89c52 frequency meter.
Hi UsbMan!
The best way is to use timer in counter mode. In this mode your counter is counting takts directly from pin of processor. Only you need is to start counter, look to its overflow flag, count overflows and stop it after exact period of time.
You can use one timer of 89c52 in 16 bit mode to count external signal takts, (it can count up to 1/24 of Fosc, so it will be enough to your 100kHz), and another timer with prescaler to count time of measuring.
After first counting, if counted frequency is too low than you can increase mesuring time by 10 and count again in more precision. So your precision will only depends of measuring time.
Best wishes & 73! Klug.