+/+…when timing is critical
C9810
Low EMI Clock Generator for Intel 810 Chipset for Mobile Applications
Advanced Information
IMI Confidential
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 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
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.).
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
%
Center 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
INTERNATIONAL MICROCIRCUITS, INC. 525 LOS COCHES ST.
MILPITAS, CA 95035, USA TEL: 408-263-6300, FAX 408-263-6571
Rev 0.4
8/31/1999
Page 9 of 15
http://www.imicorp.com
CYPRESS [ CYPRESS ]
