When using an MMCM, a fixed-phase shift offset is applied to the receiver clock MMCM to skew the clock. This performs static alignment by using the receiver clock MMCM to shift the internal version of the receiver clock such that the input data is sampled at the optimum time. The ability to shift the internal clock in small increments is critical for sampling high-speed source synchronous signals. For statically aligned systems, the MMCM output clock phase offset (as set by the phase shift value) is a critical part of the system, as is the requirement that the PCB is designed with precise delay and impedance-matching for all the GMII/MII or RGMII receiver data bus and control signals.
You must determine the best MMCM setting (phase shift) to ensure that the target system has the maximum system margin to perform across voltage, temperature, and process (multiple chips) variations. Testing the system to determine the best MMCM phase shift setting has the added advantage of providing a benchmark of the system margin based on the UI (unit interval or bit time). The System margin is defined as the following: