APPROVED PRODUCT
C9812
Low EMI Clock Generator for Intel 810E Chipset Systems
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.9A) or around the center (Fig.9B) of its resting frequency by a certain percentage (which also determines the
energy distribution bandwidth). In this device, Spread Spectrum is enabled by setting SMBUS byte0, bit3 = 1. The
default of the device at power up keeps the Spread Spectrum disabled, it is therefore, important to have SMBUS
accessibility to turn-on the Spread Spectrum function. Once the Spread Spectrum is enabled, the spread bandwidth
option is selected by SST(0:2) in SMBUS 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.9A
Fig.9B
Spread Spectrum Selection Tables
Center Frequency
Spread
%
Down Frequency
(MHz)
Spread
%
I²C BYTE5
Bit[7:5]
I²C BYTE5
Bit[7:5]
(MHz)
100
101
110
111
66/100/133.3
66/100/133.3
66/100/133.3
66/100/133.3
000
001
010
011
66/100/133.3
66/100/133.3
66/100/133.3
66/100/133.3
- 0.5
- 0.7
- 1.0
- 1.5
± 0.25
± 0.35
± 0.5
± 0.7
Table 4B
Table 4A
Cypress Semiconductor Corporation
525 Los Coches St.
Milpitas, CA 95035. Tel: 408-263-6300, Fax: 408-263-6571
http://www.cypress.com
Document#: 38-07053 Rev. **
05/03/01
Page 9 of 18
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