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AR# 18818

6.1i XST - "ERROR:Xst:1850 - Unable to insert IOBUF on port <xxx> in unit <yyy> having KEEP_HIERARCHY constraint. There are feedbacks in other units."


Urgency: Standard

General Description:

XST rejects designs that have the "Keep Hierarchy" switch set to "Yes" and have inout ports that feed back into other modules, as well as connecting directly to the top-level module. In this case, XST completely retains the hierarchy and consequently cannot make valid connections to all related inout ports.

By default the "Keep Hierarchy" switch is set to Off. To access the "Keep Hierarchy" switch, please refer to (Xilinx Answer 11088).


For your design to pass synthesis, you must turn off the "Keep Hierarchy" switch as follows:

1. Right-click the "Synthesize" process.

2. Select "Properties...".

3. Select the "Synthesis Options" tab.

4. Change the "Keep Hierarchy" setting from "Yes" to "No."

If the hierarchy must be maintained, then you must re-write your design so that all of the inout ports/related logic are on the top level and XST can properly infer IOBUF components.
AR# 18818
Date 06/29/2011
Status Archive
Type General Article