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Setting the PLL frequency
58
NS7520 Hardware Reference, Rev. D 03/2006
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