Why does my MAX_FANOUT assignment get ignored for automatically duplicated nodes?

Description

Due to a problem in the Quartus® II software versions 11.1 SP2 and earlier, the MAX_FANOUT assignment may be ignored by the duplication synthesis algorithm. This problem may occur for registers that are automatically duplicated by the Quartus II software. These nodes generally have ~SynDup appended to the original signal name. The problem occurs when for nodes that fan out to memories implemented using MLAB memory blocks. The entire MLAB instance is incorrectly calculated as one fanout instead of using the number or individual memory ALUTs for the specific memory.

Resolution

There is currently no workaround for this problem. However, the impact should be minimal because all the memory ALUT fanouts that the software treats as a single fanout are located within a single MLAB.

This problem is scheduled to be fixed in a future release of the Quartus II software.