DONT_TOUCH - 2023.1 English - UG903

Set DONT_TOUCH on a leaf cell, hierarchical cell, or net object to preserve it during netlist optimizations. DONT_TOUCH is most commonly used to:

• Prevent a net from being optimized away.

  A net with DONT_TOUCH cannot be absorbed by synthesis or implementation. This can be helpful for logic probing or debugging unexpected optimization in designs. To preserve a net with multiple hierarchical segments, place DONT_TOUCH on the net PARENT (get_property PARENT $net) which is the net segment closest to its driver.

• Prevent merging of manually replicated logic.

  Sometimes it is best to manually replicate logic, such as a high-fanout driver that spans a wide area. Adding DONT_TOUCH to the manually replicated drivers (as well as the original) prevents synthesis and implementation from optimizing these cells.