Actually if you read Helfrick Cooper (Modern Electronic Instrumentation and Measurement Techniques), he's given very good explanation of the theory behind frequency measurement. The gist of it is thus: For low frequencies measure frequencey as F = 1/Time-period, and for high frequencies measure F as number of pulses in a second or some definite interval, as per your resolution and accuracy requirements.

I cannot recall exactly now, but the cross-over frequency you use to determine what frequencies you should consider High and Low can be derived as F = square root(Fc), where Fc is the oscillator crystal frequency, which is the pulse frequency you would use to measure the time period of the input frequency. Hence for a normal 8051 running at 12 Mhz, your Fc would be 1 MHz, and your crossover freq would be sqroot(1 * 10^6) = 1000 Hz. So anything over 1000 is in the high range and anything below 1000 is low, and use appropriate methods for each range.

Kundi