CY28346
Document #: 38-07331 Rev. *C
Page 6 of 20
Dial-a-Frequency Features
SMBus Dial-a-Frequency feature is available in this device via
Byte8 and Byte9.
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 Table 2.
Dial-a-dB Features
SMBus Dial-a-dB feature is available in this device via Byte8
and Byte9.
Spread Spectrum Clock Generation (SSCG)
Spread Spectrum is a modulation technique used to
minimizing 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.
Table 3 is a listing of the modes and percentages of Spread
Spectrum modulation that this device incorporates.
Test and Measurement Set-up
For Differential CPU Output Signals
The following diagram shows lumped test load configurations
for the differential Host Clock Outputs.
Table 2. P Value
S(1:0)
P
0 0
32005333
0 1
48008000
1 0
96016000
1 1
64010667
Table 3. Spread Spectrum
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
M easurem ent Point
2pF
CPU T
M U LT SEL
T
PCB
T
PCB
CPU C
220Ω
63.4Ω
63.4Ω
475Ω
33.2Ω
33.2Ω
M easurem ent Point
2pF
Figure 1. 1.0V Test Load Termination
CPUT
MULTSEL
T
PCB
T
PCB
CPUC
33Ω
33Ω
Measurement Point
49.9Ω
49.9Ω
2pF
Measurement Point
2pF
475Ω
VDD
Figure 2. 0.7V Test Load Termination
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