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參數(shù)資料
| 型號(hào)： | CY25245 |
| 英文描述： | Clocks and Buffers |
| 中文描述： | 時(shí)鐘和緩沖器 |
| 文件頁數(shù)： | 4/11頁 |
| 文件大?。?/td> | 116K |
| 代理商： | CY25245 |
CY25245
Document #: 38-07124 Rev. *A
Page 4 of 11
Spread Spectrum Frequency Timing Generator
The device generates a clock that is frequency modulated in
order to increase the bandwidth that it occupies. By increasing
the bandwidth of the fundamental and its harmonics, the ampli-
tudes of the radiated electromagnetic emissions are reduced.
This effect is depicted in
Figure 2
.
As shown in
Figure 2
, a harmonic of a modulated clock has a
much lower amplitude than that of an unmodulated signal. The
reduction in amplitude is dependent on the harmonic number
and the frequency deviation or spread. The equation for the
reduction is
dB = 6.5 + 9*log
10
(P) + 9*log
10
(F)
where
P
is the percentage of deviation and
F
is the frequency
in MHz where the reduction is measured.
The output clock is modulated with a waveform depicted in
Figure 3
. This waveform, as discussed in
“
Spread Spectrum
Clock Generation for the Reduction of Radiated Emissions
”
by
Bush, Fessler, and Hardin, produces the maximum reduction
in the amplitude of radiated electromagnetic emissions. The
deviation selected for this chip is as described in
Table 2
.
Figure 3
details the Cypress spreading pattern. Cypress does
offer options with more spread and greater EMI reduction.
Contact your local Sales representative for details on these
devices.
Serial Data Interface
The CY25245 features a two-pin, serial data interface that can
be used to configure internal register settings that control
particular device functions. Upon power-up, the CY25245
initializes with default register settings, therefore the use of this
serial data interface is optional. The serial interface is
write-only (to the clock chip) and is the dedicated function of
device pins SDATA and SCLOCK. In motherboard applica-
tions, SDATA and SCLOCK are typically driven by two logic
outputs of the chipset. Clock device register changes are
normally made upon system initialization, if any are required.
The interface can also be used during system operation for
power management functions.
Table 3
summarizes the control
functions of the serial data interface.
Operation
Data is written to the CY25245 in eleven bytes of eight bits
each. Bytes are written in the order shown in
Table 4
.
Spread
Spectrum
Enabled
EMI Reduction
Spread
Spectrum
Non-
Frequency Span (MHz)
Down Spread
A
Figure 2. Clock Harmonic with and without SSCG Modulation Frequency Domain Representation
MAX.
MIN.
1
2
3
4
5
6
7
8
9
1
1
2
3
4
5
6
7
8
9
1
F
Figure 3. Typical Modulation Profile
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相關(guān)代理商/技術(shù)參數(shù) |
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| CY25245OXC | 制造商:Cypress Semiconductor 功能描述: |
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| CY25245PVCT | 制造商:CYPRESS 制造商全稱:Cypress Semiconductor 功能描述:Frequency-multiplying, Peak-reducing EMI Solution |
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