PCIe 5.0 architecture, like PCIe 4.0 and 3.0 architectures, supports two clock architectures:
Common REFCLK (CC): The same 100-MHz reference clock source is distributed to all components in the PCIe link — Root Complex, Retimer, and Endpoint. Due to REFCLK distribution via PCB routing, fanout buffers, cables, etc., the phase of the REFCLK will be different for all components.
Independent REFCLK (IR): Both the Root Complex and End Point use independent reference clocks and the Tx and Rx must meet stringent specifications operating in IR mode compared to the specifications under CC mode. The PCIe Base specification does not specify the properties of independent reference clocks.