Finding the Ideal Phase Shift Value - 9.0 English

Tri-Mode Ethernet MAC LogiCORE IP Product Guide (PG051)

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AMD cannot recommend a singular phase shift value that is effective across all hardware families. AMD does not recommend attempting to determine the phase shift setting empirically. In addition to the clock-to-data phase relationship, other factors such as package flight time (package skew) and clock routing delays (internal to the device) affect the clock to data relationship at the sample point (in the IOB) and are difficult to characterize.

Perform a complete sweep of phase-shift settings during your initial system test. Use a test range which covers at least half of the clock period or 128 taps. This does not imply that 128 phase-shift values must be tested. Increments of 4 (52, 56, 60, and so forth) correspond to roughly one MMCM tap at 125 MHz, and consequently provide an appropriate step size. Additionally, it is not necessary to characterize areas outside the working phase-shift range.

At the edge of the operating phase shift range, system behavior changes dramatically. In eight phase shift settings or less, the system can transition from no errors to exhibiting errors. Checking the operational edge at a step size of two (on more than one board) refines the typical operational phase shift range. After the range is determined, choose the average of the high and low working phase shift values as the default.