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Chapter 6: XST Design Constraints
If the entity my_design is instantiated several times in the design, the max_fanout=256 constraint is applied
to each instance of my_design.
To apply constraints to specific instances or signals within an entity or module, use the INST or NET keywords.
XST does not support constraints that are applied to VHDL variables.
BEGIN MODEL entityname
INST instancename constraintname = constraintvalue ;
signalname constraintname = constraintvalue ; END;
NET
XST Constraint File (XCF) Example Two
BEGIN MODEL crc32
INST stopwatch opt_mode = area ;
INST U2 ram_style = block ;
NET myclock clock_buffer = true ;
NET data_in iob = true ;
END;
For a complete list of XST synthesis constraints, see XST Specific Non-Timing Options.
Native and Non-Native User Constraint File (UCF) Constraints Syntax
All constraints supported by XST can be divided into two groups:
•
Native User Constraints File (UCF) Constraints
•
Non-Native User Constraints File (UCF) Constraints
Native User Constraints File (UCF) Constraints
Only Timing and Area Group constraints use native User Constraints File (UCF) syntax.
Use native UCF syntax, including wildcards and hierarchical names, for native UCF constraints such as:
•
Period (PERIOD)
•
Offset (OFFSET)
•
Timing Name on a Net (TNM_NET)
•
Timegroup (TIMEGRP)
•
Timing Ignore (TIG)
•
From-To (FROM-TO)
Do not use these constraints inside the BEGIN MODEL...
END construct. If you do, XST issues an error.
Non-Native User Constraints File (UCF) Constraints
For all non-native User Constraints File (UCF) constraints, use the MODEL or BEGIN MODEL...
constructs. This includes:
•
•
END;
Pure XST constraints such as:
–
Automatic FSM Extraction (FSM_EXTRACT)
–
RAM Style (RAM_STYLE)
Implementation non-timing constraints such as:
–
RLOC
–
Keep (KEEP)
If you specify timing constraints in the XST Constraint File (XCF), Xilinx® recommends that you use a forward
slash (/) as a hierarchy separator instead of an underscore (_). For more information, see Hierarchy Separator
(-hierarchy_separator).
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XST User Guide
UG627 (v 11.3) September 16, 2009