To prevent the removal of these sequential cells, set the physopt_delete_unloaded_sequential_cells variable to false before running clock tree synthesis. As a result, at the end of clock tree synthesis, you no longer see these implicit ignore pins. When you run clock tree synthesis, these unloaded sequential cells are deleted from the design.
If your design contains sequential cells with unconnected outputs, the clock pins of these cells are marked as implicit ignore pins. To see the clock tree exceptions defined for your design, generate a clock tree exceptions report by running the report_clock_tree -exceptions command or by choosing choose Clock > Report Clock Tree in the GUI and selecting Exceptions. To remove clock tree exceptions, use the remove_clock_tree_exceptions command or choose Clock > Remove Clock Tree Exceptions in the GUI. You can set both exceptions on a pin, and clock tree synthesis honors both exceptions. The don't touch subtree exception is compatible with the nonstop, exclude, float, or stop pin exception. ICC prioritizes the clock tree pin exceptions as follows: If you issue the set_clock_tree_exceptions command multiple times for the same pin, the pin keeps the highest-priority exception.
You can set clock tree exceptions on pins or hierachical pins. To define clock tree exceptions, use the set_clock_tree_exceptions command or choose Clock > set Clock Tree Exceptions in the GUI.
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