On 2009-06-19, john <[email protected]> wrote:
> I believe XST will not directly infer DDR output flops. At least it
> didn't used to be
> able to do this. I've always had to instantiate DDR output flops.

Another reason to manually instantiate a flip-flop is when the synthesis tool
is doing something suboptimal. For example, I've noticed that XST relatively
often places my critical path on the SR input of flip-flops even though that
path is quite a bit slower than the D input. In those cases I usually
instantiate the flip-flop manually to control what is going on.

(It is possible that a synthesis keep option would also help here though instead
of manually instantiating the flip-flop.)

/Andreas