I have worked with 8051 project that had 3 digit 7 segment display connected directly to port pins of 8051. A timer interrupt function running at 160 Hz is used to toggle the BP (back plane signal) on and off at a net 80 Hz rate. At each interrupt the on segments are always set to be opposite level of current BP level and off segments are set to the same level as BP. It works great and is not really that complicated, The biggest drawback I see is that it takes so many I/O pins to connect to the display. In my case a total of 23 I/O pins just for 3 digit display plus one decimal point.

If one tried to implement multiplexing in order to save I/O pins then this is where I would draw the line at trying to do it in software and instead use an LCD driver chip or just buy a module. Some of the newest modules, COG (chip on glass) or the thin sandwich types where the LCD glass is glued to a thin PC board of the same size with controller on back in a epoxee bump, are very compact and easy to design into a package. The connection cable becomes small and is a whole lot easier than the many pins of a glass only LCD, Goodness the product I mentioned above has 24 through hole pins to mess with....argh.!!

Mike Karas