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 specifications
    
    
  • FROM:TO specifications
    
    
  • PERIOD specifications
    
    
  • ALLPATHS type specifications (in PCF file only).
    
    

• 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
    
    

Net delay and Net skew specifications are analyzed independently of path delay analysis and do not interfere with one another.

If two constraints are in the same category, the user-defined priority described in the “Using the PRIORITY Keyword” section is used to determine which constraint takes precedence.