For measuring 5 Hz, I'd advice you to have a low crystal frequency, maybe 1MHz or less. The higher your crystal freq, the larger the count that represents the time period, and more complex the calculations. I don't think 16- or 32-bit calculations can be avoided in the f=1/T method, but it shouldn't be a problem if you are programming in C. The point is that you don't need a large count for accuracy, 16-bit value is more than sufficient to get a highly accurate freq measurement.

Do a search on frequency measurement on this forum and you will find a lot of discussions, including references to frequency measurement theory.

kundi