C9834
Low EMI Clock Generator for Intel 810 Chipset / Pentium II and Pentium III Systems
Preliminary
I2C Test Circuitry
+ 5V
2.2 K
Device Under
SDATA
DATAIN
+ 5V
2.2 K
SCLK
+ 5V
2.2 K
DATAOUT
CLOCK
Fig. 7
Note1: Buffer is 7407 with VCC @ 5.0 V
Spectrum Spread Clocking
Spread Spectrum is a modulation technique for reducing Electro Magnetic radiation (EMI) at the source, which in this
case is the clock. The concept is based on redistributing the energy of a frequency over a finite bandwidth (also
referred to as spread percentage), therefore reducing the measured emission. Fig8A and 8B. present actual
measurements on a spectrum analyzer showing the EMI reduction as a result of Spread Spectrum.
Spread Spectrum function is enabled through I²C byte4, Bit 2. At power up, this bit defaults to low (SST Disabled). In
order to enable the Spread Spectrum function, Bit 2 in I²C byte 4, should be programmed to a 1.
Up Spread (see table 3 below for selection) is when the modulation is directed up from the resting (non spread)
frequency. Fig.8B
This option causes the center frequency to shift up by half the spread percentage: Fcs = Fcr + (Fcr*|p|/2).
Where,
Fcs is the new center frequency
Fcr: is the center frequency of the non spread clocck
|P|: is the absolute value of the Spread Percentage
e.g.: if the non spread spectrum frequency, Fcr, is 100.2MHz and the spread is enabled with a +0.5% spread option,
then the new center frequency:
Fcs = 100.2M + (100.2M*0.005/2) = 100.45MHz
INTERNATIONAL MICROCIRCUITS, INC. 525 LOS COCHES ST.
MILPITAS, CA 95035 TEL 408-263-6300, FAX 408-263-6571
http://www.imicorp.com
Rev. 1.1
9/7/1999
Page 7 of 15
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