F-Tile Avalon® Streaming Intel® FPGA IP for PCI Express* User Guide

ID 683140
Date 4/07/2025
Public
Document Table of Contents

3.1.2.1. Guidelines for System PLL Reference Clock

One of the reference clock pins refclk[0] to refclk[7] can also be shared as the reference clock to the System PLL which generates the pld_clk and coreclkout_hip clocks. The reference clock must adhere to the following requirements:
  • If compliance to PCIe link training timing specifications is required, the reference clock to the System PLL must be available and stable before device configuration begins. You must set the Refclk is active at and after device configuration parameter in the System PLL IP to On. Derive the reference clock from an independent and free-running clock source. Alternatively, in a closed system, if the reference clock from the PCIe link is guaranteed available before device configuration starts, you can use it to drive the System PLL. Once the PCIe link refclk is alive, it can never be allowed to go down.
  • If compliance to PCIe link training timing specifications is not required and the reference clock to the System PLL may not be available before device configuration starts, you must set the Refclk is active at and after device configuration parameter in the System PLL IP to Off. In this case, you may use the reference clock from the PCIe link to drive the System PLL.
Figure 8. System PLL Reference Clock in the IP Parameter Editor

The figure below shows an example where an independent reference clock drives the System PLL (via the in_refclk_fgt_0 port). It does not share the reference clock from the PCIe link which is not available before device configuration starts.

Figure 9. Independent Refclk to System PLL

Once the reference clock for the System PLL is up, it must be stable and present throughout the device operation and must not go down. If you are not able to adhere to this requirement, you must reconfigure the device.

Note: For additional information of restrictions on reference clock and the System PLL, refer to the Clock Rules and Restrictions section in F-Tile Architecture and PMA and FEC Direct PHY IP User Guide.