AR# 38486


14.x Timing - Information regarding the PRIORITY keyword and FROM-TO constraints


The PRIORITY keyword is used to prioritize similar constraints that cover the same paths. When you run the TSI report, you might see something similar to the following:

Constraint interactions for TS_test = MAXDELAY FROM TIMEGRP "test_src" TO TIMEGRP "FFS" 10 ns PRIORITY 10;
10 paths removed by TS_test2 = MAXDELAY FROM TIMEGRP "test2_src" TO TIMEGRP"test2_dst" 8 ns PRIORITY 11;

The tools recognize that the FROM-TO with PRIORITY 10 has precedence over the constraint with PRIORITY 11. Is this correct?


Yes, this is correct.

The PRIORITY keyword can only be used to mitigate constraint overlaps between constraints with the same type and scope. In this particular case, the constraint TS_test has a user-defined FROM group and a pre-defined TO group (FFS), whereas the other constraints in question have both a user-defined FROM and TO group, which gives them higher precedence. Since these constraints have higher precedence (scope), the PRIORITY keyword will have no effect on path coverage between these constraints with different scope.

In the TSI report, you will see the following:

* Constraint coverage over a set of paths is based on the following precedence *
* order (lowest to highest precedence): *
* *
* Unconstrained path analysis *
* PERIOD or FREQUENCY, allclocknets *
* PERIOD or FREQUENCY, time group *
* OFFSET IN/OUT, global *
* OFFSET IN/OUT, time group *
* OFFSET IN/OUT, pad *
* MAXDELAY path, with pre-defined FROM and TO groups *
* MAXDELAY path, with user-defined FROM or TO group *
* MAXDELAY path, with user-defined FROM and TO groups *
* MAXDELAY path, with intermediate (THRU) points *
* PATH definition *
* TIG path *

Note that constraints with user-defined "FROMandTO" groups take precedence over constraints with "FROM or TO" groups (i.e., a constraint with a pre-defined to group).

A user-defined priority associated with a path delay constraint will be used by the Timing Wizard to resolve overlapping or duplicate timing constraints that have a similar scope. For instance, if two MAXDELAY FROM:TO specific.

AR# 38486
日期 11/13/2012
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