The best way to measure low frequency signals with 0.1% accuracy is with a Phase Lock Loop (PLL). Run The Oscillator at 1000 cps and feed that into a divide by 10. Feed your input signal into the PLL which will lock on and change the Oscillator frequency to match your input frequency except it will be 10 times greater than your input signal. Feed the Oscillator signal into the 80C51 and measure the frequency. Example, if input frequency were 60.1 cps then oscillator frequency would be 601. There are a couple of different books, which give a detail description and schematic examples at the public libraries.

Regards,

Charles Bannister