How do I count the number of high-level output of a comparator zero-crossing but only for 1 second?

[Anonymous]

 my problem is:

   

How do I count the number of high-level output of a comparator zero-crossing?

I have to count how many zero-crossing exist in a second, therefore a counter must count for 1 second and give back the number of zero-crossing and put it in a global variable.

   

any idea?

thanks all!