3.4.2. I/O Assignment Analysis
I/O assignment analysis validates I/O assignments against the complete set of I/O system and board layout rules. Full I/O assignment analysis validates blocks that directly feed or are fed by resources such as a PLL, LVDS, or gigabit transceiver blocks. In addition, the checker validates the legality of proper VREF pin use, pin locations, and acceptable mixed I/O standards
Run I/O assignment analysis during early pin planning to validate initial reserved pin assignments before compilation. Once you define design files, run I/O assignment analysis to perform more thorough legality checks with respect to the synthesized netlist. Run I/O assignment analysis whenever you modify I/O assignments.
The Fitter assigns pins to accommodate your constraints. For example, if you assign an edge location to a group of LVDS pins, the Fitter assigns pin locations for each LVDS pin in the specified edge location and then performs legality checks. To display the Fitter-placed pins, click Show Fitter Placements in the Pin Planner. To accept these suggested pin locations, you must back-annotate your pin assignments.
View the I/O Assignment Warnings report to view and resolve all assignment warnings. For example, a warning that some design pins have undefined drive strength or slew rate. The Fitter recognizes undefined, single-ended output and bidirectional pins as non-calibrated OCT. To resolve the warning, assign the Current Strength, Slew Rate or Slow Slew Rate for the reported pin. Alternatively, can assign the Termination to the pin. You cannot assign drive strength or slew rate settings when a pin has an OCT assignment.
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