Another very good approach to count very high frequencies is to use a prescaler - a digital counter like HCT595 (I hope I have remembered right) Connect the input frequency to the clock input of the counter, connect eight outputs of the counter to the CPU port and the Carry output of the counter to the CPU timer input port.
Reset the counter by the CPU port pin, enable it for the sampling time, stop the counter and read the port (low 8 bits of the count result) and also the 16 bit internal CPU counter. By combining both, you get a 24-bit counter capable of counting 24MHz with no problem (the limitation is the digital prescaler that can go up to 40 MHz). CPU internal timer counts 40MHz/256 = 156.25 kHz that can handle with no problems. When the prescaler counter is stopped, you can read a digital value from the port pins with no problems, for they are stable now. When the counter is running, pins state is changing too fast to read, but you don't need to read it when the counter is running.
I have built a frequency meter for 40MHz using 8052 and a prescaler with very high accuracy.