The UltraScale Integrated Block for PCI Express® requires three clocks: CORECLK, USERCLK, and PIPECLK. The three clocks are sourced by BUFG_GTs driven by the TXOUTCLK pin of one of the GT*_CHANNELs of the physical interface. A tight skew requirement exists between the CORCLK and PIPECLK pins and the CORECLK and USERCLK pins. To meet the skew requirement, the placer tightly controls skew as follows:
- Assigns the BUFG_GTs that drive the three PCIe clocks in groups to the upper or lower 12 BUFG_GTs in a Quad
- Assigns the clock root for all three clocks to the same clock region Note: For more information on PCIe clocking requirements, see the UltraScale Devices Gen3 Integrated Block for PCI Express LogiCORE IP Product Guide (PG156).