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AR# 9723

3.1i Virtex MAP - RPM macro's carry chain alignment is disrupted by map trimming.


Keywords: RPM, carry, XORCY, alignment, route, unroutable

Urgency: Standard

Problem description:
A case has been seen where map trimming led to the incorrect merger
of an FDCE and XORCY. This causes the carry chains in the RPM to be
non-aligned, and therefore unroutable. (This is a regression from 2.1i behavior.)


Root Cause
In the user's design, a FDCE with RLOC drives a LUT1_L and the LUT1_L
drives a XORCY through a LOCALBUF.


The mapper trimmed out both LUT1_L and LOCALBUF and put a "FAST"
property to the input pin of the XORCY during the mapping process.
Therefore, the packer legally merged the FDCE and the XORCY into
the same SLICE, which caused this unroutable situation, as the
XORCY is part of the carry chain.

2.1i doesn't cause this situation because the packer didn't
process the "FAST" property correctly. The local output packer didn't
merge the FDCE and the XORCY together as they should be. Thus, the
unroutable situation is avoided.

This problem is fixed in the latest 3.1i Service Pack available at:
AR# 9723
Date Created 07/18/2000
Last Updated 08/19/2002
Status Archive
Type General Article