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Observer
Observer
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Registered: ‎06-06-2019

Message Severity will not be overridden

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Hi everyone, I'm running into a rather strange issue - I'm receiving the error:

 

[DRC LUTLP-1] Combinatorial Loop Alert: 1 LUT cells form a combinatorial loop. This can create a race condition. Timing analysis may not be accurate. The preferred resolution is to modify the design to remove combinatorial logic loops. If the loop is known and understood, this DRC can be bypassed by acknowledging the condition and setting the following XDC constraint on any one of the nets in the loop: 'set_property ALLOW_COMBINATORIAL_LOOPS TRUE [get_nets <myHier/myNet>]'.

 

Due to the fact that this combinatorial logic loop is inside of a black box of a 3rd party, I cannot modify it. However, the 3rd party vendor has explained that they usually just set the message severity to "Warning" using the constraint "set_property SEVERITY {Warning} [get_drc_checks LUTLP-1]". I've done this and also tested after inputting it through the TCL Console - no matter what it still comes through as an error. Is there any reason this constraint would be ignored or that the message severity would not be set?

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Observer
Observer
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Registered: ‎06-06-2019

Re: Message Severity will not be overridden

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So I had tried "set_property ALLOW_COMBINATORIAL_LOOPS TRUE [get_nets <myHier/myNet>]" and had re-run synthesis and implementation between bitstream generations for each modification to the constraints. I had inherited this project from an ex-employee at my company. I found in the files for this old project a old tcl.pre* file that hadn't been included on the project. Added and it ran without issue. The best part? The only line on the tcl was "set_property SEVERITY {Warning} [get_drc_checks LUTLP-1]". Go figure. I hate Vivado sometimes.

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Xilinx Employee
Xilinx Employee
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Registered: ‎07-16-2008

回复: Message Severity will not be overridden

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Did you re-run the flow after applying the Tcl command? How?

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Moderator
Moderator
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Registered: ‎11-04-2010

Re: Message Severity will not be overridden

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Hi, @akwok2 ,

It seems that the SEVERITY of the message has been changed to ERROR in Vivado.

Generally changing SEVERITY command ("set_property SEVERITY {Warning} [get_drc_checks LUTLP-1]") cannot be used to avoid ERROR.

Please use the command in the error message to skip the checking:

set_property ALLOW_COMBINATORIAL_LOOPS TRUE [get_nets <myHier/myNet>]

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Observer
Observer
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Registered: ‎06-06-2019

Re: Message Severity will not be overridden

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So I had tried "set_property ALLOW_COMBINATORIAL_LOOPS TRUE [get_nets <myHier/myNet>]" and had re-run synthesis and implementation between bitstream generations for each modification to the constraints. I had inherited this project from an ex-employee at my company. I found in the files for this old project a old tcl.pre* file that hadn't been included on the project. Added and it ran without issue. The best part? The only line on the tcl was "set_property SEVERITY {Warning} [get_drc_checks LUTLP-1]". Go figure. I hate Vivado sometimes.

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