188.8.131.52.4. Receiver Skew Margin for Non-DPA Mode
- In DPA mode, use DPA jitter tolerance instead of the receiver skew margin (RSKM).
- In non-DPA mode, use RSKM, TCCS, and sampling window (SW) specifications for high-speed source-synchronous differential signals in the receiver data path.
Conventions used for the equation:
- 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.
You must calculate the RSKM value, based on the data rate and device, to determine if the LVDS receiver can sample the data:
- A positive RSKM value, after deducting transmitter jitter, indicates that the LVDS receiver can sample the data properly.
- A negative RSKM value, after deducting transmitter jitter, indicates that the LVDS receiver cannot sample the data properly.
Example: RSKM Calculation
- TCCS = 150 ps
- SW = 300 ps
- TUI = 1000 ps
- Total RCCS = TCCS + Board channel-to-channel skew = 150 ps + 200 ps = 350 ps
- RSKM = (TUI – SW – RCCS) / 2 = (1000 ps – 300 ps – 350 ps) / 2 = 175 ps
If the RSKM is greater than 0 ps after deducting transmitter jitter, the non-DPA receiver will work correctly.