For devices that support spread-spectrum technology, the parameter settings are located on the Bandwidth/SS page of the ALTPLL parameter editor.
The following figure shows the Spread Spectrum window.
To enable the spread-spectrum feature, turn on Use spread spectrum feature. Set the desired down spread percentage, and the modulation frequency. The table in the spread spectrum window lists the detailed descriptions of the current counter values.
The down spread percentage defines the modulation width or frequency span of the instantaneous output frequency resulting from the spread spectrum. When you use down spread, the modulation width falls at or below a specified maximum output frequency. The wider the modulation, the larger the band of frequencies over which the energy is distributed, and the more reduction is achieved from the peak. For example, with a down spread percentage of 0.5% and maximum operating frequency of 100 MHz, the output frequency is swept between 99.5 and 100 MHz.
The modulation frequency, often called sweep rate, defines how fast the spreading signal sweeps from the minimum to the maximum frequency.
Turning on spread-spectrum clocking creates no new top-level ports.
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