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Registered: ‎07-03-2009

DCM Phase Shift for GEMAC - Negative or Positive values

I am finally at the point where I can look into the setup and hold timings of the GMII port of the GEMAC on my real hardware. The user guide for the GEMAC (ug144 - Appendix C) suggest that extensive investigation of the phase shift should be carried out.


I have a working system using the DCM phase shift suggested in the example UCF (PHASE_SHIFT = -32).


The user guide recommends "a complete sweep of phase-shift settings during your initial system test. Use only positive (0 to 255) phase-shift settings, and use a test range that covers a range of no less than 128, corresponding to a total 180 degrees of clock offset."


I am a little confused by this as the example UCF uses a negative number (and works), should I be using negative or positive numbers for the phase shift ?






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