This section describes how to translate the incoming data rate and reference clock frequency into a valid NIDRU configuration.

The user configuration defines specifications for:

•Incoming data rate with associated tolerance (f_{DIN} ± ppm)

•Available reference clock frequency with associated tolerance (f_{REFCLK} ± ppm)

While f_{DIN} is given, f_{REFCLK} can be selected inside a valid range. The range upper limit comes from the necessity to close timing in the target device, and is thus device and speed grade dependent. The lower limit to f_{REFCLK} might be imposed by the PHY. For example, a SerDes typically specifies a minimum reference clock frequency. A SelectIO interface does not typically impose a lower limit to f_{REFCLK}.

The maximum f_{DIN} is typically limited by the oversampling rate O_{R }defined in This Equation:

Although the O_{R} has to be at least > 2, Xilinx recommends to keep O_{R} ≥ 3 to have enough high frequency jitter tolerance.

The theoretical high frequency jitter tolerance is related to O_{R} as defined in This Equation:

All DRUs for which This Equation is valid have an optimal J_{(TOL-HF)}.

Use This Equation and This Equation to calculate the NIDRU parameters CENTER_F, G_{1}, and G_{2}. This Equation calculates CENTER_F:

This Equation calculates G_{1} and G_{2}:

The spreadsheet nidru_transfer_function_v_1_0.xls in the reference design folder /excel_plots implements the equations listed above.

Using an equal value for G_{1} and G_{2} guarantees that the NIDRU operates in the lock-in region over the full tolerance range (ppm) of both the incoming data and the reference clock. Further reducing G_{2} increases the NIDRU bandwidth. Increasing G_{2} is not recommended because the NIDRU would operate in the pull-in region where the automatic lock is not always guaranteed.

G_{1_P} can be evaluated using the spreadsheet nidru_transfer_function_v_1_0.xls in the reference design folder /excel_plots. The G_{1_P }value should be increased until the ringing effect on the output phase becomes negligible. When G_{1} = G_{2}, setting G_{1_P} = 16 guarantees a negligible ringing effect. Thus, G_{1_P} = 16 is good for most cases.