GTS AXI Streaming Intel® FPGA IP for PCI Express* User Guide
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4.1.1. Reference Clock
There are two pairs of reference clock pins in each bank to drive the TX PLL and CDR in PMA, and the System PLL in the bank. The reference clocks can be driven from other banks through the regional reference clock network.
The reference clock to the System PLL must be available and stable before device configuration starts. Derive the reference clock from an independent and free-running clock source. Alternatively, if the reference clock from the PCIe* link is available before device configuration starts, you can use it to drive the System PLL. Once the reference clock from the PCIe* link is active, it is not allowed to go down.
The following figure shows the reference clock to drive the System PLL is from an independent oscillator. It does not share the reference clock from the PCIe* link which drives the TX PLL and CDR in PMA. The reference clock from the PCIe* link may not be available before device configuration starts.