When performing RTL simulation of the ALTPLL megafuction, the phase shift step resolution may be different than the expected value when the PLL type is set to "AUTO" by turning on the Select the PLL type automatically option in the ALTPLL MegaWizard® Plug-In Manager. In this case, the simulation model may choose a different VCO frequency than the ALTPLL megafunction, and the phase-step is incorrect.
This problem does not effect gate-level simulation using the ALTPLL megafunction.
The correct phase shift step resolution can be determined by calculation. Please refer to: Phase-Locked Loop (ALTPLL) Megafunction User Guide (PDF), in the Dynamic Phase Reconfiguration Chapter. From this User Guide, the finest phase shift step is 1/8th of the PLL's VCO frequency.