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AR# 24075

11.1 MAP/PAR - Long placement run time on designs with no timing constraints

Description

My design is in the early stages of development, and I have not yet defined any timing constraints. During the placement phase, I am experiencing very long run times. This design should implement easily without difficult timing requirements. Why does this take so long?

Solution


;When no timing constraints have been defined for a design, placement and routing default to "Performance Evaluation Mode," which automatically generates timing constraints for the design. In some cases, this feature can over-constrain the design, which results in long run times as the placer and router struggle to improve the timing results.

Both MAP and PAR support the use of the switch "-ntd", which disables the Performance Evaluation Mode and runs a non-timing driven mode, which will result in faster run time at the expense of timing performance.

NOTE: Prior to ISE version 8.1i, the "-x' switch was used to force a non-timing driven run. After 8.1i, -x is used to ignore user constraints and run Performance Evaluation Mode instead. The newer "-ntd" switch is the same functionality as the old "-x" switch.

Another solution for this problem is to apply at least one loose timing constraint to the design. This will prevent MAP and PAR from using Performance Evaluation Mode. For example, a loose PERIOD constraint on the largest clock net in the design will provide faster run times with some timing optimization and with a minimum of effort.

A CR is under investigation to improve the automatic constraint generation during Performance Evaluation Mode.
AR# 24075
Date Created 09/04/2007
Last Updated 12/15/2012
Status Active
Type General Article
Tools
  • ISE Design Suite - 11.1