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AR# 4174

1.5i 9500/XL Fitter - Uses 2 macrocells instead of 1 for combinatorial latch

Description

Keywords: CPLD, Fitter, 9500, combinatorial, latch, NOREDUCE, M1.4, M1.5i

Urgency: Standard

General Description:
For some combinatorial feedback latches, the fitter uses 2 macrocells
instead of 1, incurring an additional macrocell feedback delay.
Normally, the NOREDUCE property applied to the latches output net
should identify the break point in the feedback loop.

Solution

1

If the fitter incorrectly breaks the feedback loop in the middle of the
latch (instead of at the NOREDUCE net), you must apply COLLAPSE
properties to the intermediate nets within the latch to force them into
the same macrocell as the latches output net.

2

A design example demonstrating the technique for implementing efficient combinatorial feedback latches is provided at http://www.xilinx.com/txpatches/pub/applications/misc/latch_9k.zip The solution is expressed in 3 forms: stand-alone ABEL, VHDL instantiation and schematic with ABEL macro. Each of the 3 design examples can be restored from the ZIP file as a Foundation 1.5 project.
AR# 4174
Date Created 06/30/1998
Last Updated 06/13/2002
Status Archive
Type General Article