Clocking - 1.3 English

UltraScale+ Devices Integrated Block for PCI Express Product Guide (PG213)
Document ID
PG213
Release Date
2024-06-21
Version
1.3 English

The core requires a 100, 125, or 250 MHz reference clock input. For more information, see the Answer Records at the Solution Center for PCI Express.

The following applies:

  • The reference clock can be synchronous or asynchronous with up to ±300 PPM or 600 PPM worst case. (If spread spectrum clock (SSC) is enabled, the link must be synchronous.)
  • The PCLK is the primary clock for the PIPE interface.
  • In addition to PCLK, two other clocks (CORECLK and USERCLK) are required to support the core.
  • BUFG_GTs are used to generate the core clocks. These clocks are all driven from the TXOUTCLK pin which is a derived clock from GTREFCLK0 through a CPLL. In an application where QPLL is used, QPLL is only provided to the GT PCS/PMA block while TXOUTCLK continues to be derived from a CPLL.
  • The source of the AMD UltraScale+™ GTH reference clock must come directly from IBUFDS_GTE4.
  • To use the reference clock for FPGA general interconnect, another BUFG_GT must be used.
Figure 1. Clocking Architecture

All PCIe clocks (pipe_clk, core_clk, and user_clk) are all driven by BUFG_GT sourced from the TXOUTCLK pin. These clocks are derived clock from GTREFCLK0 through a CPLL. In an application where QPLL is used, QPLL is only provided to the GT PCS/PMA block while TXOUTCLK continues to be derived from a CPLL. All user interface signals of the core are timed with respect to the same clock (user_clk) which can have a frequency of 62.5, 125, or 250 MHz depending on the link speed and width configured (see the previous figure).

In a typical PCI Express® solution, the PCI Express reference clock is a spread spectrum clock (SSC), provided at 100 MHz. In most commercial PCI Express systems, SSC cannot be disabled. For more information regarding SSC and PCI Express, see Section 4.3.7.1.1 of the PCI Express Base Specification, rev. 3.0 .

Important: All add-in card designs must use synchronous clocking due to SSC on the reference clock of most host systems. For devices using the Slot clock, the Slot Clock Configuration setting in the Link Status register must be enabled in the AMD Vivado™ IP catalog.

Each link partner device shares the same clock source. The following figures show a system using a 100 MHz reference clock. Even if the device is part of an embedded system, if the system uses commercial PCI Express root complexes or switches along with typical motherboard clocking schemes, synchronous clocking should still be used.

Note: The clocking diagrams show high-level representations of the board layout. Ensure that coupling, termination, and details are correct when laying out a board. See UltraScale Architecture GTH Transceivers User Guide (UG576).
Figure 2. Embedded System Using 100 MHz Reference Clock
Figure 3. Open System Add-In Card Using 100 MHz Reference Clock

The PCIe core checks for GT power to be stable before the clock is enabled.

  • This results in a logic driven CE (rather than VCC) for the BUFG_GT that is driven by IBUFDS_GTE4 (PCIe ref clock).
  • Before this change in CE, if you had another (parallel) BUFG_GT connected to the IBUFDS_GTE4 with CE driven by VCC, the BUFG_GT_SYNC inserted by opt_design/MLO could drive both BUFG_GTs.
  • If there is a parallel BUFG_GT that does not share the same CE as the PCIe BUFG_GT clock, then two BUFG_GT_SYNC are inserted by opt_design/MLO.
  • Because you can only have one BUFG_GT_SYNC for IBUFDS_GTE4 drivenBUFG_GTs, the router does not know how to handle the second BUFG_GT_SYNC and does not route the IBUFDS_GTE4/ODIV2 driven clock net.
  • You must ensure that the BUFG_GTs driven by the IBUFDS_GTE4 have the same CE/CLR pins.