First: use an INT0 or any other HW interrupt. For Frequencies below a certain frequency I always measure the period using for ex. the T2 or T0 counter in 16bit mode (beware of overflows just count them), in parallel you can count the frequency in the interrupt routine on a regular interval depending also on the desired range. The rest is a bit of math. This will run smoothly also if you write the code in C

Werner