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/
100#
0
0
0
0
0
0
0
0
1
1
1
1
1
1
1
1
S2
0
0
0
0
1
1
1
1
0
0
0
0
1
1
1
1
S1
0
0
1
1
0
0
1
1
0
0
1
1
0
0
1
1
S0
0
1
0
1
0
1
0
1
0
1
0
1
0
1
0
1
CPU
(MHz)
83.33
90.00
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
Default N
(Decimal)
402
434
363
482
482
535
578
610
643
366
353
402
322
322
644
644
Table 2
C2
Max Frequency (MHz)
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
Resolution (KHz)
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
3
3
3
2
2
2
2
2
2
2
2
2
3
2
3
3
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
2
determines the energy distribution bandwidth). In this device, Spread Spectrum is enabled by setting I C Byte0, bit1 = 1.
2
The default of the device at power up keeps the Spread Spectrum disabled, it is therefore, important to have I C
accessibility to turn-on the Spread Spectrum function. Once the Spread Spectrum is enabled, the spread bandwidth
2
option is selected by I C 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 SEMICONDUCTOR ]
