I have used similar principal as San Bergmans mentioned here, for decoding a remote control signal mesurement.

You might not need to use any external component. Just a 89C2051 would do.
Just do the following steps.

1- You make a time base of 1/10 (100ms) seconds, by using one of the timer. Keep the timer on non-running state.

2- Apply your signal (to be measured) on any of port pin (lets say P1.0).

3- When you want to measure the signal, start timer and poll P1.0 pin and check for how long the signal is high and low. calculate the value with reference of your internal time base.

That is a descrete example. You can modify it according to your requirement.

Goodluck.

Rauf