C9827H
High Performance Pentium® 4 Clock Synthesizer
Cypress Semiconductor Corporation
http://www.cypress.com
Document#: 38-07106 Rev. *A
12/26/2002
Page 6 of 25
Approved Product
Dial-a-Frequency
™
Feature
SMBus Dial-a-frequency feature is available in this device via Byte8 and Byte9. See our App Note AN-0025 for details
on our Dial-a-Frequency™ feature.
P is a large value PLL constant that depends on the frequency selection achieved through the hardware selectors
(S1, S0). P value may be determined from the following table:
S(1:0)
P
0 0
32005333
0 1
48008000
1 0
96016000
1 1
64010667
Table 1
Dial-a-dB™ Features
SMBus Dial-a-dB™ feature is available in this device via Byte8 and Byte9. See our App Note AN-0026 for details on
the Dial-a-dB™.
Spread Spectrum Clock Generation (SSCG)
Spread Spectrum is a modulation technique used to minimizing Electro-Magnetic Interference (EMI) radiation
generated by repetitive digital signals.
A clock presents the greatest EMI energy at the center frequency it is
generating. Spread Spectrum distributes this energy over a specific and controlled frequency bandwidth therefore
causing the average energy at any one point in this band to decrease in value. This technique is achieved by
modulating the clock away from its resting frequency by a certain percentage (which also determines the amount of
EMI reduction). In this device, Spread Spectrum is enabled by setting specific register bits in the SMBus control
Bytes. See applications note AN-0024 for a more in depth description of Spread spectrum modulation and see the
SMBus register section of this data sheet for the exact bit and byte functionally. The following table is a listing of the
modes and percentages of Spread Spectrum modulation that this device incorporates.
SS2
SS1
SS0
Spread Mode
Spread %
0
0
0
Down
+0.00, -0.25
0
0
1
Down
+0.00, -0.50
0
1
0
Down
+0.00, -0.75
0
1
1
Down
+0.00-1.00
1
0
0
Center
+0.13, -0.13
1
0
1
Center
+0.25, -0.25
1
1
0
Center
+0.37, -0.37
1
1
1
Center
+0.50 -1.50
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