Did you start by asking on this forum, or have you spent any time reading up on comparators and thinking about what would happen if you use a comparator with your 8051?

By the way - why do you want to give a sinus signal to the processor? Because you want to know the frequency? Or the curve form? Or analyze for distortion? Or measure the voltage?

Always start with a full explanation of your final intentions, instead of jumping directly to questions about a specific solution.