The following known limitations can prevent XST from inferring counters:
- The counter load signal is connected to a comparator.
- The counter count statement comes before the load control signal.
- The counter counts by a power of 2.
- Not all count bits are connected.
- The count statement consists of the concatenation operator and the count value.
In each situation listed above, XST produces sub-optimal timing results. The only way to work around this issue is to avoid the above conditions when inferring counters.