MAX_FANOUT instructs Vivado synthesis on the fanout limits for registers and
signals. You can specify this either in RTL or as an input to the project. The value is
an integer that indicates what the fanout must be. A value of -1 tells the tool not to
perform any replication.
This attribute only works on registers and combinatorial signals.
Note: The attribute replicates the register or the driver of
the combinatorial signal to achieve the specified fanout. It does not support black
boxes, EDIF files, or Native Generic Circuit (NGC) files.
Recommended: Using
MAX_FANOUT
attributes on global high fanout signals leads to sub-optimal replication in synthesis.
For this reason, AMD recommends only using
MAX_FANOUT inside the hierarchies on local signals with medium to
low fanout.