OK first, you dont have a problem of Duty Cycle ! You just want to measure Duty Cycle.

Fine, the first post already talks about MCU's with PCA.

And you want 100KHz max. That is 10 MicroSec - well not a very comfortable value unless you are clocking really fast. We dont know what your clock is and what your processor is. The knowledge of that could help to help you.

Before you do anything I do hope that you have read the PCA Cook Book by Intel in the Links of this site. It has the works - how to measure period, frequency, duty cycle and what not.

After all this if you still need help - you just have to ask again.

Cheers

Raghu