Hello!
It's still unclear. Do you want to measure the frequency of the 230 V line? Just for your
info, it's a known value, and quite accurately.
Now if you're using a frequency variation for a synchrone motor, this is another story.
You can measure it with any micro controller. Even if your micro controller is a 1MHz device,
it will show 20 000 pulses for each period (at 50 Hz), and therefore its accuracy will be
around 1/20 000. More than enough.
Do this with a timer. Everytime you detect a rising 0 crossing of the sine wave, check your
timer, and you will get an exact value of the period.
Dora.