You use digital logic... a multiplexer (mux) chip allows you to control several lines using a single pin. But you can only use one line at a time, and you must spend a few more pins to select which line you wish to control at that time.

With n selector lines, you can address 2^n data lines, e.g. with 3 selector lines, you can mux 8 data lines onto a single pin.

Hence you must decide if its worthwhile using a mux.. if you need to mux 2 pins onto one, you need one selector pin as well as one data pin, which again is the same thing with extra hardware. A slightly better scenario is muxing 4 lines onto one pin, so you need one data pin and 2 selector pins, in which case you gain a single extra line.

I haven't used any digital ICs in a reeeally long time so I can't give you any names/numbers, but there should be plenty of material available on the net. People generally prefer using CPLDs for these sort of thing nowadays, but I have no real knowledge about them whatsoever.

kundi