Answers Database
M1.5: Trce: Negative slack reported in OFFSET IN AFTER constraints.
Record #4994
Product Family: Software
Product Line: FPGA Implementation
Product Part: Timing Analyzer
Product Version: 1.5i
Problem Title:
M1.5: Trce: Negative slack reported in OFFSET IN AFTER constraints.
Problem Description:
Urgency: Standard
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 1:
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 ac
cordingly.
2- Remove the constraint overridding the PERIOD constraint.
This is fixed in the next release of the software, which is 2.1i.
End of Record #4994 - Last Modified: 05/19/99 14:25 |
