+/+…when timing is critical
C9824
Low EMI Clock Generator for Intel 133MHz/3 DIMM Chipset Systems
Preliminary
I2C Test Circuitry
+ 5V
2.2 K
Device under Test
DATAIN
SDATA
SCLK
+ 5V
2.2 K
+ 5V
DATAOUT
2.2 K
CLOCK
Fig.7
Note: Buffer is 7407 with VCC @ 5.0 V
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 clocks. A clock accumulates EM energy at the center frequency it
is generating. Spread Spectrum distributes this energy over a small frequency band, therefore distributing this energy
over a wider spectrum. This technique is achieved by modulating the clock either down (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 modulation is enabled by setting I2C byte0, bit3 = 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. The spread bandwidth option is selected by SST(0:2) in I2C byte 5, bits 5, 6 & 7 following tables 4A, and 4B
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.).
INTERNATIONAL MICROCIRCUITS, INC. 525 LOS COCHES ST.,
MILPITAS, CA 95035, USA TEL: 408-263-6300, FAX 408-263-6571
http://www.imicorp.com
Rev 1.0
2/15/2000
Page 9 of 17
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