Intel® Agilex™ General-Purpose I/O and LVDS SERDES User Guide

ID 683780
Date 10/29/2021
Public

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4.7.2. Receiver Skew Margin

Different modes of SERDES receivers use different specifications, which determine the ability to sample the received serial data correctly.
  • In non-DPA mode, use RSKM, TCCS, and sampling window (SW) specifications for high-speed source-synchronous differential signals in the receiver data path.
  • In DPA and Soft-CDR modes, use DPA jitter tolerance instead of the receiver skew margin (RSKM).
The RSKM equation expresses the relationship between RSKM, TCCS, and SW:
Figure 78. RSKM Equation


The equation uses the following conventions:

  • RSKM—the timing margin between the clock input of the receiver and the data input sampling window, and the jitter induced from core noise and I/O switching noise.
  • Time unit interval (TUI)—time period of the serial data.
  • SW—the period of time that the input data must be stable to ensure that the LVDS receiver samples the data successfully. The SW is a device property and varies according to device speed grade.
  • TCCS—the timing difference between the fastest and the slowest output edges across channels driven by the same PLL. The TCCS measurement includes the tCO variation, clock, and clock skew.
Note: If there is additional board channel-to-channel skew, consider the total receiver channel-to-channel skew (RCCS) instead of TCCS. Total RCCS = TCCS + board channel-to-channel skew.

You must calculate the RSKM value, based on the data rate and device, to determine if the LVDS SERDES receiver can sample the data:

  • A positive RSKM value, after deducting transmitter jitter, indicates that the LVDS SERDES receiver can sample the data properly.
  • A negative RSKM value, after deducting transmitter jitter, indicates that the LVDS SERDES receiver cannot sample the data properly.
Figure 79. Differential High-Speed Timing Diagram and Timing Budget
This example shows the RSKM calculation for Intel® Agilex™ devices at 1 Gbps data rate with a 200 ps board channel-to-channel skew.
  • TCCS = 100 ps
  • SW = 300 ps
  • TUI = 1000 ps
  • Total RCCS = TCCS + Board channel-to-channel skew = 100 ps + 200 ps = 300 ps
  • RSKM = (TUI – SW – RCCS) / 2 = (1000 ps – 300 ps – 300 ps) / 2 = 200 ps
The non-DPA receiver works correctly if the RSKM is greater than 0 ps after deducting transmitter jitter.