Hi Oren,

the 8051 can only count XTAL/24, e.g. 1MHz on the AT89C2051 at 24MHz.

I would suggest to use a prescaler 512 to get 200MHZ / 512 = 390kHz.

But for an accuracy of 5kHz a standard crystal was to inaccurate. You need an oven controlled oscillator and connect them to the XTAL1 input of your 8051.

To get a good feeling you should update the display at about 0.5sec. So you can use a measuring time of 0.5sec.
One timer use to count the measuring time and the other to count the input frequency.

Then calculate:

f_input = f_crystal * prescaler * input_count / time_count / 12

and voila, you get it.


Peter