+/+…when timing is critical
C9829B
Low EMI Clock Generator for Intel 133MHz/3DIMM Chipset Systems
Approved Product
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, 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. Once the Spread Spectrum is enabled, the spread bandwidth option is selected by 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.).
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 Tables
Spread
%
Spread
%
I²C BYTE5
Bit[7:5]
I²C BYTE5
Bit[7:5]
000
001
010
011
- 0.5
- 0.7
N/A
100
101
110
111
± 0.25
± 0.35
± 0.5
- 0.25
+/- 0.125
Table 4A
Table 4B
INTERNATIONAL MICROCIRCUITS, INC. 525 LOS COCHES ST.,
MILPITAS, CA 95035, USA. TEL: 408-263-6300, FAX 408-263-6571
http://www.imicorp.com
Rev 1.3
4/12/2000
Page 12 of 20
CYPRESS [ CYPRESS ]
