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AR# 4994

M1.5: Trce: Negative slack reported in OFFSET IN AFTER constraints.

Description

General Description:

Design contains the following constraints.

NET "clk" PERIOD = 30.5ns

NET "input" OFFSET = IN 15ns AFTER COMP "clk" ;

NET "input*" TNM = GROUP;

TIMESPEC TS_01 = TO : GROUP : 20;

The result of having the TS_01 constraint is that all registers connected to

the input net are removed from the PERIOD analysis.

This causes the following to happened:

The input net delay is: 10.785ns

The clk net delay is: 3.040ns

The period is: 0.0ns

The offset is: 15.0ns

Required Pad2Setup = Period - Offset + clk net delay = -11.96ns

Actual Pad2Setup = Input net delay = 10.785ns

Slack = Required - Actual = -22.745ns

Since this path has been removed from the PERIOD analysis it is

calculated as 0ns in the OFFSET equation resulting in a negative slack.

Solution

There are a couple of resolutions to this:

1- Change the OFFSET IN AFTER to an OFFSET IN BEFORE. The designer will need to adjust his offset accordingly.

2- Remove the constraint overridding the PERIOD constraint.

This is fixed in the next release of the software, which is 2.1i.

AR# 4994
Date Created 08/31/2007
Last Updated 01/18/2010
Status Archive
Type General Article