Answers Database 1.5i 9500/XL Fitter - Uses 2 macrocells instead of 1 for combinatorial latch Record #4174 Product Family: Software Product Line: EPLD Core Product Part: epld-fit Problem Title: 1.5i 9500/XL Fitter - Uses 2 macrocells instead of 1 for combinatorial latch Problem Description: 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. Solution 2: A design example demonstrating the technique for implementing efficient combinatorial feedback latch es is provided at ftp://ftp.xilinx.com/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. End of Record #4174 - Last Modified: 12/09/99 16:08

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