IS field is ‘b10 for all standard frequencies from 36.9MHz to 55.3MHz.

The ND field can be changed to 16 different values based on the PLLCNT. The next table shows the 16 frequencies that can be produced by changing the PLLCNT field in the PLL Control register.

MHz

PLLCNT

IS

ND+1

PLL Settings register

Notes

 

 

 

 

 

 

23.0

0

01

00101

0x00000084

 

 

 

 

 

 

 

27.6

1

01

00110

0x00000085

 

 

 

 

 

 

 

32.3

2

01

00111

0x00000086

 

 

 

 

 

 

 

36.9

3

10

01000

0x00000087

1

 

 

 

 

 

 

41.5

4

10

01001

0x00000108

 

 

 

 

 

 

 

46.1

5

10

01010

0x00000109

2

 

 

 

 

 

 

50.7

6

10

01011

0x0000010A

 

 

 

 

 

 

 

55.3

7

10

01100

0x0000010B

3

 

 

 

 

 

 

59.9

8

10

01101

0x0000010C

 

 

 

 

 

 

 

64.5

9

10

01110

0x0000010D

 

 

 

 

 

 

 

69.1

A

10

01111

0x0000010E

 

 

 

 

 

 

 

73.7

B

11

10000

0x0000010F

 

 

 

 

 

 

 

78.3

C

11

10001

0x00000110

 

 

 

 

 

 

 

82.9

D

11

10010

0x00000111

 

5 8

N S 7 5 2 0 H a r d w a r e R e f e r e n c e , R e v . D 0 3 / 2 0 0 6

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Image 72
Digi NS7520 manual MHz, ND+1 PLL Settings register