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AR# 12318

XST - "ERROR:Xst:742 - Unexpected 'Z' expression found"

Description

Keywords: XST, expression, 742

Urgency: Standard

General Description:
When I synthesize a design in XST, the following error appears:

"ERROR:Xst:742 - Unexpected 'Z' expression found."

How do I avoid this error?

Solution

1

One way to fix this problem is to modify the XST State Machine Encoding option. Go to "Synthesis Properties" and select the HDL Options tab; then, change the FSM Encoding Algorithm option to "None".

2

This could also be the result of poor FSM coding:

Bad Code:

when hclk_state =>
//missing default output values here = bad coding
if(HCLK = '1')
then
nDREQ <= '0';
NextState <= data_state;
else
NextState <= idle_state;
end if;

Good Code: (all values covered)

when hclk_state =>
DATA <= DATAX;
D_ZERO <= D_ZEROX;
if(HCLK = '1')
then
nDREQ <= '0';
NextState <= data_state;
else
NextState <= idle_state;
end if;
AR# 12318
Date Created 08/29/2007
Last Updated 10/21/2008
Status Archive
Type General Article