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W155

更新时间: 2024-11-20 22:14:55
品牌 Logo 应用领域
赛普拉斯 - CYPRESS /
页数 文件大小 规格书
6页 89K
描述
Spread Spectrum Frequency Timing Generator

W155 数据手册

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W155  
Spread Spectrum Frequency Timing Generator  
Features  
Table 1. Frequency Selection (14.318-MHz Reference)  
• 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)  
SYSCLK  
FS3  
0
FS2  
0
FS1  
0
FS0  
0
(Output Freq.)  
133.3 MHz  
120 MHz  
100 MHz  
74.77 MHz  
70 MHz  
0
0
0
1
0
0
1
0
0
0
1
1
0
1
0
0
0
1
0
1
66.6 MHz  
60 MHz  
0
1
1
0
0
1
1
1
50 MHz  
1
0
0
0
40 MHz  
1
0
0
1
33.33 MHz  
30 MHz  
Overview  
1
0
1
0
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.  
1
0
1
1
25 MHz  
1
1
0
0
20 MHz  
1
1
0
1
16.67 MHz  
12 MHz  
1
1
1
0
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.  
1
1
1
1
10 MHz  
Table 2. Spread Percentage Selection  
SS%  
Spread Percentage  
–1.25%  
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.  
0
1
–3.75%  
[1]  
Pin Configuration  
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).  
VDD  
X1  
1
2
3
4
5
6
7
8
16  
15  
14  
13  
12  
11  
10  
9
TEST  
VDD  
X2  
USBCLK/SS%*  
GND  
FS3*  
VDD  
FS2*  
FS1*  
GND  
SYSCLK  
GND  
FS0*  
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. **  

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