CY28346
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
S(1:0)
00
01
10
11
P
32005333
48008000
96016000
64010667
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.
Table 3. Spread Spectrum
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
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
T
PCB
CPUT
Test and Measurement Set-up
For Differential CPU Output Signals
The following diagram shows lumped test load configurations
for the differential Host Clock Outputs.
M easurem ent P oint
2p F
M U LT S E L
CPUC
T
PCB
2 pF
M easurem ent P oint
Figure 1. 1.0V Test Load Termination
T
PCB
VDD
CPUT
2pF
Measurement Point
MULTSEL
T
PCB
CPUC
2pF
Measurement Point
Figure 2. 0.7V Test Load Termination
Rev 1.0, November 24, 2006
Page 6 of 19
SPECTRALINEAR [ SPECTRALINEAR INC ]
