
C9827J
High Performance Pentium 4 Clock Synthesizer
Cypress Semiconductor Corporation
525 Los Coches St.
Milpitas, CA 95035. Tel: 408-263-6300, Fax: 408-263-6571
http://www.cypress.com
Document#: 38-07107 Rev. **
5/24/2001
Page 6 of 25
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)
0 0
0 1
1 0
1 1
P
32005333
48008000
96016000
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
0
0
0
0
1
1
1
1
SS1
0
0
1
1
0
0
1
1
SS0
0
1
0
1
0
1
0
1
Spread Mode
Down
Down
Down
Down
Center
Center
Center
Center
Spread %
+0.00, -0.25
+0.00, -0.50
+0.00, -0.75
+0.00 -1.00
+0.13, -0.13
+0.25, -0.25
+0.37, -0.37
+0.50 -1.50