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Timing Constraints
FROM:TO (Multi-Cycle) Constraints
This section discusses FROM:TO (Multi-cycle) Constraints and includes:
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About FROM:TO (Multi-Cycle) Constraints
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False Paths or Timing Ignore (TIG) Constraint
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Paths Covered by FROM:TO Constraints
About FROM:TO (Multi-Cycle) Constraints
A multi-cycle path is path that is allowed to take multiple clock cycles. These types of paths
are typically covered by a PERIOD constraint by default. They may cause errors since a
PERIOD is a one-cycle constraint. To eliminate these errors, remove the paths from the
PERIOD constraint by putting a specific multi-cycle constraint on the path.
A multi-cycle constraint is applied by using a FROM:TO constraint.
FROM:TO constraints:
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Have a higher priority than a PERIOD constraint,
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Removes the specified paths from the PERIOD to the FROM:TO constraint
Multicycle constraints:
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Have a higher priority than PERIOD and OFFSET constraints. It pulls paths out of the
lower priority constraints and the paths are analyzed by the multicycle constraints.
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Can be tighter or looser than lower priority constraints.
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Constrain a specific path
The specific path can be within the same clock domain, but have a different requirement
than the PERIOD constraint. Alternatively, the specific path with a data path, which
crosses clock domains are constrained with a multicycle constraint.
A FROM:TO constraint begins at a synchronous element and ends at a synchronous
element. For example, if a portion of the design needs to run slower than the PERIOD
requirement, use a FROM:TO constraint for the new requirement. The multi-cycle path can
also mean that there is more than one cycle between each enabled clock edges. When using
a FROM:TO constraint, you must specify the constrained paths by declaring the start and
end points, which must be pre-specified time groups (such as PADS, FFS, LATCHES,
RAMS), user-specified time groups, or user-specified synchronous points (see TPSYNC).
FROM or TO is optional when constraining a specific path. A FROM multicycle constraint
covers a from or source time group to the next synchronous elements or pads elements. A
TO multicycle constraint covers all previous synchronous elements or pad elements to a to
or destination time group. Following are some possible combinations:
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FROM:TO
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FROM:THRU:TO
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THRU:TO
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FROM:THRU
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FROM
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TO
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FROM:THRU:THRU:THRU:TO
A FROM:TO constraint can cover the multi-cycle paths that cover the path between clock
domains. For example, one clock covers a portion of the design and another clock covers
TIming Constraints User Guide
UG612 (v 13.2) July 6, 2011
www.xilinx.com
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