Grouping critical logic to avoid crossing SLR or I/O columns can help improve the critical path of a design. The following figure shows two examples of a large FIFO implemented with 29 FIFO36E2 primitives. The critical path is from the WRRSTBUSY pin of every FIFO36E2 in the group through 5 LUTs to the WREN pin of every FIFO36E2 in the group.
- On the left, the example shows that the placer was unable to find the most optimal placement of the path, because block RAM utilization was high. FIFO36E2 primitives are marked in red.
- On the right, the example shows that the placer was able to meet timing, because the FIFO36E2 blocks were grouped in a rectangle that avoided the configuration column crossing. FIFO36E2 primitives are marked in green.