> I think that the period of a 20Hz signal is too long for the internal
> timers of the 8052. The counters will overflow.

You should consider to configure a timer for gated counter mode,
feed EXTERNAL clock to corresponding counter input, and signal
being measured -- to corresponding gate input. Standard
8051 timers are capable for that.

Only disadvantage of such technique is that signal duty cycle
should be constant for proper period determination. If it isn't,
the frequency should be divided by two before feeding it to
gate input.

The lower limit of frequency measured in this way is limited
only by its period, for not having measurement time too long.

By the way, very convenient would be to use AT89C52, since
it has additional timer capable to generate signal with desired
frequency at T2OUT pin. This pin should be connected directly
to counter input. Then the frequency of EXTERNAL clock mentioned
above can be changed quickly and very broad range of frequencies
with moderate accuracy can be measured.

===
Tomas