C9830
133 MHz Clock Generator for Intel 820 Chipset
Preliminary
Frequency Stepping Description (Cont.)
For example if the select lines are set to 1101, then the N value defaults to 322, which results in a CPU frequency of
100.22726MHz. The next N value is 323, and from the table 1 it can be found that the resolution is 0.31126478 MHz (or
311.26478KHz). Therefore, the new CPU output frequency will be 100.538525 MHz.
Frequency Stepping Table
Sel133/ S2 S1 S0
100#
CPU
(MHz)
Default N
(Decimal)
C2
Max Frequency (MHz)
Resolution (KHz)
83.33
90.00
402
434
363
482
482
535
578
610
643
366
353
402
322
322
644
644
3
3
3
2
2
2
2
2
2
2
2
2
3
2
3
3
133.33
133.33
133.33
200.00
200.00
200.00
200.00
200.00
200.00
200.00
200.00
200.00
133.33
200.00
133.33
133.33
207.50986
207.50986
207.50986
311.26478
311.26478
311.26478
311.26478
311.26478
311.26478
311.26478
311.26478
311.26478
207.50986
311.26478
207.50986
207.50986
0
0
0
0
0
0
0
0
1
1
1
1
1
1
1
1
0
0
0
0
1
1
1
1
0
0
0
0
1
1
1
1
0
0
1
1
0
0
1
1
0
0
1
1
0
0
1
1
0
1
0
1
0
1
0
1
0
1
0
1
0
1
0
1
75.33
150.00
150.00
166.67
180.00
190.00
200.00
114.00
110.00
125.00
66.80
100.20
133.60
133.60
Table 2
Spread Spectrum Clock Generation (SSCG)
Spread Spectrum is a modulation technique applied here for maximum efficiency in minimizing Electro-Magnetic
Interference radiation generated by repetitive digital signals, mainly clocks. A clock accumulates EM energy at the
center frequency it is generating. Spread Spectrum distributes this energy over a small frequency bandwidth therefore
distributing an even amount of energy over a wider spectrum. This technique is achieved by modulating the clock either
down (Figure 5a) or around the center (Figure 5b) of its resting frequency by a certain percentage (which also
determines the energy distribution bandwidth). In this device, Spread Spectrum is enabled by setting I2C Byte0, bit1 = 1.
The default of the device at power up keeps the Spread Spectrum disabled, it is therefore, important to have I2C
accessibility to turn-on the Spread Spectrum function. Once the Spread Spectrum is enabled, the spread bandwidth
option is selected by I2C Byte 5, bits 6 & 7 following table 3.
In Down Spread mode the center frequency is shifted down from its rested (non-spread) value by ½ of the total spread
%. (e.g.: assuming the center frequency is 100MHz in non-spread mode; when down spread of –0.5% is enabled, the
center frequency shifts to 99.75MHz.).
In Center Spread mode, the Center frequency remains the same as in the non-spread mode.
INTERNATIONAL MICROCIRCUITS, INC. 525 LOS COCHES ST
MILPITAS, CA 95035, USA TEL: 408-263-6300. FAX 408-263-6571
http://www.imicorp.com/
Rev 1.1
4/25/2000
Page 8 of 14
CYPRESS [ CYPRESS ]
