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AR# 63663

2014.4 Vivado Timing - Clock uncertainty shows up as "positive number" even when a negative value is applied

Description

In the synthesized design of the example design "BFT Core", the following command returns a Clock uncertainty of 0.035ns:

report_timing -name test -from [get_clocks bftClk] -to [get_clocks bftClk]  -setup

When I try to set the Clock uncertainty to -0.100 and rerun the report_timing command, I find the Clock uncertainty is 0.065ns, even though it is expected to be -0.065ns.

set_clock_uncertainty -0.100 [get_clocks bftClk]
report_timing -name test -from [get_clocks bftClk] -to [get_clocks bftClk]  -setup

Is this only a display problem?

 

Solution

This is only a display issue in the GUI. The timing engine takes the correct UNCERTAINTY value in the path calculation.

If you check the "UNCERTAINTY" property of the timing path, you will find that the value is the expected negative value.

get_property UNCERTAINTY [get_timing_paths -from [get_clocks bftClk] -to [get_clocks bftClk] ]

-0.065

This issue is fixed in the 2015.1 release of Vivado Design Suite.

AR# 63663
Date Created 02/22/2015
Last Updated 03/04/2015
Status Active
Type General Article
Tools
  • Vivado Design Suite - 2014.4