When Spread Spectrum (SS) is selected, CLK_OUT3 and CLK_OUT4 are not available. Divide values of these outputs are used for SS modulation frequency generation. There are four modes available for Spread Spectrum Mode:
- DOWN_LOW
- DOWN_HIGH
- CENTER_LOW
- CENTER_HIGH
The available modulation frequency range is 25 – 250 kHz.
Spread Spectrum calculation details are described in the following figures:
Figure 1. Spread Spectrum Mode (Center Spread)
Figure 2. Spread Spectrum Mode (Down Spread)
Note: Input_clock_frequency is in Hz.
For spread:
- If (SS_Mode = CENTER_HIGH):
- If (SS_Mode = CENTER_LOW):
- If (SS_Mode = DOWN_HIGH):
- If (SS_Mode = DOWN_LOW):
In the above equatons, M is CLKFBOUT_MULT_F, D is DIVCLK_DIVIDE, and O is respective CLKOUTX_DIVIDE.
For Modulation Frequency:
- O2 and O3 are calculated by the write_device_image in implementation. Same calculation is done in the wizard to get actual modulation frequency value.
- Then based on what O2 and O3 is calculated, the actual
modulation frequency is calculated:
- If (SS_Mode = CENTER_HIGH or SS_Mode = CENTER_LOW)
- If (SS_Mode = DOWN_HIGH)
- If (SS_Mode = DOWN_LOW)
- If (SS_Mode = CENTER_HIGH or SS_Mode = CENTER_LOW)
Important: Actual modulation
frequency may deviate within +/- 10% of the requested modulation frequency for some
settings.