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W155 데이터시트 PDF




Cypress Semiconductor에서 제조한 전자 부품 W155은 전자 산업 및 응용 분야에서
광범위하게 사용되는 반도체 소자입니다.


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부품번호 W155 기능
기능 Spread Spectrum Frequency Timing Generator
제조업체 Cypress Semiconductor
로고 Cypress Semiconductor 로고


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W155 데이터시트, 핀배열, 회로
W155
Spread Spectrum Frequency Timing Generator
Features
• Generates a spread spectrum timing signal (SYSCLK)
and a non-spread signal (USBCLK)
• Requires a 14.318-MHz crystal for operation
• Supports MIPS microprocessor clock frequencies
• Reduces peak EMI by as much as 12 dB
• Integrated loop filter components
• Cycle-to-cycle jitter = 250 ps (max)
• Operates with a 3.3 or 5.0V power supply
• Spread output is selectable from 10 to 133 MHz
• TEST mode supports modulation off (High-Z) and spe-
cial test input reference frequency
• Guaranteed 45/55 duty cycle
• Packaged in a 16-pin, 300-mil-wide SOIC (Small Outline
Integrated Circuit)
Overview
The W155 incorporates the latest advances in PLL-based
spread spectrum frequency synthesizer technology. By fre-
quency modulating the SYSCLK output with a low-frequency
carrier, peak EMI can be greatly reduced in a system. Use of
this technique allows systems to pass increasingly difficult EMI
testing without resorting to costly shielding or redesign.
In a system that uses the W155, not only is EMI reduced in the
various clock lines, but also in all signals which are synchro-
nized to SYSCLK. Therefore, the benefits of using this tech-
nique increase with the number of address and data lines in
the system.
The W155 is specifically targeted toward MIPS microproces-
sor based systems where EMI is of particular concern. Each
device uses a single 14.318-MHz crystal to generate a select-
able spread spectrum output and an unmodulated 48-MHz
USB Output.
The spreading function can be disabled by taking the SSON#
pin high. Spread percentage can be selected with the SS%
input (see Table 2 below).
Table 1. Frequency Selection (14.318-MHz Reference)
SYSCLK
FS3 FS2 FS1 FS0 (Output Freq.)
0 0 0 0 133.3 MHz
0001
120 MHz
0010
100 MHz
0 0 1 1 74.77 MHz
0100
70 MHz
0101
66.6 MHz
0110
60 MHz
0111
50 MHz
1000
40 MHz
1 0 0 1 33.33 MHz
1010
30 MHz
1011
25 MHz
1100
20 MHz
1 1 0 1 16.67 MHz
1110
12 MHz
1111
10 MHz
Table 2. Spread Percentage Selection
SS%
0
1
Spread Percentage
–1.25%
–3.75%
[1]
Pin Configuration
VDD
X1
X2
GND
FS3*
VDD
FS2*
FS1*
1
2
3
4
5
6
7
8
16 TEST
15 VDD
14 USBCLK/SS%*
13 GND
12 SYSCLK
11 GND
10 FS0*
9 SSON#^
Note:
1. Internal pull-up resistor present on inputs marked with ‘*’ and pull-down
resistor present on input marked with ‘^’.
Cypress Semiconductor Corporation • 3901 North First Street • San Jose • CA 95134 • 408-943-2600
September 29, 1999, rev. **




W155 pdf, 반도체, 판매, 대치품
W155
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 am-
plitudes of the radiated electromagnetic emissions are re-
duced. 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*log10(P) + 9*log10(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 Emissionsby
Bush, Fessler, and Hardin produces the maximum reduction
in the amplitude of radiated electromagnetic emissions. The
deviation selected for this chip is specified in Table 2. Figure 3
details the Cypress spreading pattern. Cypress does offer op-
tions with more spread and greater EMI reduction. Contact
your local sales representative for details on these devices.
Spread Spectrum clocking is activated or deactivated by se-
lecting the appropriate values for pin 9.
EMI Reduction
Spread
Spectrum
Enabled
Non-
Spread
Spectrum
Figure 2. Clock Harmonic with and without SSCG Modulation Frequency Domain Representation
MAX
MIN
Figure 3. Typical Modulation Profile
4

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