Constraints Priority

In some cases, two timing specifications cover the same path. For cases where the two timing specifications on the path are mutually exclusive, the following constraint rules apply.

• Priority depends on the file in which the constraint appears. A constraint in a file accessed later in the design flow replaces a constraint in a file accessed earlier in the design flow. Priority is as follows (first listed is the highest priority, last listed is the lowest).
  
  
  • Constraints in a Physical Constraints File (PCF)
    
    
  • Constraints in a User Constraints File (UCF)
    
    
  • Constraints in a Netlist Constraints File (NCF)
    
    
  • Attributes in a schematic
    
    

• If two timing specifications cover the same path, the priority is as follows (first listed is the highest priority, last listed is the lowest).
  
  
  • Timing Ignore (TIG)
    
    
  • FROM THRU TO
    
    
  • FROM TO
    
    
  • Specific OFFSET
    
    
  • Group OFFSET
    
    
  • Global OFFSET
    
    
  • PERIOD
    
    
  • ALLPATHS.
    
    

• FROM THRU TO or FROM TO statements have a priority order that depends on the type of source and destination groups included in a statement. The priority is as follows (first listed is the highest priority, last listed is the lowest).
  
  
  • Both the source group and the destination group are user-defined groups
    
    
  • Either the source group or the destination group is a predefined group
    
    
  • Both the source group and the destination group are predefined groups
    
    
• OFFSET constraints take precedence over more global constraints such as the ALLPATHS constraints.
  
  If two specific OFFSET constraints at the same level of precedence interact, an OFFSET with a register qualifier takes precedence over an OFFSET without a qualifier; if otherwise equivalent, the latter in the constraint file takes precedence.
  
  
• Net delay and Net skew specifications are analyzed independently of path delay analysis and do not interfere with one another.