Intel® Cyclone® 10 GX Transceiver PHY User Guide

ID 683054
Date 4/14/2023
Document Table of Contents

3.11.3. Implementing PLL Cascading

In PLL cascading, the output of the first PLL feeds the input reference clock to the second PLL.

For example, if the input reference clock has a fixed frequency, and the desired data rate was not an integer multiple of the input reference clock, the first PLL can be used to generate the correct reference clock frequency. This output is fed as the input reference clock to the second PLL. The second PLL generates the clock frequency required for the desired data rate.

The transceivers in Cyclone® 10 GX devices support fPLL to fPLL cascading. Only maximum two PLLs are allowed in the cascading chain.
Note: When the fPLL is used as a cascaded fPLL (downstream fPLL), a user recalibration on the fPLL is required. Refer to the "User Recalibration" section for more information.
Figure 145. PLL Cascading

Steps to implement fPLL to fPLL cascading:

  1. Instantiate the fPLL IP core.
  2. Set the following configuration settings for the fPLL IP core in the Parameter Editor:
    • Set the fPLL Mode to Cascade Source.
    • Set the Desired output clock frequency.
  3. Instantiate the fPLL IP core (the second PLL in PLL cascading configuration).
  4. Configure the second fPLL IP core for the desired data rate and the reference clock frequency. Set reference clock frequency for the second fPLL same as the output frequency of the first fPLL.
  5. Connect the fPLL IP core (cascade source) to fPLL IP core (transceiver PLL) as shown in the above figure. Ensure the following connections:
    • The fPLL has an output port hssi_pll_cascade_clk. Connect this port to the second fPLL's pll_refclk0 port.
  6. Set the source (upstream) fPLL bandwidth to Low setting and the destination (downstream) fPLL bandwidth to High setting.
  7. If the input reference clock is available at device power-up, the first PLL is calibrated during the power-up calibration. The second PLL need to be recalibrated. Refer to the User Recalibration section. If the input reference clock is not available at device power-up, then re-run the calibration for the first PLL. After the first PLL has been calibrated, re-calibrate the second PLL.
Note: No special configuration is required for the Native PHY instance.