C9707
Clock Generator for VIA VT8371/Athlon (K7) Chipset Systems
Preliminary
Dial-a-Frequency™ Feature (Cont.)
Where N and R values are programmed in binary into byte 4 for N and byte 7 for R. See table below for min and max
allowed values.
R
Min N
66
67
69
70
72
73
75
77
Max N
109
112
115
117
120
122
125
130
130
133
42
43
44
45
46
47
48
49
50
51
78
80
P is a large value PLL constant that depends on the frequency selection achieved through the hardware selectors (S3,
S2, S1, S0) or through the software selectors (byte0 , bits 6,5,4,2,1). P value may be determined from the following
table:
S(4:0)
0100X, 00XXX
01010, 100XX, 10111,
11000, 11111
P
96016000
64010666.67
01011, 011XX, 10101,
10110, 11001, 1101X,
11101, 11110
48008000
38406400
10100, 11100
6
Therefore, if a 145MHz (use 145x10 ) value is desired, then we should apply 145 into equation 1, and start by choosing
R to be 47 (assume the last frequency selection has the value P = 96016000):
6
145X10 = 96016000 X N
47
=> N = 70.97775371
Since this N number must be entered in Binary, it can only be an integer, so it must be rounded up or down. Here we
can rounded it up to 71, which will give us an exact CPU frequency of:
Fcpu = 96016000 X N = 145.045 MHz (accuracy + 310 ppm)
47
If the above frequency is not accurate enough, then you must choose another R value and start from the beginning. For
example choose R = 49 and this will yield an N = 73.99808365, which is rounded to 74. If the 74 is applied in the
formula 1, then Fcpu = 145.0038 MHz (accuracy + 26 ppm).
Other R values within the above limits may also be evaluated.
INTERNATIONAL MICROCIRCUITS, INC. 525 LOS COCHES ST.
MILPITAS, CA 95035 TEL: 408-263-6300 FAX 408-263-6571
http://www.imicorp.com
Rev 1.2
4/25/2000
Page 11 of 17
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