Sorry, didn't see the duty cycle requirment.

In your case it is easyest to use a gated counter. Count the processor's crystal frequency only when a gate input is high (or low). This will give you the time of the pulse. This same principle can be used to measure the total time of one cycle as well by simply dividing the input frequency by 2 using a flip-flop.
Dividing the 2 results by each other will give you the duty cycle.
Knowing the cycle time it's fairly easy to calculate the frequency of the signal.

regards, San.