Using the Fractional PLL Clock Calculator

The PLL Clock Calculator provides a graphical way for you to set up the advanced PLL block. When you open the calculator, the GUI appears in automatic mode, which provides basic settings. You can:

  • Choose the feedback mode (Local, Core, or External).
  • Turn signals on (gray x) or off (green arrow) by clicking the icons next to the signal.
  • Specify the signal names.
  • Specify the phase shift.
  • Choose which clock has feedback (for core feedback mode).

The calculator has a manual mode. Refer to Manually Configuring the PLL for details.

The Topaz PLL supports dynamic phase shifting. To enable it, click the Dynamic button for the clock output. The calculator adds three additional pins that you use to control the dynamic shifting. You need to specify the pin names.

As you make selections, the calculator determines the values for the pre-divider, multiplier, post divider, and clock dividers that meet your settings. The GUI prompts you if you make selections that are impossible to solve.

In manual mode, the interface displays the PLL's internal block diagram, and provides boxes for you to set the values for the pre-divider, multiplier, post divider, and clock dividers. As you adjust the values, the calculator prompts you if you make settings that result in FVCO values that are out of range or are impossible to solve. When you turn manual mode off, the calculator adjusts the output clock frequencies to match the manual settings. If you have incorrect settings for the pre-divider, multiplier, post divider, and clock dividers, when you turn manual mode off, the calculator adjusts the values to ones that allow a valid solution.

When you are finished using the calculator, click Finish to save your settings and close the GUI.

Programmable Duty Cycle and Fractional Feedback

CLKOUT1 supports programmable duty cycle and fractional modes; you cannot use both modes at the same time. To turn these options on:

  1. Click the settings icon next to the Clock 1 options in the clock calculator.
  2. In the CLKOUT1 Feedback Mode dialog box, choose the mode:
    • Disable—Do not use either mode (default).
    • Programmable Duty Cycle—When you choose this mode you can set the target duty cycle. The dialog box shows the actual duty cycle the PLL can achieve based on your other settings.
    • Use as Fractional Feedback—Choose this mode to allow the CLKOUT 1 clock to have a fractional part as well as the integer part. If you want to see the fractional divider equations this mode enables, turn on Manual Mode and click the f icon in the CLK Divider 0-4 boxes.