Dear Jeff,

Glad to note that you want to write your own code.

At the same time there is no point in reinventing the wheel.

To the left of this page you have a links button. Click that. Scroll down to 'Interesting sites'. There you will find an article " Intel PCA cookbook "

In it you will find actual code ( from Intel - can you beat that ? ) for measuring pulse width, frequency and the whole works.
But they use the PCA special function registers. But maybe you can gain a lot from the discussion even if you dont have access to a chip with PCA.

Good Luck

Raghu