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AR# 5889

1.5i Virtex Map - MAP is dropping some FF paths from Period constraint.

Description

Keywords : map, pcf, ffs, missing, worst case, timing, errors, period, constraints

Urgency: Standard

Problem Description:
On Virtex design Worst case paths are not always covered by the PERIOD constraint.

Solution

1

MAP is dropping some FFS from the timing groups. Hence under
timing analysis worst case path is not always shown.

A work around is to use a FFS to FFS constraint instead.

This problem is fixed for the 2.1i release scheduled to be released
in June, 1999.

2

Do custom timing analysis in the Timing Analyser to cover the FFs to FFs path.

3

The missing paths may not just be FF to FF but can also
be from FFs to BlockRams. So the period timespec has to be
replaced by 4:
From:To's - FF->FF, FF->Ram, Ram->FF,
Ram->Ram. Although this can be
reduced to 1 timespec with some use of time groups.

Also the period spec always has lower priority than
from:to so when you replace it you need to start using explicit
PRIORITY values to preserve multicycle from:to definitions.

In 1.5i the priority scheme is: tig, from:to, and period.
In 2.1i the priority scheme is: tig, from:thru:to, from:to, offset, and then period.

4

Add the missing entries to the PCF file time groups so that these paths are considered by PAR.
AR# 5889
Date Created 03/11/1999
Last Updated 04/25/2000
Status Archive
Type General Article