C9730
Low EMI Clock Generator for SiS730/ AMD K7 Applications
Preliminary
Spread Spectrum Clock Generation (SSCG)
Spread Spectrum is a modulation technique applied here for maximum efficiency in minimizing Electro-Magnetic
Interference radiation generated from 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
spreading the same amount of energy over a spectrum. This technique is achieved by modulating the clock down from
(Fig.8A) or around the center (Fig.8B) 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
MBS(0:1) in I²C byte 5, bit6 and bit 7, and SSTS Byte1, Bit6 following table 4 below.
In Down Spread mode the center frequency is shifted down from its rested (non-spread) value by ½ of the total spread
%. (eg.: 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.
Down Spread
Center Spread
Fig.8A
Fig.8B
Spread Spectrum Selection Table
SSTS
MBS 1
MBS 0
Spread%
-0.25%
-1 %
-0.7%
-0.5%
+/-0.125
+/-0.5
+/-0.35
+/-0.25
0
0
0
0
1
1
1
1
0
0
1
1
0
0
1
1
0
1
0
1
0
1
0
1
Table 4
INTERNATIONAL MICROCIRCUITS, INC. 525 LOS COCHES ST.
MILPITAS, CA 95035, USA TEL: 408-263-6300, FAX 408-263-6571
Rev 1.0
4/26/2000
Page 11 of 19
http://www.imicorp.com
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