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AR# 10427

12.1 Constraints - When should I use a FROM:TO constraint?

Description

When should I use a FROM:TO constraint in my design?

Solution

When using a FROM:TO constraint, you must specify the constrained path(s) by declaring the start and end points, which must be pads, flip-flops, latches, RAMs, or user-specified sync points (see TPSYNC).

To group a set of endpoints together, you can attach a TNM attribute to the object (or to a net that is an input to the object). With a macro, the TNM traverses the hierarchy to tag all relevant objects. A TIMEGRP is a method for combining two or more sets of TNMs or other TIMEGRPs together, or, alternatively, for creating a new group by pattern-matching (grouping a set of objects that all have output nets that begin with a given string).

Figure 1 illustrates a simple example of what can be covered by a FROM:TO constraint:

Figure 1 - Schematic Example of a FROM-TO Constraint
Figure 1 - Schematic Example of a FROM-TO Constraint

You should use FROM:TO constraints for a fast or slow exception from the PERIOD constraint (e.g., if a portion of the design needs to run slower than the PERIOD requirement, use a FROM:TO constraint for the new requirement).

For more details on timing constraints, please see the Timing Constraints User Guide: http://www.xilinx.com/support/documentation/sw_manuals/xilinx12_1/ug612.pdf

AR# 10427
Date Created 08/29/2007
Last Updated 02/21/2013
Status Active
Type General Article