DONT_TOUCH - 2023.2 English

Vivado Design Suite User Guide: Using Constraints (UG903)

Document ID
UG903
Release Date
2023-11-01
Version
2023.2 English

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.

Note: Use reset_property to reset the DONT_TOUCH property. Setting the DONT_TOUCH property to 0 does not reset the property.
Tip: Avoid using DONT_TOUCH on hierarchical cells for implementation as Vivado IDE implementation does not flatten logical hierarchy. Use KEEP_HIERARCHY in synthesis to maintain logical hierarchy for applying XDC constraints.