Receiver Skew Margin - Intel Agilex User Manual

5. Intel Agilex High-Speed SERDES I/O Architecture
UG-20214 | 2019.04.02
For the Intel Agilex devices, perform PCB trace compensation to adjust the trace
length of each SERDES channel to improve channel-to-channel skew when interfacing
with non-DPA receivers at data rate above 840 Megabits per second (Mbps). The Intel
Quartus Prime software Fitter Report panel reports the amount of delay you must add
to each trace for the Intel Agilex device. You can use the recommended trace delay
numbers shown under the Transmitter/Receiver Package Skew Compensation panel
and manually compensate the skew on the PCB board trace to reduce channel-to-
channel skew, thus meeting the timing budget between SERDES channels.

5.4.2. 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 37. 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 t
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.
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