AR# 18924

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7.1i CPLDFit XC9500 CoolRunner/-II - "WARNING:Cpld:179 - The signal(s) '%s' are in combinational feedback loops"

Description

When my design contains a combinational feedback loop, a message similar to the following occurs:

WARNING:Cpld:179 - The signal(s) 'test' are in combinational feedback loops.
These signals may cause hazards/glitches. Logic should include hazard reduction circuitry to avoid hazards/glitches. Apply the NOREDUCE parameter to the hazard reduction circuitry.

Solution

This warning message reminds you to check for glitches or hazards. Hazard reduction circuitry is redundant in logic optimization. 

To prevent the CPLD Fitter from optimizing this redundant logic away, add a NOREDUCE attribute to the equation to instruct the CPLD Fitter to not optimize this equation.

For more details on applying the NOREDUCE attribute, see the "Constraints Guide", which can be accessed at: 

https://www.xilinx.com/support/documentation/sw_manuals/xilinx14_7/cgd.pdf

AR# 18924
Date 06/13/2017
Status Active
Type General Article
Devices
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