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AR# 14644

14.x Timing Analyzer - How do I set Timing Analyzer to report asynchronous set/reset paths? (reg_sr_r & reg_sr_o)


How do I configure the Timing Analyzer so that it reports asynchronous S/R paths, when the default analysis does not report asynchronous paths?


To configure Timing Analyzer so that it reports asynchronous S/R paths, you must enable the reg_sr_r (async sr recovery/removal paths) and reg_sr_o (async sr propagation paths) switches.

NOTE: This can be an important time check for coming out of reset (even asynchronous reset). For example, you might enable this switch if you want all counters and FSMs to begin on the same cycle.

To turn this option on from inside Timing Analyzer, follow these steps:
  1. Go to the "Path Tracing" tab of any "Analyze" window.
  2. Under the "Path Tracing" tab, go to the "Path Types" section. If your design contains asynchronous paths, path types called reg_sr_r (async sr recovery paths) and reg_sr_o (async sr propagation paths) will be present, and are unchecked by default.
  3. Check these path types and run the analysis. The asynchronous paths will be reported.

The path tracing control reg_sr_q is obsolete. These paths are now controlled by reg_sr_r (async sr recovery paths) and reg_sr_o (async sr propagation paths).

You can also turn this switch on by placing an "ENABLE" attribute in the UCF or PCF files, as follows:




TIMEGRP <grp_name> ENABLE=reg_sr_q;

By performing the two resolutions, an asynchronous path from a Q output of the flip-flops to the Asynchronous CLR port of another flip-flop can be constrained with a PERIOD constraint on the clock that is clocking both of these flip-flops.

This also applies to all synchronous elements in a common clock domain.

To enable these paths, add the following to the UCF:

ENABLE = reg_sr_r;
ENABLE = reg_sr_o;

For more details on timing constraints, please see the Timing Constraints User Guide: http://www.xilinx.com/support/documentation/sw_manuals/xilinx14_1/ug612.pdf
AR# 14644
Date Created 08/29/2007
Last Updated 09/05/2012
Status Active
Type General Article
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