CY28346 Cypress Semiconductor, CY28346 Datasheet - Page 6

• Manufacturer Part Number: CY28346
• Description: Clock Synthesizer with Differential CPU Outputs
• Manufacturer: Cypress Semiconductor

Document #: 38-07331 Rev. *B
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.
Table 2. P Value
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
VDD
S(1:0)
0 0
0 1
1 0
1 1
M U LT S E L
MULTSEL
C P U C
C P U T
CPUC
CPUT
32005333
48008000
96016000
64010667
Figure 1. 1.0V Test Load Termination
Figure 2. 0.7V Test Load Termination
P
T
T
PC B
PC B
T
T
PCB
PCB
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.
Table 3. Spread Spectrum
Test and Measurement Set-up
For Differential CPU Output Signals
The following diagram shows lumped test load configurations
for the differential Host Clock Outputs.
SS2
0
0
0
0
1
1
1
1
SS1
2pF
2pF
0
0
1
1
0
0
1
1
Measurement Point
Measurement Point
SS0
0
1
0
1
0
1
0
1
2p F
2 pF
Spread Mode
M ea surem ent P o in t
M e asu rem en t P oint
Center
Center
Center
Center
Down
Down
Down
Down
CY28346
+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
Spread%
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