W155

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 special test input reference frequency Guaranteed 45/55 duty cycle Packaged in a 16-pin, 300-mil-wide SOIC (Small Outline Integrated Circuit) Table 1. Frequency Selection (14.318-MHz Reference) FS3 0 0 0 0 0 0 0 0 1 1 1 1 1 1 1 1 FS2 0 0 0 0 1 1 1 1 0 0 0 0 1 1 1 1 FS1 0 0 1 1 0 0 1 1 0 0 1 1 0 0 1 1 FS0 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 SYSCLK (Output Freq.) 133.3 MHz 120 MHz 100 MHz 74.77 MHz 70 MHz 66.6 MHz 60 MHz 50 MHz 40 MHz 33.33 MHz 30 MHz 25 MHz 20 MHz 16.67 MHz 12 MHz 10 MHz Overview The W155 incorporates the latest advances in PLL-based spread spectrum frequency synthesizer technology. By frequency 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 synchronized to SYSCLK. Therefore...