Another question:

I am also looking at measuring the frequency of a square wave varying between 20Hz and 400Hz. I was thinking about using a frequency to voltage converter like the LM2917 chip, feed the resulting signal to an A/D converter and put that on the bus.

Is there a better way of doing this?

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

Sergiu Tofanel