I'm trying to figure out the best way to floorplan registered adder trees, such as those used in FIR filters. The (Xilinx) placer seems to have very little idea what to do with these. Even when I use an area constraint around the whole tree, the individual adders are not optimally placed, so the result is that my critical path is always between adders, and not the adder carry chain itself. I always have to manually place each adder in the tree to get good results. My questions are:
- Is there a way to get adder trees to work without manual placement?
- Is the best placement for an adder tree a tree structure (wide at one end and narrow at the other) or some more rectangular arrangement?