PIC Processor's - Understanding Multiplexing

A problem that occurs frequently in pic projects requiring readout displays is lack of enough IO pins to control more than one or two displays directly. Fortunately, this is a fairly easy problem to work around! - with a bit of clever programming and a minimal bit of external hardware.

The obvious, direct scheme for running multiple displays is to assign one output pin to each segment of each display. For example, to run 3 displays with 7 segments each will require 21 IO's. How inefficient!

Enter multiplexing - the process of using the same 7 IO pins for each segment of each digit, plus one or more addressing lines to control which display is currently on. The general idea is based on persistance of vision - that if we scan through the digits fast enough, they will appear to be continuously on. So how exactly does this work?

Let's use a 3 digit readout for the rest of this tutorial. How many IO's will we need? 1 for each segment, plus 3 addressing lines (there are further improvements, and it's possible to do it with 2 addressing lines for up to four digits, but for the sake of simplicity in introduction, we'll use 3 addressing lines). That gives us a total of 10 IO lines. To see more clearly how this works, look at the animation below:

Lines 1, 2 & 3 conrol the on/off state of each digit. Lines a-g control which segment of the currently on digit are lit up. If we change the state of lines a-g to correspond to which digit is currently on as we scan through the digits (by scanning through lines 1-3), and we do this fast enough, then each digit will appear to be solidly on, and will be displaying it's own value.

 

So there's the theory of it - now it's time to actually implement it!

 

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