+/+…when timing is critical
C9866
Low EMI Clock Generator for Intel 815 Chipset Systems
Advanced Information
IMI Confidential
Dial-a-Frequency™ Feature
I2C Dial-a-frequency feature is available in this device via Bytes 6, 7 and 8.
These bytes allow the user to enter the N and R values that will enable them to program any CPU frequency desired
following the formula:
P × N
Fcpu =
R
Where N and R values are programmed in binary into Bytes 6 & 7 for N and Byte 8 for R. See table below for min
and max allowed values.
R
Min N
44
45
46
47
48
49
50
51
Max N
87
90
92
94
96
98
100
102
104
107
42
43
44
45
46
47
48
49
50
51
52
53
P is a large value PLL constant that depends on the last frequency selection achieved through the hardware selectors
(SEL3, SEL2, SEL1, SEL0) or through the software selectors (Byte0, Bits7, 6, 5, 4). P value may be determined from
the following table:
SEL(3:0)
P
0000
0001, 1100
32005333
48008000
64010666
96016000
0010, 0011, 0100, 1000, 1111
0110, 0111, 0101, 1001, 1010, 1011, 1101, 1110
Therefore, if a 145MHz (use 145x106) 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):
145X106 = 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
INTERNATIONAL MICROCIRCUITS, INC. 525 LOS COCHES ST.,
MILPITAS, CA 95035, USA. TEL: 408-263-6300, FAX 408-263-6571
http://www.imicorp.com
Rev 0.02
4/13/2000
Page 10 of 19
CYPRESS [ CYPRESS ]
