There was a thread about ring oscillators recently. I tried to build one in a Xilinx V2Pro and found, as I suspected, that the ISE tools collapsed the ring of inverters into a single inverter. I'm sure there is a simple way to preserve the logic from being pruned by the tools by means of a constraint, but I can't figure it out. What's the secret, short of routing each inverter's output to an IOB to prevent pruning?
The frequency of the oscillator is not critical so I'm not worried about placement or routing.
-Kevin