ALTPLL (Phase-Locked Loop) IP Core User Guide

ID 683732
Date 6/16/2017
Document Table of Contents

Parameter Settings

The parameter settings to configure the bandwidth of the ALTPLL IP core are located on the Bandwidth/SS page of the ALTPLL parameter editor.

The following figure shows the bandwidth configuration options on the Bandwidth/SS page.

Figure 14. Bandwidth Configuration Options

The following list describes the preset values that you can choose:

  • Low—PLL with a low bandwidth has better jitter rejection but a slower lock time.
  • High—PLL with a high bandwidth has a faster lock time but tracks more jitter.
  • Medium—A medium bandwidth offers a balance between lock time and jitter rejection.

If you select Auto, the ALTPLL parameter editor chooses the best possible bandwidth values to achieve the desired PLL settings. In some cases, you can get a bandwidth value outside the Low and High preset range.

To set the bandwidth manually, select Custom, and specify the value. The compiler attempts to achieve the value that you specify, or the closest possible value to achieve your desired setting. You can check the bandwidth value in the compilation report.

The table on the right in the Bandwidth/SS page shows the values of the charge pump current, loop filter resistance and capacitance, and the M counter.

An advanced level of control is also possible for precise control of the PLL loop filter characteristics. This level allows you to select the charge pump current, and loop filter resistance and capacitance values explicitly. The advanced parameters are: charge_pump_current, loop_filter_r, and loop_filter_c.

You can use the programmable bandwidth feature with the clock switchover or spread-spectrum features to get the PLL output settings that you desire. You must set the bandwidth to Auto if you want to enable the spread-spectrum feature.

These parameter settings create no additional top-level ports.