APPROVED PRODUCT
C9777
Low EMI Clock Generator for 440BX, ALi1631 & Via Pro133+ / Pentium®III Systems
Spectrum Spread Clocking
Center Spread
Down Spread
Spectrum Spread OFF
Spectrum Spread ON
Fig.5a
Fig.5B
Description
Spread Spectrum is a modulation technique for distributing clock period over a certain bandwidth (called Spread
Bandwidth). This technique allows the distribution of the energy (EMI) over a range of frequencies therefore reducing
the radiation generated from clocks. As the spread is a percentage of the center (non-spread) frequency, it is effective
at the fundamental and all its harmonics.
In this device Spread Spectrum is enabled through pin 28 (SSON#). As the name suggests, spread spectrum is
enabled when SSON# is low. This pin has an internal pull up, therefore, defaults to a high (Spread Spectrum disabled)
unless externally forced to a low state.
SSON#
SMBW
SDWN#
Spread %
0
Direction
Spread OFF
1
0
0
0
0
x
0
0
1
1
x
0
1
0
+/-0.125
+/-0.35
-0.5
Center
Center, See note 1 below
Down
1
+/- 0.25
Center, default, See note 2 below
Table 5
Note1: This spread selection is NOT applicable when S (2:0) = 010. The device will be in TEST MODE.
Note 2: When frequency selection is 000, 011 and 101, the spread is –0.5%, otherwise it is +/- 0.25% as listed it Table 5 above.
Cypress Semiconductor Corporation
525 Los Coches St.
Document#: 38-07048 Rev. **
05/02/2001
Milpitas, CA 95035. Tel: 408-263-6300, Fax: 408-263-6571
http://www.cypress.com
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