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AR# 9912

3.1i NGDBuild - "WARNING:nd300 - The signal(s) '<signal_name>' are in combinational feedback"

Description

Keywords: NOREDUCE, VHDL, Verilog, HDL, loop, ignore

Urgency: Standard

General Description:
NGDBuild issues the following warning when signals are in a combinatorial loop:

"WARNING:nd300 - The signal(s) 'N108, N111' 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."

Even after applying the NOREDUCE attribute through my HDL in XST, XST appears to ignore the attribute.

Solution

This is a known problem in XST. To work around this issue, flatten the hierarchy during the synthesis process:

1. Right-click on "Synthesize".
2. Select "Properties".
3. Select the "Synthesis Options" tab.
4. Check the "Flatten Hierarchy" box.
5. Click the "OK" button.

The NOREDUCE attribute will now work when you run your design.
AR# 9912
Date Created 08/14/2000
Last Updated 04/29/2003
Status Archive
Type General Article