The Vivado Design Suite respects the DONT_TOUCH property during physical optimization. It does not perform physical optimization on nets or cells with these properties. To speed up the net selection process, nets with DONT_TOUCH properties are pre-filtered and not considered for physical optimization. Additionally, Pblock assignments are obeyed such that replicated logic inherits the Pblock assignments of the original logic. Timing exceptions are also copied from original to replicated cells.
For more information, see section Synthesis Attributes in the Vivado Design Suite User Guide: Synthesis (UG901).
The DONT_TOUCH property is typically placed on leaf cells to prevent them from being optimized. DONT_TOUCH on a hierarchical cell preserves the cell boundary, but optimization can still occur within the cell.
The tools automatically add DONT_TOUCH properties of value TRUE to nets that have MARK_DEBUG properties of value TRUE. This is done to keep the nets intact throughout the implementation flow so that they can be probed at any design stage. This is the recommended use of MARK_DEBUG. However, there might be rare occasions on which the DONT_TOUCH is too restrictive and prevents optimizations such as replication and retiming, leading to more difficult timing closure. In those cases DONT_TOUCH can be set to a value of FALSE while keeping MARK_DEBUG TRUE.
The recommended approach for managing MARK_DEBUG usage is the
config_flows -mark_debug option, which allows
you to control optimization of objects with MARK_DEBUG without modifying source
files or constraints. The following three values are supported:
- enable
- Do not optimize MARK_DEBUG nets. This is a default value.
- disable
- Allow both synthesis and implementation to freely optimize MARK_DEBUG nets.
- synthesis_only
- Synthesis will not optimize MARK_DEBUG nets so that they are available at the beginning of implementation, but MARK_DEBUG nets can be freely optimized during implementation.